The URL can be used to link to this page

APPROVED-STM-BLD2020-0564+Storm_Drainage_ReportC � ENGINEERING 250 4th Ave S Ste 200 Edmonds, WA 98020 Phone: (425) 778-8500 Fax: (425) 778-5536 civil & structural engineering & planning RECEIVED Jul 13 2020 CITY OF EDMONDS DEVELOPMENT SERVICES DEPARTMENT BLD2020-0564 DRAINAGE REPORT Larsen Residence 7329 158th St SW Edmonds, WA 98026 APPLIES WITH APPLICABLE CITY STROMWATER CODE "'.1 1'0�z�- 0613012020 CG Project No.: 20039.20 Table of Contents Section I — Project Overview Section II — Minimum Requirements Section III — Off -Site Analysis Section IV — Permanent Stormwater Control Plan Section V — Construction Stormwater Pollution Prevention Section VI — Special Reports and/or Studies Section VII — Other Permits Section VIII — Bond Quantities and Operation and Maintenance Manual C GM ENGINEERING 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Larsen Residence - CG #20039.20 June 30, 2020 Drainage Report Section I, Page 1 Section I — Project Overview Section I Summary Overview Existing Condition Developed Condition nilorvicini This drainage report has been written for proposed additions to an existing single-family residence (SFR) in Edmonds, WA. The subject property recently went through a short plat process (PLN 2018-0033, approval 10/21/2019) to divide the property into three lots. The overall parcel is part of the Meadowdale Beach Supplemental Plat Block and is 1.57 ac total. The divided parcel consists of Lot 1 (access from 751h PI W), which contains an existing SFR, Lot 2 (subject property with access from 158th St SW), which contains an existing SFR and Lot 3 (access from 72nd Ave W), which is undeveloped. Lot 2 exists entirely within the Earth Subsidence Landslide Hazard Area (ESLHA). The existing house on Lot 2 is a single story with a one -car garage built into the house and an entry on the west face to a daylight basement. The east side of the house is higher in elevation and consists of a gravel parking area, concrete walkways and entry to the main floor of the house. The proposed development consists of the addition of an attached garage with an associated gravel driveway at the main floor level on the east side, a deck on the west side at the main floor level and new roof overhangs totaling in approximately 3,360 sf of new/replaced hard surfaces. The project must comply with ECDC 18.30 — Stormwater Management Code, the 2014 Stormwater Management Manual of Western Washington (DOE Manual), and the 2017 Edmonds Stormwater Addendum. Per ECDC 18.30.060.D.5.b, the project is classified as a Category 1 (ECDC 18.30.060.D.5.b.ii) and it also meets the Direct Discharge Requirement (ECDC 18.30.060.D.5.b.iv) since the site is within the Puget Sound Piped Watershed and runoff would be discharged to Puget Sound through less than 1,000 lineal feet of conveyance pipes. The Direct Discharge Requirement is assumed to govern overthe Category 1 requirement in this case and a BMP feasibility review can be found in Section IV. This report is based on the steps recommended in Chapter 7 of the Edmonds Stormwater Addendum. Site Address: 7329 158th St SW Tax Parcel Number: 00-5133-0000-2700 Zoning: RS-20 Watershed: Puget Sound Piped Watershed Existing Condition Lot 2 looks trapezoidal in shape and is 26,712 sf (0.61 ac) in area. The lot is already developed with a house, gravel parking areas, concrete walkways and landscaping. Roof downspouts have been observed on the house and according to the property owner, the house's drains are connected to the storm system in 1581h St SW. The existing hard surface areas are on the western half of the lot and are approximately 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ENGINEERING ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Larsen Residence - CG #20039.20 June 30, 2020 Drainage Report Section I, Page 2 4,224 sf (0.10 ac), which takes up about 16% of the property. The remainder of the lot consists of a relatively small keystone block wall, landscaping and a steep slope which extends through the east property line and is characterized as the ESLHA. The steep slope covers more than half of the lot and has an average slope of about 70% with an elevation change of about 100 ft. According to the geotechnical engineering report, two borings were conducted on the lot to depths of approximately 46 to 51 feet. The underlying soils generally consist of what was interpreted as "surficial undocumented fill and historic granular landslide debris" underlain by "native Whidbey Formation deposits". See Geotechnical Engineering Evaluation in Section VI for more. Lot 2 is bordered by 158th St SW on the south edge, Lot 1 on the west edge, an undeveloped lot on the north edge, and Lot 3 on the east edge. There are existing catch basins and storm and sanitary sewer manholes in 158th St SW. The site is within the Puget Sound Piped Watershed. Existing utilities are in -use on Lot 2. The house has an existing side sewer line connection to an existing main in 158th St SW. There is an existing shared water service line between Lots 1 and 2, which is proposed to be abandoned and replaced by a new water meter and service line just for Lot 2. This was proposed under the Short Plat permit (PLN2018-0033) and is referenced on the civil plans for this project. See Section III and the civil plans for more. The existing land coverage is as follows: Roof: 1,954 sf (0.045 ac) Gravel, flat: 1,635 sf (0.038 ac) Walkway, flat: 635 sf (0.014 ac) Lawn, flat: 5,390 sf (0.124 ac) Forest, steep: 17,098 sf (0.392 ac) Total: 26,712 sf (0.613 ac) Developed Condition The proposed development consists of the addition of an attached garage with an associated gravel driveway at the main floor level on the east side of the house and a main floor -level covered deck on the west side totaling in approximately 3,360 sf of new/replaced hard surfaces. On -site Stormwater Management BMPs were evaluated for a project that discharges directly to Puget Sound through the City's MS4. All dispersion and infiltration BMPs were found to be infeasible due to the site being located in the ESLHA. See Section IV for feasibility review. The proposed land cover is as follows: Roof: 4,246 sf (0.097 ac) Asphalt Driveway, flat: 1,033 sf (0.024 ac) Gravel, flat: 1,171 sf (0.027 ac) Lawn, flat: 3,164 sf (0.073 ac) Forest, steep: 17,098 sf (0.392 ac) Total: 26,712 sf (0.613 ac) C GM ENGINEERING 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Larsen Residence - CG #20039.20 Drainage Report June 30, 2020 Section I, Page 3 15700 a 15722 � WI 15701 r4 � 15706 1511; 1571 1572 2as, 157"/S 15730 ---158TH ST SW i 153Uh i •.GO ti� k 1 i 7324 15808 158 APPROXIMATE ti °p 15911 LOT LINES 158 ' 5825- 155 15908� tj n� f j �I 59250 (� 15910 159 V + 15910 f i IS917 a aw 15912 r �' 7309 n 15920 7429 �s -" 16000 3 .NMEADOWDALE RD a � h � a e � Figure 1-1: Vicinity map (from Edmonds GIS Map) 250 4th Avenue South, Suite 200 0 CM Edmonds, WA 98020 ENGINEERING ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Larsen Residence - CG #20039.20 Drainage Report June 30, 2020 Section I, Page 4 Figure 1-2: Aerial image (from Google Maps) 250 4th Avenue South, Suite 200 0 CM Edmonds, WA 98020 ENGINEERING ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Larsen Residence - CG #20039.20 June 30, 2020 Drainage Report Section II, Page 1 Section II — Minimum Requirements Section II Summary Narrative The project must comply with ECDC 18.30 — Stormwater Management Code, the 2014 Stormwater Management Manual of Western Washington (DOE Manual), and the 2017 Edmonds Stormwater Addendum (Stormwater Addendum). It is classified as a Category 1 project per ECDC 18.30.060.D.5.b.ii and must meet Minimum Requirements #1-5 because the proposed amount of new plus replaced hard surfaces totals between 2,000 and 5,000 sf. Minimum Requirement #1: Preparation of Stormwater Site Plans: The stormwater site plan consists of this report and the civil drawings and is prepared in accordance with Chapter 3 of Volume I of the DOE Manual and the requirements in the Edmonds Stormwater Addendum. Minimum Requirement #2: Construction Stormwater Pollution Prevention Plan (SWPPP): The SWPPP shall include a narrative and drawings. The SWPPP narrative shall include documentation that addresses the 13 elements of Construction Stormwater Pollution Prevention. See Section V and the civil drawings. Minimum Requirement #3: Source Control of Pollution: All known, available and reasonable source control BMPs are required for all projects approved by the City. All single-family residential projects shall, at a minimum, incorporate required BMPs from SWMMWW Volume IV, 5411— BMPs for Landscaping and Lawn/Vegetation Management. The Operation & Maintenance Manual found in Section VII addresses Lawn/Vegetation management. Minimum Requirement #4: Preservation of Natural Drainage Systems and Outfalls: Natural drainage patterns shall be maintained, and discharges from the project site shall occur at the natural location, to the maximum extent practicable. The manner by which runoff is discharged from the project site must not cause a significant adverse impact to downstream receiving waters and down -gradient properties. All projects shall submit an off -site qualitative analysis. A qualitative analysis of the upstream and downstream system entering the site is presented in Section III. Minimum Requirement #5: On -Site Stormwater Management: The proposed project discharges directly to Puget Sound through the City's MS4. Per ECDC 18.30.060.D.5.b.iv, it must implement BMP T5.13 Post - Construction Soil Quality and Depth and evaluate BMPs T5.10A Downspout Full Infiltration Systems, T5.10B Downspout Dispersion Systems, T5.10C Perforated Stub -out Connections, T5.11 Concentrated Flow Dispersion, and T5.12 Sheet Flow Dispersion. On -Site Stormwater BMPs were found to be infeasible for all hard surfaces due to the site being located entirely within the ESLHA. See Section IV for more. C GM ENGINEERING 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Larsen Residence - CG #20039.20 Drainage Report June 30, 2020 Section II, Page 2 Dkfcs the project result in 2,ODO Square feet, or greater, of new plus replaced hard surface area? OR Does the land disturbing activity total 710O0 square feet or greater? Yes No Minimum Rcquircmcnls No. 1 through 5 apply Minimum Rcyuircmcni No. 2 applies Nex�--ti— Lh)cs the project add 5,000 square feet or mare of new plus replaced hard surfaces? OR Convert 0.75 acres or mnrc of vegetation to lawn or landscaped areas? OR Convert 2.5 acres or more of native vegetation to pasture? Yes >riu No Tes 7`711ia road related project? Does the project add 5,000 square feet or No more of new hard surfaces? Yes All Minimum Rcquirerrxnts apply to the new and replaced Yes Do new hard surfaces add 50%or hard surfaces and converted more to the existing hard surfaces vegetation areas_ within the project limits? All Minimum Requirements No No additional apply to 0K- new hard surfaces and converted vegetation areas. nquircmcnts. Figure II-1: Flow Chart for Determining Requirements for Redevelopment (Figure 3.1 from the Edmonds Stormwater Addendum) C CM ENGINEERING 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Larsen Residence - CG #20039.20 June 30, 2020 Drainage Report Section III, Page 1 Section III — Off -Site Analysis Section III Summary Task 1 — Define and map the study area Task 2 — Review all available information of the study area Task 3 — Field inspect the area Task 4 - Describe the drainage system, and its existing and predicted problems The off -site analysis for this report will be focused on Lot 3. Task 1— Define and map the study area An initial qualitative analysis shall document potential off -site impacts of stormwater discharges for each upstream drainage system entering a site, and each downstream drainage system leaving a site according to Section 6.2 of the Edmonds Stormwater Addendum. Runoff from the site enters the City's MS4 within one -quarter mile and therefore must evaluate and document downstream conditions up to and including runoff entry into the City's MS4. O O �7ti O �PD1-8 1 \_ O > O 00 Q — 0 15718 Z 7119 o N O O O O 0 1571 15 57; 1 , ? 1 1 Puget>Sound Pi red ' i 2 15125 Watershed 1 711,4 7329 1 i 7 i (1 � P D1-4 a400 7324 CB 1-144 (per 15812 Edmor)os GIS Map) t55zt - 15325 Figure III-1: Study area 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ENGINEERING ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Larsen Residence - CG #20039.20 June 30, 2020 Drainage Report Section III, Page 2 Task 2 — Review all available information on the study area Existing stormwater improvements were determined from the survey and the City GIS map. As can be seen in Figure III-1, there is a public storm main that runs around the subject parcel. According to the property owner, the existing house's drains are tied into CB 1-144 (CB T-1 on the survey). From there, and through a series of 12" and 18" CMP and PVC conveyance pipes, runoff is conveyed through about 800 lineal feet of pipes to the outfall (Figure III-1). Critical areas were evaluated using the City GIS map. Lot 2 sits entirely within the ESLHA and contains severe erosion hazard areas (15%-40%) and landslide hazard areas (40%). Task 3 — Field inspect the study area A site visit was performed on the afternoon of January 31, 2020. The weather was cloudy, and it had lightly rained earlier in the day. Most of the lot and surrounding area is highly vegetated and heavy concentrations of runoff are not expected. There are existing catch basins and manholes near Lot 2 in 1581" St SW. Photos can be seen on the following pages that show the site and existing stormwater structures on -site and downstream to the outfall. Task 4 — Describe the drainage system, and its existing and predicted problems Existing stormwater improvements were determined from the survey and the City GIS map. This is described in Task 2. Due to the site location in the ESLHA, infiltration and dispersion techniques have been determined to be infeasible (see Section IV for more). Runoff from the proposed house additions is proposed to be tied into the house's existing roof drains which are known to be tied into the 158t" St SW storm system. There do not appear to be any issues with the existing storm system, and it is not expected that any issues will arise as a result of the project. Although flow control is not required to be addressed for this project, due to comments that arose during the Short Plat permitting, it was determined that the site should be modeled in WWHM in order to analyze new peak runoff flows from the proposed development. The runoff generated by the proposed development from this project was found to be about a 0.02 cfs increase (using 15-minute time steps) in flow from existing to developed conditions for the 100-year event (see Section IV for WWHM output). From the survey and City GIS, the flow path for runoff consists of 12 to 18-inch pipes. There is also a condition downstream that results in two diverging downstream conveyance systems. On the intersection of 75t" and 1581", the City GIS notes a vertical control structure. Most of the runoff is expected to go southerly along 75t", but there is an overflow that will also convey stormwater to the north if need be. All surveyed pipes appear to have at least 1.0% slope and further downstream topography indicates well - sloping pipes. The capacity of a 12-inch pipe sloping at 1.0% is 3.57 cfs (see Figure IV-1 in Section IV). The 0.02 cfs increase makes up only about 0.5% of the capacity of a 12-inch pipe sloping at 1.0%. Due to such a small percentage of increased capacity, there are no predicted problems associated with runoff generated by this project's development. C GM ENGINEERING 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Larsen Residence - CG #20039.20 Drainage Report June 30, 2020 Section III, Page 3 The photos on the following pages in Figures III-2 through III-9 show portions of Lot 3, locations of existing drainage facilities, and the downstream path of runoff from Lot 2 to Puget Sound. Drainage structures were found to be in good repair and there were no potential problems to be seen. C CM ENGINEERING Figure III-2: Facing NE at existing house (from SW property corner) 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Larsen Residence - CG #20039.20 Drainage Report June 30, 2020 Section III, Page 4 Figure III-3: Bottom of steep slope, looking up (east of house) Figure III-4: Facing west (from 158th St SW, just south of house) C CM ENGINEERING 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Larsen Residence - CG #20039.20 Drainage Report June 30, 2020 Section III, Page 5 Figure III-5: Conveyance connection in 751" PI W MANHOLE Figure III-6: Downstream flow continues on 75`' PI W 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ENGINEERING ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Larsen Residence - CG #20039.20 Drainage Report June 30, 2020 Section III, Page 6 Figure III-7: Runoff conveyed down slope through pipes toward Puget Sound Figure III-8: Catch basin east of Puget Sound. Outfall west to Puget Sound 250 4th Avenue South, Suite 200 0 GM Edmonds, WA 98020 ENGINEERING ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Larsen Residence - CG #20039.20 Drainage Report June 30, 2020 Section 111, Page 7 Figure III-9: Outfall at beach. Pipe not visible here (behind/under rocks) C CM ENGINEERING 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Larsen Residence - CG #20039.20 June 30, 2020 Drainage Report Section IV, Page 1 Section IV — Permanent Stormwater Control Plan Section IV Summary Narrative BMP Feasibility Review WWHM Report Conveyance Capacity Table Airyrrryti o The project must address Minimum Requirement #5: On -Site Stormwater Management. Per ECDC 18.30.060.D.5.b.iv, projects that directly discharge to Puget Sound through the City's MS4 must implement BMP T5.13 Post -Construction Soil Quality and Depth and evaluate BMPs T5.10A Downspout Full Infiltration Systems, T5.1013 Downspout Dispersion Systems, T5.10C Perforated Stub -out Connections, T5.11 Concentrated Flow Dispersion, and T5.12 Sheet Flow Dispersion. The WWHM report in this section supports the claims in Section III about the increase in flows from existing to developed conditions. For Flow Frequency modeling purposes, the site areas were modeled as follows: Existing: Roof: 1,954 sf (0.045 ac) Parking, flat: 1,635 sf (0.038 ac) Walkway, flat: 912 sf (0.014 ac) C, Lawn, flat: 5,390 sf (0.124 ac) C, Forest, steep: 17,098 sf (0.392 ac) Total: 26,712 sf (0.613 ac) Proposed: Roof: 4,246 sf (0.097 ac) Parking, flat: 2,204 sf (0.051 ac) C, Lawn, flat: 3,164 sf (0.073 ac) C, Forest, steep: 17,098 sf (0.392 ac) Total: 26,712 sf (0.613 ac) BMP Feasibility Review Per ECDC 18.30.060.D.5.b.iv, projects that discharge directly to Puget Sound through the City's MS4 must implement BMP T5.13 Post -Construction Soil Quality and Depth and evaluate BMPs T5.10A Downspout Full Infiltration Systems, T5.10B Downspout Dispersion Systems, T5.10C Perforated Stub -out Connections, T5.11 Concentrated Flow Dispersion, and T5.12 Sheet Flow Dispersion to the maximum extents feasible. C GM ENGINEERING 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Larsen Residence - CG #20039.20 Drainage Report June 30, 2020 Section IV, Page 2 1. BMP T5.13 Post -Construction Soil Quality and Depth: Feasible. All new landscaping will meet the Post -Construction Soil Quality and Depth in accordance with BMP T5.13 of the DOE Manual. 2. BMP T5.10A Downspout Full Infiltration Systems: Infeasible. The site is located within the ESLHA. 3. BMP T5.10B Downspout Dispersion Systems: Infeasible. The site is located within the ESLHA. 4. BMP T5.10C Perforated Stub -out Connections: Infeasible. The site is located within the ESLHA. 5. BMP T5.11 Concentrated Flow Dispersion: Infeasible. The site is located within the ESLHA. 6. T5.12 Sheet Flow Dispersions: Infeasible. The site is located within the ESLHA. Stormwater BMPs have been determined to be infeasible due to the lot being located within the Earth Subsidence and Landslide Hazard Area. Therefore, runoff from the proposed development is proposed to be tied into the house's existing roof drains, which are known to be connected to the public storm system in 158t" St SW. See civil plans for more. 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ENGINEERING ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Larsen Residence - CG #20039.20 Drainage Report June 30, 2020 Section IV, Page 3 WWHM Report WWHM2012 PROJECT REPORT (15-minute time steps) Project Name: Larsen Residence Flow Frequencies Site Name: Larsen Residence Site Address: 7329 158t' St SW City Edmonds Report Date: 4/29/2020 MGS Regoin Puget East Data Start 1901/10/01 00:00 Data End : 2058/09/30 00:00 DOT Data Number: 03 Version Date: 2018/10/10 Version : 4.2.16 Low Flow Threshold for POC 1 : 50 Percent of the 2 Year High Flow Threshold for POC 1: 50 year PREDEVELOPED LAND USE Name : Basin 1 Bypass: No GroundWater: No Pervious Land Use acre C, Forest, Steep .392 C. Lawn, Flat .124 Pervious Total 0.516 Impervious Land Use acre ROOF TOPS FLAT 0.045 SIDEWALKS FLAT 0.014 PARKING FLAT 0.038 Impervious Total 0.097 Basin Total 0.613 Element Flows To: Surface Interflow MITIGATED LAND USE Name : Basin 1 Bypass: No Groundwater 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ENGINEERING ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Larsen Residence - CG #20039.20 Drainage Report June 30, 2020 Section IV, Page 4 GroundWater: No Pervious Land Use acre C, Forest, Steep .392 C. Lawn, Flat .073 Pervious Total 0.465 Impervious Land Use acre ROOF TOPS FLAT 0.097 PARKING FLAT 0.051 Impervious Total 0.148 Basin Total 0.613 Element Flows To: Surface Interflow Groundwater ANALYSIS RESULTS Stream Protection Duration Predeveloped Landuse Totals for POC #1 Total Pervious Area:0.516 Total Impervious Area:0.097 Mitigated Landuse Totals for POC #1 Total Pervious Area:0.465 Total Impervious Area:0.148 Flow Frequency Return Periods for Predeveloped. POC #1 Return Period Flow(cfs) 2 year 0.037928 5 year 0.05527 10 year 0.069264 25 year 0.090137 50 year 0.108231 100 year 0.128711 Flow Frequency Return Periods for Mitigated. POC #1 Return Period Flow(cfs) 2 year 0.051667 5 year 0.072272 10 year 0.088052 25 year 0.110592 50 year 0.129383 100 year 0.149989 (0.021 cfs increase) 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ENGINEERING ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Larsen Residence - CG #20039.20 Drainage Report June 30, 2020 Section IV, Page 5 Perind and Impind Changes No changes have been made. This program and accompanying documentation are provided 'as -is' without warranty of any kind. The entire risk regarding the performance and results of this program is assumed by End User. Clear Creek Solutions Inc. and the governmental licensee or sublicensees disclaim all warranties, either expressed or implied, including but not limited to implied warranties of program and accompanying documentation. In no event shall Clear Creek Solutions Inc. be liable for any damages whatsoever (including without limitation to damages for loss of business profits, loss of business information, business interruption, and the like) arising out of the use of, or inability to use this program even if Clear Creek Solutions Inc. or their authorized representatives have been advised of the possibility of such damages. Software Copyright © by Clear Creek Solutions, Inc. 2005-2020; All Rights Reserved. C CM ENGINEERING 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Larsen Residence - CG #20039.20 Drainage Report June 30, 2020 Section IV, Page 6 Conveyance Capacity Table Manning's Equation - Developed Conditions for: Larsen Residence Conveyance 1.49 2 1 Q - *A*R3*S2 npvc= 0.013 '.%ianning's Equation By TAF Date 4 29 2020 Developed Conditions C!4d Dal ENGINEERING ale N.T.S. sreec\o 2504th Ave. South Suite 200 lob No 200�9 20 Edmonds. NV A 98020 Larsen Residence Conveyance Capaci • 10-inch 1_'-inch Slope (ft�ft) 0.005 0.13 O.�10 0.37 1.55 2.52 0.010 0.19 0.56 1.23 2.20 3.51 0.015 0.23 0.69 1.50 2.69 4.37 0.0'_0 0.27 0.79 1.7�1 3.11 5.05 0.025 0.30 O.S9 1.94 3.47 5.65 0.030 0.33 0.97 2.13 3.31 6.13 0.035 0.36 1.05 2.30 �l.11 6.63 0.040 0.3S 1.12 2.�16 �1.39 7.1�! 0.045 O.�10 1.19 2.60 �1.66 7.57 0.050 0.43 1.26 2.75 4.91 7.93 0.055 0.45 132 2.33 5.15 3.37 0.060 O.�17 1.33 3.01 5.33 8.75 0.065 O.�19 1.43 3.13 5.60 9.10 0.070 0.50 1.49 3.25 5.31 9.45 0.075 0.52 1.S�1 3.36 6.02 US 0.080 O.S�I 1.59 3.�17 6.21 10.10 0.085 0.56 1.6-t 3.53 6.40 10.�11 0.090 0.57 1.63 3.63 6.59 10.71 0.095 0.59 1.73 3.73 6.77 11.01 0.100 0.60 1.73 3.38 6.95 11.290 GM Figure IV-1. Conveyance capacity table using Manning's Equation. 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ENGINEERING ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Figure IV-1. Conveyance capacity table using Manning's Equation. 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ENGINEERING ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Larsen Residence - CG #20039.20 June 30, 2020 Drainage Report Section V, Page 1 Section V — Construction Stormwater Pollution Prevention Section V Summary Narrative Erosion control details are provided consistent with the Edmonds Stormwater Addendum and the DOE Manual. Erosion control plan sheets are provided in full size as a part of the civil drawing set. A Construction SWPPP is not required by the Department of Ecology because the proposed area of work is under one acre (the land -disturbing activity threshold which requires the completion of their SWPPP document and Construction Stormwater General Permit). The elements for construction pollution prevention are discussed as follows: Element 1: Mark Clearing Limits To protect adjacent properties and to reduce the area of soil exposed to construction, the limits of construction will be clearly marked before land -disturbing activities begin. Clearing limits will be to the extents of necessary land disturbance for the proposed house additions. The BMPs relevant to marking the clearing limits that will be applied for this project include: High Visibility Plastic or Metal Fence (BMP C103) Element 2: Establish Construction Access Construction access or activities occurring on unpaved areas shall be minimized; yet, where necessary, access points shall be stabilized to minimize the tracking of sediment onto public roads. If required, a stabilized construction entrance should be placed over the existing gravel parking area on the east side of the house. The BMPs relevant to establishing construction access include: Stabilized Construction Entrance (BMP C105) Element 3: Control Flow Rates The site is small and flat enough that a silt fence used for Element 4 may be used to control flow rates. Element 4: Install Sediment Controls All stormwater runoff from disturbed areas shall pass through an appropriate sediment removal BMP before leaving the construction site or prior to being discharged. Silt fence will be installed around the perimeter of the work area. This can be seen on the Temporary Erosion Control Plan, C2.1. Pollution prevention facilities on the erosion control plan must be constructed prior to or in conjunction with all clearing and grading to ensure that the transport of sediment to surface waters and adjacent properties is minimized. The specific BMPs to be used for controlling sediment on this project include: 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ENGINEERING ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Larsen Residence - CG #20039.20 Drainage Report June 30, 2020 Section V, Page 2 Silt Fence (BMP C233) Element 5: Stabilize Soils Leave existing ground cover undisturbed to the maximum extent feasible. Exposed and unworked soils shall be stabilized with the application of effective BMPs to prevent erosion throughout the life of the project. Soils must not remain exposed and unworked for more than the time periods set forth below to prevent erosion: • During the dry season (May 1— September 30): 7 days • During the wet season (October 1— April 30): 2 days The specific BMPs for soil stabilization that shall be used on this project include: Temporary and Permanent Seeding (BMP C120) Mulching (BMP C121) Nets and Blankets (BMP C122) Plastic Covering (BMP C123) Sodding (BMP C124) Topsoil ing/Composting (BMP C125) Surface Roughening (BMP C130) Dust Control (BMP C140) Element 6: Protect Slopes Existing ground cover should be left undisturbed to the maximum extent feasible. Exposed slopes shall be stabilized with BMPs found in Element 5. Element 7: Protect Drain Inlets Drain inlets adjacent to the site and those made operable on -site must be protected from sedimentation. Stormwater shall not enter the conveyance system without first being filtered or treated to remove sediment. Inlet protection devices shall be cleaned or removed and replaced when sediment has filled one-third of the available storage (or as specified by the manufacturer). The specific BMPs to be used for protecting drain inlets are: Storm Drain Inlet Protection (BMP C220) Element 8: Stabilize Channels and Outlets Conveyance channels are not located on or in the immediate vicinity of the site. C GM ENGINEERING 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Larsen Residence - CG #20039.20 Drainage Report June 30, 2020 Section V, Page 3 Element 9: Control Pollutants Design, install, implement and maintain effective pollution prevention measures to minimize the discharge of pollutants. The suggested BMPs are: Concrete Handling (BMP C151) Sawcutting and Surfacing Pollution Prevention (BMP C152) Material Delivery, Storage and Containment (BMP C153) Element 10: Control De -Watering De -watering is not anticipated to be necessary on this project. Element 11: Maintain BMPs All temporary and permanent erosion and sediment control BMPs shall be maintained and repaired as needed to ensure continued performance of their intended function. Element 12: Manage the Project • Phase development projects to the maximum degree practicable and consider seasonal work limits. • Inspection and monitoring— Inspect, maintain, and repair all BMPs as needed to assure continued performance of their intended function. Conduct site inspections and monitoring in accordance with the Construction Stormwater General Permit or local plan approval authority. • Maintain an Updated Construction SWPPP - This SWPPP shall be retained on -site or within reasonable access to the site. The SWPPP shall be modified whenever there is a change in the design, construction, operation, or maintenance at the construction site that has, or could have, a significant effect on the discharge of pollutants to waters of the state. The SWPPP shall be modified if, during inspections or investigations conducted by the owner/operator, or the applicable local or state regulatory authority, it is determined that the SWPPP is ineffective in eliminating or significantly minimizing pollutants in stormwater discharges from the site. The SWPPP shall be modified as necessary to include additional or modified BMPs designed to correct problems identified. Revisions to the SWPPP shall be completed within seven (7) days following the inspection. Element 13: Protect Low Impact Development BMPs N/A — All infiltration and dispersion BMPs were found to be infeasible. C GM ENGINEERING 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Larsen Residence - CG #20039.20 Drainage Report June 30, 2020 Section VI, Page 1 Section VI — Special Reports and/or Studies Section VI Summary: Narrative The following reports are included in this section: 1. Preliminary Geotechnical Engineering Evaluation by Nelson Geotechnical Associates, Inc, dated February 28, 2018. 2. NRCS Soil Resource Report dated April 29, 2020. C CM ENGINEERING 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com MNINE', .- -1 =_ Main Office 17311 — 135th Ave NE, A-500 Woodinville, WA 98072 (425) 486-1669 • FAX (425) 481-2510 February 28, 2018 Carl and Sharon Larsen 9832 Marine View Drive Mukilteo, Washington 98275 NELSON GEOTECHNICAL AssociATEs, INC. ENGiNEERs :. • -- Preliminary Geotechnical Engineering Evaluation Larsen Property Short Plat and Residence Underpinning 15729 — 75r' Place West Edmonds, Washington NGA File No. 999017 Dear Mr, and Mrs. Larsen: Engineering -Geology Branch 5526 Industry Lane, 02 East Wenatchee, WA 98802 (509) 665-7696 - FAX (509) 665-7692 We are pleased to submit the attached report titled "Preliminary Geotechnical Engineering Evaluation — Larsen Property Short Plat and Residence Underpinning — 15729 - 75"' Place West — Edmonds, Washington." This report summarizes the existing surface and subsurface conditions within the project site, and provides geotechnical recommendations for design and construction of the proposed site improvements. Our services were completed in general accordance with the proposal signed by you on June 23, 2017. The site is currently occupied by an existing single-family residence and detached garage within the lower eastern portion of the property and an existing single-family residence with a daylight basement within the central portion of the property. The ground surface within the site generally slopes gently to moderately up to the east from 75t" Place West along the western property line to the toe of a steep to very steep west -facing slope within the central portion of the property. The steep to very -steep west -facing slope ascends up to a relatively level to gently sloping bench area within the very eastern portion of the property. We were informed that the existing residence within the central portion of the property has experienced some foundation settlement throughout the years. We understand that the proposed development plan consists of underpinning the central residence foundation to mitigate potential future settlement of the structure. A detached garage is also proposed to be constructed along the eastern portion of the central residence location between the toe of the steep slope and the residence. We also understand that you wish to subdivide the existing property into three separate residential parcels. The existing residences within the western and central portion of the property would be located on two of the parcels and the upper bench area would be located on the third parcel. The upper eastern parcel will likely be developed with a new single-family residence sometime in the future. Specific development plans and stormwater handling plans were not available at the time this report was prepared. The site is mapped as part of the large historic/prehistoric Meadowdale Landslide complex area now known as the North Edmonds Earth Subsidence & Landslide Hazard Area (ESHLA). The overall property is located in the designated "Zones B through D" generally located above 75`" Avenue West extending east past the top of the steep to very steep west -facing slope. This report provides information and discussion to fulfill the requirements of the City of Edmonds for construction within this area. Preliminary Geotechnical Engineering Evaluation NGA File No. 999017 Larsen Property Short Plat and Residence Underpinning February 28, 2018 Edmonds, Washington Summary - Page 2 We explored the site subsurface soil conditions with four drilled soil borings on August 7, 2017. Our explorations indicated that the lower central portion of the site is underlain by undocumented fill and historic landslide debris at shallow depths, with competent native Whidbey Formation deposits at depth. The upper eastern portion of the site is underlain by competent native glacial soils at shallow depths with competent native Whidbey Formation deposits at depth. Based on our site reconnaissance and explorations, and our understanding of the proposed plans, we have concluded that the proposed short plat subdivision, central residence underpinning, central property garage construction and upper eastern lot development on this site should be feasible from a geotechnical standpoint, provided that our recommendations are incorporated into the overall design and construction of this project. We did not observe signs of recent deep-seated slope failures or earth subsidence on the property. The proposed development area appears to be relatively stable under current conditions. The recommendations presented in this report should aid in maintaining and/or improving the current stability conditions observed at the site. Due to the relatively loose nature of the undocumented fill soil and landslide debris encountered in our explorations within the lower portion of the site within the vicinity of the existing central residence, it is our opinion that the undocumented fill and landslide debris soils have likely contributed to the settlement and distress that has been observed within the central residence. Without foundation improvements, the settlement and settlement -related distress are likely to continue. To limit future potential impacts to the residence as a result of continued settlement, we recommend that all interior and exterior foundations, along with all interior slabs -on -grade be supported on driven pin piles advancing through the undocumented fill and landslide debris and terminating within the underlying native competent soils. Due to tight access constraints within portions of the residence, some of these improvements may be difficult to implement. We therefore should consult with the structural engineer and contractors on adequate stabilization measures as plans are finalized. In the attached report, we have included recommendations for pin pile foundation underpinning support. We also recommend that any new foundations and slabs associated with the proposed garage within the central property be supported on a deep foundation system consisting of driven pin piles. Due to the proposed garage being located in close proximity to the toe of the steep slope within the upper eastern portion of the property, we also recommend that the proposed garage be protected against potential slope movement on the eastern uphill side by incorporating some type of a debris wall on that side. This is further discussed in the attached report. For the proposed development within the upper eastern parcel, we recommend that any proposed structure foundations be designed to utilize conventional spread footings extending down to competent native soil or structural fill extending to these soils. We recommend that any proposed structures be set back at least 60 feet from the top of the steep east -facing slope. In the attached report, we have included recommendations for site grading, retaining walls, foundation support, and site drainage. We strongly recommend that all runoff generated within this site, including roof downspouts, driveways, yard and footing drains, and all runoff entering the property from the road or adjacent properties, be directed into the existing drainage system that is located along the southern portion of the property. No water should be infiltrated or dispersed within the site or near the site slopes. Such activity may lead to instability within the site slopes. We should be retained to review final grading and drainage plans prior to construction. We also recommend that NGA be retained to provide monitoring and consultation services during construction to confirm that the conditions encountered are consistent with those indicated by the explorations, to provide recommendations for design changes should the conditions revealed during the work differ from those anticipated, and to evaluate whether or not earthwork and foundation installation activities comply with contract plans and specifications. Preliminary Geotechnical Engineering Evaluation Larsen Property Short Plat and Residence Underpinning Edmonds, Washington NGA File No. 999017 February 28, 2018 Summary - Page 3 We appreciate the opportunity to provide service to you on this project. Please contact us if you have any questions regarding this report or require further information. Sincerely, NELSON GEOTECHNICAL ASSOCIATES, INC. Khaled M. Shawish, PE Principal Table of Contents INTRODUCTION.......................................................................................................................... l SCOPE............................................................................................................................................ 2 SITECONDITIONS......................................................................................................................2 SurfaceConditions.................................................................................................................... 2 SubsurfaceConditions..............................................................................................................3 HydrogeologicConditions........................................................................................................4 SENSITIVEAREA EVALUATION............................................................................................5 SeismicHazard......................................................................................................................... 5 ErosionHazard......................................................................................................................... 6 Landslide Hazard/Slope Stability. 6 CONCLUSIONS AND RECOMMENDATIONS.......................................................................7 General...................................................................................................................................... 7 Erosion Control and Slope Protection Measures....................................................................10 Site Preparation and Grading.................................................................................................. I I Temporary and Permanent Slopes.......................................................................................... 12 StructureSetbacks.................................................................................................................. 12 FoundationImprovements...................................................................................................... 12 ShallowFoundations..............................................................................................................14 RetainingWalls......................................................................................................................15 StructuralFill.......................................................................................................................... 16 Slab-on-Grade......................................................................................................................... 17 PavementSubgrade................................................................................................................ 18 Site Drainage Improvements.................................................................................................. 18 USEOF THIS REPORT.............................................................................................................19 LIST OF FIGURES Figure 1 —Vicinity Map Figure 2 — Site Plan Figure 3 — Cross -Section A -A' Figure 4 — Soil Classification Chart Figures 5 through 8 — Boring Logs NELSON GEOTECHN/CAL ASSOCIATES, /NC. Preliminary Geotechnical Engineering Evaluation Larsen Property Short Plat and Residence Underpinning 15729 — 75th Place West Edmonds, Washington INTRODUCTION This report presents the results of our preliminary geotechnical engineering evaluation for the proposed Larsen Property Short Plat and Residence Underpinning project. The address for the site is 15729 - 75th Place West in Edmonds, Washington, as shown on the Vicinity Map in Figure 1. The purpose of the study is to explore and characterize the surface and subsurface conditions at the site and provide general geotechnical recommendations for site development and improvements. For our use in preparing this report, we have been provided with an undated, untitled boundary and topographic showing the existing site conditions. The site is currently occupied by an existing single-family residence and detached garage within the lower western portion of the property and an existing single-family residence with a daylight basement within the central portion of the property. The ground surface within the site generally slopes gently to moderately up to the east from 751h Place West along the western property line to the toe of a steep to very steep west -facing slope within the central portion of the property. The steep to very -steep west -facing slope ascends up to a relatively level to gently sloping bench area within the very eastern portion of the property. We were informed that the existing residence within the central portion of the property has experienced some foundation settlement throughout the years. We understand that the proposed development plan consists of underpirming the central residence foundation to mitigate potential future settlement of the structure. A detached garage is also proposed to be constructed along the eastern portion of the central residence location between the toe of the steep slope and the residence. We also understand that you wish to subdivide the existing property into three separate residential parcels. The existing residences within the western and central portion of the property would be located on two of the parcels and the upper bench area would be located on the third parcel. The upper eastern parcel will likely be developed with a new single-family residence sometime in the future. Stormwater plans have not been finalized at this time, but we anticipate that runoff generated on this site will be tighlined to discharge into an existing stormwater system within the southern portion of the property. The current site layout is shown on the Site Plan in Figure 2. The site is mapped as part of the large historic/prehistoric Meadowdale Landslide complex area now known as the North Edmonds Earth Subsidence & Landslide Hazard Area (ESHLA). The overall property is located in the designated "Zones B through D" generally located above 75tt' Avenue West extending east past the top of the steep to very steep west -facing slope. Slide movement from the large- scale slide complex and smaller localized slides within the complex can both affect this property. This NELSON CEOTECHNICAL ASSOCIATES, INC. Preliminary Geotechnical Engineering Evaluation Larsen Property Short Plat and Residence Underpinning Edmonds, Washington NGA File No. 999017 February 28, 2018 Page 2 report provides information and discussion to fulfill the requirements of the City of Edmonds for construction within this area. SCOPE The purpose of this study is to explore and characterize the site surface and subsurface conditions, and provide opinions and recommendations for the proposed site development. Specifically, our scope of services includes the following: 1. Review available soil and geologic maps of the area. 2. Explored the subsurface soil and groundwater conditions within the site with four geotechnical borings using a limited access drill rig. The drill rig was subcontracted by NGA. 3. Map the conditions on the slope and evaluate current slope stability conditions and perform hand explorations within the steep slope area and interior portion of the central residence, as needed. 4. Perform laboratory classification and analysis of soil samples, as necessary. 5. Provide recommendations for earthwork, foundation support, and slabs -on -grade in accordance with the City standards for development in the North Edmonds ESLHA. 6. Provide recommendations for temporary and permanent slopes. 7. Provide recommendation for deep foundation support, as needed. 8. Provide recommendations for the existing residence underpinning support. 9. Provide recommendations for retaining walls. 10. Provide recommendations for subgrade preparation. 11. Provide recommendations for site drainage and erosion control. 12. Document the results of our findings, conclusions, and recommendations in a written geotechnical report. SITE CONDITIONS Surface Conditions The western and central portions of the property are generally situated on gently to moderately sloping ground above and to the east of 75"' Place West. The western portion of the property is occupied within an existing residence and detached garage, while the central portion of the property is occupied with an existing single-family residence with a daylight basement. The ground surface within the western and central portions of the property slopes gently to moderately up to the east from 75" Place West to the toe of the steep to very steep west -facing slope within the eastern portion of the property at gradients in the range of approximately 2 to 18 degrees (3 to 32 percent). A series of concrete block retaining walls are located within the sloping areas between the existing residences. The western and central portions of the property are generally vegetated with grass and landscaping plants. NELSON GEOTECHN/CAL ASSOC/A TES, INC Preliminary Geotechnical Engineering Evaluation NGA File No. 999017 Larsen Property Short Plat and Residence Underpinning February 28, 2018 Edmonds, Washington Page 3 From the lower central portion of the site, the ground surface slopes steeply to very steeply up to the east to a relatively level to gently sloping upper bench area at gradients in the range of 35 to 45 degrees (70 to 100 percent). The existing residence within the central portion of the site is located approximately 37 to 47 feet from the toe of the steep to very steep west -facing slope. The overall height of the steep slope is approximately 100 feet. The existing site conditions, site topography, and interpreted subsurface conditions are presented on Cross Sections A -A' in Figure 3. The steep slope and upper bench area are generally vegetated with dense underbrush and young to mature trees. We did not observe any surface water during our site visit on August 7, 2017. We also did not observe any groundwater emitting from the site slopes, however we were informed that water seepage on the steep slope has been observed in the past. We did not observe any indications of recent slope movement such as deep-seated landsliding, however some minor erosion and sloughing was observed within portion of the upper most part of the steep west -facing slope area. The lower western and central portion of the property is accessed via an existing asphalt driveway located along the southern portion of the property extending from 751h Place West. The upper eastern portion of the property is accessed via 72nd Avenue West. The site is bordered to the north, south and east by existing residential properties, and to the west by 751h Place West. The existing and proposed site conditions are shown on the Site Plan in Figure 2. Subsurface Conditions Geology: The geologic units for the site vicinity are shown on the Preliminary Geologic Map of the Edmonds East and Edmonds West Quadrangles Snohomish and King Counties, Washington, by Mackey Smith (U.S.G.S., 1975). The lower western and central portion of the site is mapped as old landslides (Qols) while the upper eastern portion of the property is mapped as Esperance Sand deposits (Qe). The old landslide deposits are described as large slumps that occurred during the ablation of the Puget Lobe of the Vashon ice sheet by lowering of water -table level. The Esperance sand is described as thinly bedded sand. We generally encountered undocumented fill and granular landslide debris underlain by fine- grained silt and clay within the lower central portion of the property that we interpreted as native Whidbey Formation soils. Our explorations within the upper eastern portion of the site generally encountered native glacial till and Esperance sand soils with fine-grained sand, silt and clay soils that we interpreted as native Whidbey Formations soils. Explorations: The subsurface conditions within the site were explored on August 7, 2017 by drilling four borings using a track -mounted limited -access drill rig to depths of approximately 46.5 to 51.5 feet below the existing ground surface. The approximate locations of our explorations are shown on the Site Plan in Figure 2. A geologist from NGA was present during the explorations, examined the soils and geologic conditions encountered, obtained samples of the different soil types, and maintained logs of the borings. NELSON GEOTECHN/CAL ASSOC/A TES, INC Preliminary Geotechnical Engineering Evaluation NGA File No. 999017 Larsen Property Short Plat and Residence Underpinning February 28, 2018 Edmonds, Washington Page 4 A Standard Penetration Test (SPT) was performed on each of the samples during drilling to document soil density at depth. The SPT consists of driving a 2-inch outer -diameter, split -spoon sampler 18 inches using a 140-pound hammer with a drop of 30 inches. The number of blows required to drive the sampler the final 12 inches is referred to as the "N" value and is presented on the boring logs. The N value is used to evaluate the strength and density of the deposit. The soils were visually classified in general accordance with the Unified Soil Classification System, presented in Figure 4. The logs of our borings are attached to this report and are presented as Figures 5 through 8. We present a brief summary of the subsurface conditions in the following paragraphs. For a detailed description of the subsurface conditions, the boring logs should be reviewed. Underlying the ground surface within Borings B-1 and B-2 within the lower central portion of the property, we generally encountered very loose to medium dense, gray -brown to gray silty fine to medium sand with varying amounts of gravel and organics that we interpreted as surficial undocumented fill and historic granular landslide debris. Underlying the surficial undocumented fill and historic granular landslide debris in B-2 at a depth of approximately 40 feet below the ground surface, we encountered very stiff blue -gray silty with varying amounts of fine sand and clay that we interpreted as native Whidbey Formation deposits. Boring B-1 was terminated within the historic granular landslide deposits at a depth of 46.5 feet below the existing ground surface, while Boring B-2 was terminated within the native fine- grained Whidbey Formation deposit at a depth of approximately 51.5 feet below the existing ground surface. Below the ground surface within Borings B-3 and B-4 within the upper eastern portion of the property generally encountered approximately 20.0 to 25.0 feet of medium dense to dense, gray to orange -brown silty fine to medium sand that we interpreted as native glacial soils. Underlying the native glacial soils in Borings B-3 and B-4, we encountered medium dense/very stiff to very dense/very hard blue -gray to gray silty fine sand and silt with varying amounts of sand that we interpreted as native Whidbey Formation Deposits. Borings B-3 and B-4 were terminated within the native fine-grained Whidbey Formation deposit at a depth of approximately 46.5 feet below the existing ground surface. Hydrologic Conditions Groundwater seepage was not encountered in our explorations. However, wet soil conditions were encountered within the upper granular soils above the fine-grained siltier soils encountered at depth. It is our opinion that any groundwater seepage encountered within the site would be perched water. Perched water occurs when surface water infiltrates through less dense, more permeable soils and accumulates on top of relatively low permeability materials. The more permeable soils on this site would consist of the granular soils found within the landslide debris and the upper native glacial soils. The low NELSON GEOTECHNICAL ASSOC/A TES, INC Preliminary Geotechnical Engineering Evaluation NGA File No. 999017 Larsen Property Short Plat and Residence Underpinning February 28, 2018 Edmonds, Washington Page 5 permeability soil consists of underlying siltier soils. Perched water does not represent a regional groundwater "table" within the upper soil horizons. Perched water tends to vary spatially and is dependent upon the amount of rainfall. We would expect the amount of perched groundwater to decrease during drier times of the year and increase during wetter periods. However, due to the large area of recharge, there may be areas of seepage and wet soils on this site even in the drier times of the year. SENSITIVE AREA EVALUATION Seismic Hazard Older landslide debris and medium stiff to hard cohesive soils were encountered underlying the site at depth within the western and central portions of the property, while medium dense/very stiff to very dense/very hard native glacial soils and Whidbey Formation deposits were encountered within the upper eastern portion of the property. Based on the 2015 International Building Code (IBC), the site conditions best fit the description for Site Class E within the lower western and central portions of the property while the site conditions best fit the description for Class D soils within the upper eastern portion of the property. Tables 1 and 2 below provides seismic design parameters for the site that are in conformance with the 2015 IBC, which specifies a design earthquake having a 2% probability of occurrence in 50 years (return interval of 2,475 years), and the 2008 USGS seismic hazard maps. Table 1— 2015 IBC Seismic Design Parameters (Lower Western and Central Portions of the Site) Site Class Spectral Acceleration Spectral Acceleration Site Coefficients Design Spectral at 0.2 sec. (g) at 1.0 sec. (g) Response SS S I Parameters Fa Fv SDs SDI E 1.323 0.520 0.90 2.40 0.794 0.832 Table 2 — 2015 IBC Seismic Design Parameters (Upper Eastern Portion of the Site) Site Class Spectral Acceleration Spectral Acceleration Site Coefficients Design Spectral at 0.2 sec. (g) at 1.0 sec. (g) Response Ss Si Parameters Fa Fv SDs SDI D 1.324 0.521 1.00 1.500 1 0.883 0.521 The spectral response accelerations were obtained from the USGS Earthquake Hazards Program Interpolated Probabilistic Ground Motion website (2008 data) for the project latitude and longitude. NELSON CEOTECHN/CAL ASSOC/A TES, INC. Preliminary Geotechnical Engineering Evaluation NGA File No. 999017 Larsen Property Short Plat and Residence Underpinning February 28, 2018 Edmonds, Washington Page 6 Hazards associated with seismic activity include liquefaction potential and amplification of ground motion. Liquefaction is caused by a rise in pore pressures in a loose, fine sand deposit beneath the groundwater table. It is our opinion that the competent glacial and fine-grained soils interpreted to underlie the site have a low potential for liquefaction or amplification of ground motion. The glacial soils interpreted to form the core of the steep slopes within the site are considered stable with respect to deep-seated slope failures. All steep slopes have the potential for shallow sloughing failures during seismic events. Such events should not significantly affect the planned development provided the foundations are designed with the recommended setback values, debris protection systems, and the slope and drainage systems are maintained as described in this report. Erosion Hazard The criteria used for determining the erosion hazard for the site soils includes soil type, slope gradient, vegetation cover, and groundwater conditions. The erosion sensitivity is related to vegetative cover and the specific surface soil types, which are related to the underlying geologic soil units. The Soil Survey of Snohomish County Area, Washington by the Soil Conservation Service (SCS), was reviewed to determine the erosion hazard of the on -site soils. The site surface soils were classified using the SCS classification system as Everett very gravelly sandy loam, 15 to 30 percent slopes within the lower western portion of the site, Alderwood-Everett gravelly sandy loams, 25 to 70 percent slopes within the central portion of the site and Alderwood-Urban land complex, 2 to 8 percent slopes within the upper eastern portion of the site. These soils are described as having a slight to high erosion hazard. It is our opinion that the site soils should have a slight to moderate hazard for erosion in areas that are not disturbed and where vegetation cover is not removed. Landslide Hazard/Slope Stability The criteria used for the evaluation of landslide hazards include soil type, slope gradient, and groundwater conditions. The western and central portions of the site slope gently to moderately down to the west. A steep west -facing slope is located above and to the east of the existing central residence within the eastern portion of the property. Groundwater seepage was not observed on the sloping portions of the property during site visit, however we understand that groundwater seepage has been observed on the steep slope area in the past. This site and the overall site vicinity lies within a known ancient landslide area. The site and vicinity have been relatively stable for a very long period of time, and development in the area has taken place in the form of single-family residences, roadways, and underground utilities. Although the likelihood of the ancient slide to become active in the foreseeable future is very low, extreme environmental conditions NELSON GEOTECHNICAL ASSOCIATES, INC. Preliminary Geotechnical Engineering Evaluation NGA File No. 999017 Larsen Property Short Plat and Residence Underpinning February 28, 2018 Edmonds, Washington Page 7 coupled with inadequate human practices could, in theory, re -activate the ancient landslide. Such external factors could include severe and elongated weather events and/or significant seismic activity. The site falls within "Zones B through D" of the North Edmonds Earth Subsidence and Landslide Hazard Area Report prepared by Landau Associates for the City of Edmonds. This designation requires that certain features be included (or excluded) in the design. Such features include the restriction of cuts and fills, the need for tightlining runoff into an approved system, the need to design foundations and retaining walls to withstand high lateral earth pressures and potential loss of soil beneath parts of the foundation, the need to vegetate slopes with deeply rooted drought -tolerant vegetation, and the elimination of any and all irrigation systems. We have addressed all of these requirements in the remainder of this report. We encountered what we interpreted as older granular landslide debris to depths of about 40 feet below the existing ground surface within the lower (central) portion of the property. Medium dense/stiff undisturbed native soils were encountered below this material, which we interpret the core of the site slopes at depth to consist primarily of this relatively stable material. Potential deep-seated slide planes were reported to be up to 100 feet deep below ground surface within the area; however, we did not observe signs of recent deep-seated slope failures on the property and do not anticipate that such failures will occur during the expected life of the existing and proposed structures. Localized areas of surface instability and surface sliding can occur on steep slopes, particularly where modified through grading activities. Backwasting (movement of near -surface soil) through soil erosion processes or local surface slides is common to slopes, particularly where the soils are exposed to weathering. Normal surface erosion and shallow sloughing failures should be expected to continue on the steeper portions of the site, but we have provided geotechnical recommendations for deep -foundation support, debris catchment, erosion control, structure setbacks and other development considerations that should reduce the potential impact of site development on the site slopes. CONCLUSIONS AND RECOMMENDATIONS General It is our opinion, from a geotechnical standpoint, that the site should be compatible with the proposed development and improvements, provided that the geotechnical recommendations presented in this report are incorporated into project plans and followed during construction. The proposed development area appears to be relatively stable under current conditions. However, the historic landslide mapped at the site could be reactivated by extreme natural conditions and/or poor grading and/or drainage activities by other human activity in the general site vicinity. We consider this potential to be low. NELSON GEOTECHNICAL ASSOCIA TES, INC. Preliminary Geotechnical Engineering Evaluation NGA File No. 999017 Larsen Property Short Plat and Residence Underpinning February 28, 2018 Edmonds, Washington Page 8 In general, the site and steep slopes within the site currently appear generally stable with respect to deep- seated movement. However, the site vicinity is mapped within an older landslide complex and our explorations encountered evidence of past landslide debris and ground movement within the lower central and western portions of the property. The potential for landslide and erosion hazards within this site will greatly depend on how the site is graded and how surface water and near surface water are controlled. We recommend that site grading be kept to a minimum and that grading and site drainage plans be subjected to geotechnical engineering review prior to construction. This review would include specific stability analysis of proposed cuts and fills and would provide recommendations for accomplishing the desired grading while maintaining long-term stability. In our opinion, based on our observations and our understanding of the prevailing building conditions, the settlement and distress to the central residence is likely a result of long-term consolidation (settlement) of the relatively loose undocumented fill and landslide debris soils that were encountered in our explorations and is interpreted to underlie portions of the residence. It appears that this full was likely placed during the initial development of this property. Due to the varying nature of the overall density of the fill encountered in our explorations, it is likely that some of the fill material was placed in a loose condition at the time of residence construction, and throughout the years has been settling under the building loads. It is likely that the foundations and slabs -on -grade will continue to experience steady settlement and subsidence under the current conditions resulting in additional distress to the foundation, slabs and utilities within the structure. To mitigate this condition, we recommend that the all exterior and interior foundations along with all slabs -on -grade affected by the ongoing settlement be supported on deep foundations consisting of driven pin piles to transfer building loads through the undocumented fill and landslide debris down to the underlying competent native soils. Lateral loads on the residence should be resisted by helical anchor tiebacks. The exact location and extent of the pin piles and helical anchor tiebacks should be determined by the structural engineer. We should be retained to discuss with the structural engineer the details of the stabilization plans. We have provided recommendations for driven pin piles and helical anchor tiebacks in the Foundation Improvements subsection of this report. We also recommend that after the pin pile and helical anchor installation is completed, the remainder of the residence be closely monitored for any signs of further distress, and additional stabilization measures be installed if signs of future distress are observed. We recommend that if a slab -on -grade is utilized in the lower portions of the proposed central residence, that the slab be designed as a structural slab and be supported on the deep foundation system. Other hard surfaces, such as paved areas or walkways that are supported on the existing soil have some risk of future settlement, cracking, and the need for maintenance. To reduce this risk, we recommend over -excavating NELSON GEO TECHNICAL ASSOCIATES, INC. Preliminary Geotechnical Engineering Evaluation NGA File No. 999017 Larsen Property Short Plat and Residence Underpinning February 28, 2018 Edmonds, Washington Page 9 a minimum of two feet of the upper soil from the slab and pavement areas and replacing this material with compacted pit run or crushed rock structural fill. This recommendation is only for hard surfaces to be supported on grade and does not apply for the lower floor structural slab. Even with the recommended treatment, some settlement of the underlying loose material should be anticipated. In addition to the foundation improvements, we recommend that the drainage picture surrounding the existing central residence be improved. Such improvements should include confirming and/or implementing the placement of footing drains around the building, and routing all downspouts and runoff from all hard surfaces into a permanent discharge system. We recommend that these pipes be extended to discharge to an approved system. This is further discussed in the Drainage Improvements subsection of this report. It is also our opinion that the proposed garage development along the eastern side of the existing central residence is feasible from a geotechnical standpoint. We recommend that the foundations and slabs associated with the proposed garage structure be supported on a deep foundation system consisting of driven pin piles. We understand that the garage will be constructed along or very near the toe of the steep west -facing slope. In our opinion, this should be feasible provided the foundation walls along the upslope side of the garage be designed as recommended in this report. We recommend that the upslope portions of the garage foundation be designed as debris walls and extend a minimum of four feet above finished ground surface to protect the structure against potential failures on this slope. This is intended to provide a catchment measure should any sloughing debris travel towards the garage during extreme weather or as a result of an earthquake. Alternatively, a separate debris protection structure or fence could be utilized. It is our opinion that the planned residence development within the upper eastern portion of the site is feasible from a geotechnical standpoint. It is also our opinion that the soils that underlie the upper eastern portion of the site and form the core of the steep west -facing slope should be stable with respect to deep- seated earth movements, due to their inherent strength and slope geometry. However, shallow failures could occur on the slopes in the loose surficial soil, especially during adverse weather or a significant seismic event. Proper structure setbacks along with erosion and drainage control measures as recommended in this report should reduce this potential. Our explorations indicated that the upper eastern portion of the site is underlain by medium dense or better native glacial soils at relatively shallow depths. The native glacial soils within the proposed upper eastern development area should provide adequate support for foundation, slab, and pavement loads. We recommend that the structures within the upper eastern portion of the site be designed utilizing shallow foundations. Footings should extend through any loose surficial soil and be keyed into the underlying competent native soils. These soils should be encountered roughly two to five feet below the existing ground surface within the planned development area, with some potential localized areas of deeper loose soils in unexplored areas of the site. To protect NELSON GEOTECHN/CAL ASSOCIATES, INC. Preliminary Geotechnical Engineering Evaluation NGA File No. 999017 Larsen Property Short Plat and Residence Underpinning February 28, 2018 Edmonds, Washington Page 10 the structures against potential failures on the slopes, we recommend that any proposed structures be set back at least 60 feet from the top of the steep west -facing slopes. We should be retained to review final structure locations and to observe foundation excavations prior to placing forms. The control of surface and near -surface water is very important for the long-term stability of the site and on the steeper portions of the site slopes. We highly recommend that temporary and final site grading be designed to direct surface water away from the structures and away from the site slopes. Final drainage plans have not been developed at this time, but we understand that all stormwater generated on the site will be collected in tightlines and transported into an existing stromwater system along the southern portion of the property. No water should be infiltrated or dispersed within the site. We discuss general site drainage in the Site Drainage subsection of this report. The soils encountered within our explorations are considered extremely moisture sensitive and will disturb easily when wet. We recommend that construction take place during extended periods of dry weather if possible. If construction takes place during wet weather, additional expenses and delays should be expected due to the wet conditions. Additional expenses could include the need to export on - site soil, the import of clean, granular soil for fill, and the need to place a blanket of rock spalls or crushed rock in the construction traffic areas and on exposed subgrades prior to placing structural fill or structural elements. In this report, we have also provided recommendations for drainage, erosion control, and other development considerations intended to reduce the potential impact of development on the site and the site slopes. We should be retained to review final project plans prior to construction. We also recommend that NGA be retained to provide monitoring and consultation services during construction to confirm that the conditions encountered are consistent with those indicated by the explorations, to provide recommendations for design changes should the conditions revealed during the work differ from those anticipated, and to evaluate whether or not earthwork and foundation installation activities comply with contract plans and specifications. Erosion Control and Slope Protection Measures The erosion hazard for the on -site soils is considered moderate to high, but the actual hazard will be dependent on how the site is graded and how water is allowed to concentrate. Best Management Practices (BMPs) should be used to control erosion. Areas disturbed during construction should be protected from erosion. Erosion control measures may include diverting surface water away from the stripped or disturbed areas. Silt fences and/or slope waddles should be erected to prevent muddy water from leaving the site or flowing over the site slopes. Disturbed areas should be planted as soon as practical and the vegetation should be maintained until it is established. The erosion potential for areas not stripped of NELSON GEO TECHNICAL ASSOCIA TES, INC. Preliminary Geotechnical Engineering Evaluation NGA File No. 999017 Larsen Property Short Plat and Residence Underpinning February 28, 2018 Edmonds, Washington Page 11 vegetation should be low to moderate. Also, irrigation systems should not be installed within the property. Protection of the site slopes should be performed as required by the City of Edmonds. Specifically, we recommend that the slopes not be disturbed or modified through placement of any fill or future structures outside the planned development areas. No additional material of any kind should be placed on any portion of sloping ground, such as excavation spoils and soil stockpiles. It may be possible for trees to be cut down and removed from the slopes as long as a mitigation plan is developed for maintaining slope stability, such as the replacement of vegetation for erosion protection. A vegetation cover should be preserved on the slopes. Replacement of vegetation should be performed in accordance with the City of Edmonds code. Under no circumstances should water be allowed to concentrate on the slopes. Any sloping areas disturbed during construction should be planted with vegetation as soon as practical to reduce the potential for erosion. Site Preparation and Grading Plans for site grading should be devised such that cuts and fills are kept to a minimum. Site preparation should consist of excavating the upper eastern residence and garage footprints down to planned elevations. Site preparation should also consist of stripping any organic topsoil and/or loose/soft soils in areas that will support foundations, slabs -on -grade, pavement, or structural fill. The stripped material should not be stockpiled within the site. If the exposed soils are loose/soft, they should be compacted to a non -yielding condition. Areas observed to pump or weave during compaction should be over -excavated and replaced with rock spalls. If significant surface water flow is encountered during construction, this flow should be diverted around areas to be developed and the exposed subgrade maintained in a semi -dry condition. In wet conditions, the exposed subgrade should not be compacted, as compaction of a wet subgrade may result in further disturbance of the soils. A layer of crushed rock may be placed over the prepared areas to protect them from further disturbance. The site soils are considered extremely moisture sensitive and will disturb easily when wet. We recommend that earthwork construction take place during periods of extended dry weather, and suspended during periods of precipitation. If work is to take place during periods of wet weather, extreme care should be taken during site preparation as to not to disturb the site soils. This can be accomplished by utilizing large excavators equipped with smooth buckets and wide tracks to complete earthwork, and diverting surface and groundwater flow away from the prepared subgrades. Also, construction traffic should not be allowed on the exposed subgrade. A blanket of rock spalls should be used in construction access areas if wet conditions are prevalent. The thickness of this rock spall layer should be based on subgrade performance at the time of construction. For planning purposes, we recommend a minimum one -foot thick layer of rock spalls. NELSON GEOTECHNICAL ASSOCIATES, INC. Preliminary Geotechnical Engineering Evaluation NGA File No. 999017 Larsen Property Short Plat and Residence Underpinning February 28, 2018 Edmonds, Washington Page 12 Temporary and Permanent Slopes Temporary cut slope stability is a function of many factors, including the type and consistency of soils, depth of the cut, surcharge loads adjacent to the excavation, length of time a cut remains open and the presence of surface or groundwater. It is exceedingly difficult under these variable conditions to estimate a stable, temporary, cut slope angle. Therefore, it should be the responsibility of the contractor to maintain safe slope configurations since they are continuously at the job site, able to observe the nature and condition of the cut slopes, and able to monitor the subsurface materials and groundwater conditions encountered. The following information is provided solely for the benefit of the owner and other design consultants and should not be construed to imply that Nelson Geotechnical Associates, Inc. assumes responsibility for job site safety. Job site safety is the sole responsibility of the project contractor. For planning purposes, we recommend that temporary cuts in the on -site material be no steeper than two units horizontal to one unit vertical (2H: IV). If groundwater seepage is encountered, we would expect that flatter inclinations would be necessary. We should be retained to specifically review proposed geometry for significant cuts planned on this site. We recommend that cut slopes be protected from erosion. Erosion control measures may include covering cut slopes with plastic sheeting and diverting surface water runoff away from the top of cut slopes. We do not recommend vertical slopes for cuts deeper than four feet, if worker access is necessary. We recommend that cut slope heights and inclinations conform to appropriate OSHA/WISHA regulations. Permanent cut and fill slopes should be no steeper than 3H:1 V. However, flatter inclinations may be required in areas where loose soils are encountered. Permanent slopes should be covered with erosion control matting and vegetated. The vegetative cover should be maintained until established. We should specifically review all plans for grading on this project. We do not recommend placing irrigation systems near the slopes. Structure Setbacks Uncertainties related to building along steep slopes are typically addressed by the use of building setbacks. The purpose of the setback is to establish a "buffer zone" between the structure and the top and toe of the slope so that ample room is allowed for normal slope recession during a reasonable life span of the structure. In a general sense, the greater the setback distance, the lower the risk of slope failures impacting the structure. From a geological standpoint, the setback dimension is based on the slope's physical characteristics, such as slope height, surface angle, material composition, and hydrology. Other factors such as historical slope activity, rate of regression, and the type and desired life span of the development are important considerations as well. NELSON GEOTECHN/CAL ASSOC/A TES, INC Preliminary Geotechnical Engineering Evaluation NGA File No. 999017 Larsen Property Short Plat and Residence Underpinning February 28, 2018 Edmonds, Washington Page 13 We recommend that any proposed structures within the upper eastern portion of the property be set back at least 60 feet from the top of the steep west -facing slope. We also understand that a proposed garage structure may be constructed along the toe of the steep west -facing slope within the central portion of the property. In our opinion, this is feasible provided that the uphill eastern side of the garage is designed as a debris catchment wall and extends a minimum of four feet above the surrounding ground surface. We should be retained to review final residence location and to observe foundation excavations prior to placing forms. Any proposed development within the setback area, should be the subject of a specific geotechnical evaluation. Under no circumstances should water be allowed to concentrate on the slopes, during or after construction. Foundation Improvements Driven Pin Piles: We recommend that all interior and exterior foundation lines along with all impacted slabs -on -grade associated with the central residence be supported on driven pin piles to transfer foundation loads to the underlying native competent native materials found below the fill and landslide debris. We also recommend that the foundations and slabs for the proposed garage associated with the central residence also be supported on deep foundations consisting of driven pin piles. Due to the limited access to the existing residence area, we anticipate that all interior areas to be underpinned will be supported on 2-inch pin piles. However, the exterior foundations and foundations for the planned garage could be supported by either 2-inch pin piles or 4-inch pin piles, if larger equipment can access these areas. We recommend that the foundations and slabs associated with the proposed garage be supported on 4-inch pin piles. We have provided recommendations for the two pin pile sizes, in this section. Our explorations did not encounter any significant debris within the existing fill or landslide debris, however there is a possibility that some obstructions within the fill may impede some of the piles. There should be contingencies in the budget and design for additional/relocated piles to replace piles that may be obstructed by debris. We also recommend that excavation equipment be available on site during pile installation so that shallow obstructions can be removed from the planned pile locations. For 2-inch diameter pipe piles driven to refusal using a hand-held, 140-pound jackhammer, we recommend a design axial compression capacity of three tons for each pile. The refusal criterion for this pile and hammer size is defined as less than one inch of movement during 60 seconds of continuous driving. We recommend using galvanized extra strong (Schedule 80) steel pipe for the 2-inch pin piles. We recommend that the four -inch pipe piles, if utilized, be driven using a tractor -mounted hydraulic hammer, with an energy rating of at least 1,100 foot -lb. For this pile and hammer size, we recommend a design capacity of eight tons for each pile driven to refusal. The refusal criterion for this pile and hammer size is defined as less than one -inch of movement during 15 seconds of continuous driving at a rate of 550 blows per minute or higher. We recommend using galvanized schedule 40 pipe for the 4-inch pin piles. NELSON GEO TECHNICAL ASSOCIATES, INC. Preliminary Geotechnica) Engineering Evaluation NGA File No. 999017 Larsen Property Short Plat and Residence Underpinning February 28, 2018 Edmonds, Washington Page 14 Maintaining these recommendations for minimum hammer size and refusal criteria is essential for obtaining a successful outcome. Final pile depths should be expected to vary somewhat and will depend on the depth of the loose material and landslide debris, the nature of the underlying competent soils. The pin piles should penetrate a minimum of five feet into the competent native glacial soils below the fill material in order to develop the design capacity. Based on our explorations within the vicinity of the central residence, we anticipate that piles may need to be driven to a minimum depth of 50 feet below the ground surface to achieve the minimum embedment depth into the competent native soils. Piles that do not meet this minimum embedment criterion should be rejected, and replacement piles should be driven after consulting with the structural engineer regarding the new pile locations. The piles should be spaced a minimum of two feet apart to avoid a grouping effect on the piles. We should also be retained to observe pin pile installation during construction. Due to the relatively small slenderness ratio of pin piles, maintaining pin pile confinement and lateral support is essential in preventing pile buckling. The brackets and pin pile connections to the existing and proposed foundation and slabs should be designed by the structural engineer. Vertically driven pin piles do not provide meaningful lateral capacity. Due to the rigid pile support, friction between the foundation and subgrade soil should not be considered for resisting lateral pressures on this structure. We recommend that all lateral loads be resisted on the helical anchor tiebacks. Shallow Foundations For the upper eastern portion of the development, conventional shallow spread foundations should be placed on undisturbed medium dense or better native soils. Medium dense to dense soils should be encountered roughly two to five feet below the ground surface based on our explorations; however, deeper areas of loose soil may be encountered in unexplored areas of the site. Where undocumented fill or less dense soils are encountered at footing bearing elevation, the subgrade should be over -excavated to expose suitable bearing soil. Footings should extend at least 18 inches below the lowest adjacent finished ground surface for frost protection and bearing capacity considerations. Foundations should be designed in accordance with the 2015 IBC. Footing widths should be based on the anticipated loads and allowable soil bearing pressure. Water should not be allowed to accumulate in footing trenches. All loose or disturbed soil should be removed from the foundation excavation prior to placing concrete. NELSON GEOTECHNICAL ASSOCIATES, INC. Preliminary Geotechnical Engineering Evaluation NGA File No. 999017 Larsen Property Short Plat and Residence Underpinning February 28, 2018 Edmonds, Washington Page 15 For foundations constructed as outlined above, we recommend an allowable design bearing pressure of not more than 2,000 pounds per square foot (psf) be used for the footing design for footings founded on the medium dense or better native soils or structural fill extending to the native competent material. The foundation bearing soil should be evaluated by a representative of NGA. We should be consulted if higher bearing pressures are needed. Current IBC guidelines should be used when considering increased allowable bearing pressure for short-term transitory wind or seismic loads. Potential foundation settlement using the recommended allowable bearing pressure is estimated to be less than one inch total and 1/2 inch differential between adjacent footings or across a distance of about 20 feet, based on our experience with similar projects. Lateral loads may be resisted by friction on the base of the footing and passive resistance against the subsurface portions of the foundation. A coefficient of friction of 0.30 may be used to calculate the base friction and should be applied to the vertical dead load only. Passive resistance may be calculated as a triangular equivalent fluid pressure distribution. An equivalent fluid density of 150 pounds per cubic foot (pcf) should be used for passive resistance design for a level ground surface adjacent to the footing. This level surface should extend a distance equal to at least three times the footing depth. These recommended values incorporate safety factors of 1.5 and 2.0 applied to the estimated ultimate values for frictional and passive resistance, respectively. To achieve this value of passive resistance, the foundations should be poured "neat" against the native medium dense soils or compacted fill should be used as backfill against the front of the footing. We recommend that the upper one -foot of soil be neglected when calculating the passive resistance. Retaining Walls Final grading and development plans were not available at the time this report was prepared but retaining walls will likely be needed for the proposed development. We recommend that the uphill foundation wall for the proposed garage within the central portion of the property be extended a minimum of four feet above the finished ground surface to protect the structure against potential failures from the slope above. This is intended to provide a catchment measure should any sloughing debris travel towards the garage structure during extreme weather or as a result of an earthquake. Alternatively, a separate debris protection structure or fence could be utilized. We also recommend that no window or door openings be located on the uphill eastern side of the proposed garage within the central portion of the property. The lateral pressure acting on subsurface retaining walls is dependent on the nature and density of the soil behind the wall, the amount of lateral wall movement which can occur as backfill is placed, wall drainage conditions, the inclination of the backfill, and other possible surcharge loads. For walls that are free to yield at the top at least one thousandth of the height of the wall (active condition), soil pressures will be less than if movement is limited by such factors as wall stiffness or bracing (at -rest condition). We NELSON CEOTECHN/CAL ASSOC/A TES, INC Preliminary Geotechnical Engineering Evaluation NGA File No. 999017 Larsen Property Short Plat and Residence Underpinning February 28, 2018 Edmonds, Washington Page 16 recommend that walls supporting horizontal backfill and not subjected to hydrostatic forces be designed using a triangular earth pressure distribution equivalent to that exerted by a fluid with a density of 45 pcf for yielding (active condition) walls, and 65 pcf for non -yielding (at -rest condition) walls. If the uphill side of the proposed garage is intended to act as a debris wall, we recommend the above ground portion of the garage wall be designed to resist an active pressure of 100 pcf. These recommended lateral earth pressures are for a drained granular backfill and are based on the assumption of a maximum 2H:1 V backfill inclinations and do not account for additional surcharge loads. Additional lateral earth pressures should be considered for surcharge loads acting adjacent to subsurface walls and within a distance equal to the subsurface height of the wall. This would include the effects of surcharges such as traffic loads, floor slab and foundation loads, or other surface loads. We are available to provide consultation regarding additional loads on retaining walls during final design, if needed. The lateral pressures on walls may be resisted by friction between the foundation and subgrade soil, and by passive resistance acting on the below -grade portion of the foundation. Recommendations for frictional and passive resistance to lateral loads are presented in the Foundations subsection of this report. All wall backfill should be well compacted as outlined in the Structural Fill subsection of this report. Care should be taken to prevent the buildup of excess lateral soil pressures, due to over -compaction of the wall backfill. This can be accomplished by placing wall backfill in thin loose lifts and compacting it with small, hand -operated compactors within a distance behind the wall equal to at least one-half the height of the wall. The thickness of the loose lifts should be reduced to accommodate the lower compactive energy of the hand -operated equipment. The recommended level of compaction should still be maintained. Permanent drainage systems should be installed for retaining walls. Recommendations for these systems are found in the Subsurface Drainage subsection of this report. We recommend that we be retained to evaluate the proposed wall drain backfill material and drainage systems. Structural Fill General: Fill placed beneath foundations, pavements, and other settlement -sensitive structures, or behind retaining walls should be placed as structural fill. Structural fill, by definition, is placed in accordance with prescribed methods and standards and is monitored by an experienced geotechnical professional or soils technician. Field monitoring procedures would include the performance of a representative number of in -place density tests to document the attainment of the desired degree of relative compaction. The area to receive the fill should be prepared as outlined in the Site Preparation and Grading subsection of NELSON GEOTECHN/CAL ASSOC/A TES, INC Preliminary Geotechnical Engineering Evaluation NGA File No. 999017 Larsen Property Short Plat and Residence Underpinning February 28, 2018 Edmonds, Washington Page 17 this report. Sloping areas to receive fill should be benched prior to fill placement. The benches should be level and at least four feet wide. Materials: Structural fill should consist of a good quality, granular soil, free of organics and other deleterious material and be well graded to a maximum size of about three inches. All-weather fill should contain no more than five -percent fines (soil finer than U.S. No. 200 sieve, based on that fraction passing the U.S. 3/4-inch sieve). We do not anticipate placement of significant volumes of structural fill for this project. The on -site soils consist of moisture -sensitive silty materials and slide debris. We recommend that the on -site material not be used as structural fill. We should be retained to evaluate the suitability of proposed structural fill materials at the time of construction. Fill Placement: Following subgrade preparation, placement of structural fill may proceed. All filling should be accomplished in uniform lifts up to eight inches thick. Each lift should be spread evenly and be thoroughly compacted prior to placement of subsequent lifts. All structural fill underlying building areas and pavement subgrade should be compacted to a minimum of 95 percent of its maximum dry density. Maximum dry density, in this report, refers to that density as determined by the ASTM D-1557 Compaction Test procedure. The moisture content of the soils to be compacted should be within about two percent of optimum so that a readily compactable condition exists. It may be necessary to over - excavate and remove wet soils in cases where drying to a compactable condition is not feasible. All compaction should be accomplished by equipment of a type and size sufficient to attain the desired degree of compaction. Slab -on -Grade As mentioned earlier, we recommend that slabs -on -grade associated with the central portion of the property be designed as a structural slab and fully supported on the deep foundation system consisting of driven pin piles. Slabs associated with the upper eastern portion of the property should be supported on subgrade soils prepared as described in the Site Preparation and Grading subsection of this report. We recommend that all floor slabs be underlain by at least six inches of free -draining gravel with less than three percent by weight of the material passing Sieve #200 for use as a capillary break. We recommend that the capillary break be hydraulically connected to the footing drain system to allow free drainage from under the slab. A suitable vapor barrier, such as heavy plastic sheeting (6-mil minimum), should be placed over the capillary break material. An additional 2-inch-thick moist sand layer may be used to cover the vapor barrier. This sand layer is optional, and is intended to be used to protect the vapor barrier membrane and to aid in curing the concrete. NELSON GEOTECHN/CAL ASSOC/A TES, INC Preliminary Geotechnical Engineering Evaluation NGA File No. 999017 Larsen Property Short Plat and Residence Underpinning February 28, 2018 Edmonds, Washington Page 18 Pavement Subgrade Pavement subgrade preparation should be completed as recommended in the Site Preparation and Grading and Structural Fill subsections of this report. Depending on the tolerance to pavement cracking, we recommend that the upper two feet of the existing material be removed and replaced with granular structural fill or crushed rock. The pavement subgrade should be proof -rolled with a heavy, rubber -tired piece of equipment, to identify soft or yielding areas that may require repair prior to placing any structural fill and prior to placing the pavement base course. We should be retained to observe the proof -rolling and recommend repairs prior to placement of the asphalt or hard surfaces. The pavement section should be thickened to further reduce the effects of settlement due to the landslide debris, but potential long-term cracking should still be expected. Site Drainage Improvements Surface Drainage: The finished ground surface should be graded such that stormwater is directed to an appropriate stormwater collection system. Water should not be allowed to collect in any area where footings, slabs, or retaining walls are to be constructed. Final site grades should allow for drainage away from the structure and away from the site slopes. We suggest that the finished ground be sloped at a minimum gradient of three percent, for a distance of at least 10 feet away from the structure and slopes. Surface water should be collected by permanent catch basins and drain lines, and be discharged into an appropriate discharge system. Under no circumstances should water be allowed to flow uncontrolled over the site slopes or excavation walls. Existing drains within the property should be investigated and repaired, as needed, to ensure all runoff generated on this site is routed away from the building foundation and into the approved discharge system. We recommend that all stormwater generated on the site, including roof downspouts, footing drains, pavement and yard drains, and any water flow from the road, be tightlined to the existing stormwater system within the southern portion of the site. Subsurface Drainage: If groundwater is encountered during construction, we recommend that the contractor slope the bottom of the excavations and collect water into ditches and small sump pits where the water can be pumped out of the excavations and routed into an appropriate outlet. We recommend the use of footing drains around the planned structure and behind retaining walls. Footing drains should be installed at least one -foot below planned finished floor elevation. The drains should consist of a minimum four -inch -diameter, rigid, slotted or perforated, PVC pipe surrounded by free -draining material, such as washed rock, wrapped in a filter fabric. We recommend that an 18-inch- wide zone of clean (less than three -percent fines), granular material be placed along the back of the walls above the drain. Washed rock is an acceptable drain material, or drainage composite may be used instead. NELSON GEOTECHN/CAL ASSOC/A TES, INC. Preliminary Geotechnical Engineering Evaluation NGA File No. 999017 Larsen Property Short Plat and Residence Underpinning February 28, 2018 Edmonds, Washington Page 19 The free -draining material should extend up the wall to one -foot below the finished surface. The top foot of backfill should consist of low permeability soil placed over plastic sheeting or building paper to minimize the migration of surface water or silt into the footing drain. Footing drains should discharge into tightlines leading to an appropriate collection and discharge point with convenient cleanouts to prolong the useful life of the drains. Roof drains should not be connected to footing drains. Roof drains should also be installed around the structure. Roof drains should consist of gutters and downspouts collecting stormwater runoff from the roof. The downspouts should discharge to catch basins and 4-inch minimum diameter, rigid, PVC tightline pipes. The drains should be directed into catch basins and then into the controlled drainage system. The footing and roof drains should discharge via independent (separate) tightlines into catch basins/cleanouts leading to the stormwater system. Surface water from the driveway and yard areas should also be collected in a catch basin and tightlined separately to the stormwater system. USE OF THIS REPORT NGA has prepared this report for Mr. Carl Larsen and his agents, for use in the planning and design of the development planned on this site only. The scope of our work does not include services related to construction safety precautions and our recommendations are not intended to direct the contractors' methods, techniques, sequences, or procedures, except as specifically described in our report for consideration in design. There are possible variations in subsurface conditions between the explorations and also with time. Our report, conclusions, and interpretations should not be construed as a warranty of subsurface conditions. A contingency for unanticipated conditions should be included in the budget and schedule. We recommend that we be retained to review final project plans and provide consultation regarding specific structure placement, site grading, foundation support, and drainage. We also recommend that NGA be retained to provide monitoring and consultation services during construction to confirm that the conditions encountered are consistent with those indicated by the explorations, to provide recommendations for design changes should the conditions revealed during the work differ from those anticipated, and to evaluate whether or not earthwork and foundation installation activities comply with contract plans and specifications. We should be contacted a minimum of one week prior to construction activities and could attend pre -construction meetings if requested. All people who own or occupy homes on or near hillsides should realize that landslide movements are always a possibility. The landowner should periodically inspect the slope, especially after a winter storm. If distress is evident, a geotechnical engineer should be contacted for advice on remedial/preventative measures as soon as possible. The probability that landsliding will occur is substantially reduced by the NELSON CEOTECHN/CAL ASSOC/A TES, //VC. Preliminary Geotechnical Engineering Evaluation Larsen Property Short Plat and Residence Underpinning Edmonds, Washington NGA File No. 999017 February 28, 2018 Page 20 proper maintenance of drainage control measures at the site (the runoff from the impervious surfaces should be led to an approved discharge point). Therefore, the homeowner should take responsibility for performing such maintenance. Within the limitations of scope, schedule, and budget, our services have been performed in accordance with generally accepted geotechnical engineering practices in effect in this area at the time this report was prepared. No other warranty, expressed or implied, is made. Our observations, findings, and opinions are a means to identify and reduce the inherent risks to the owner. NELSON CEOTECHN/CAL ASSOC/A TES, /NC. Preliminary Geotechnical Engineering Evaluation Larsen Property Short Plat and Residence Underpinning Edmonds, Washington NGA File No. 999017 February 28, 2018 Page 21 We appreciate the opportunity to provide service to you on this project. If you have any questions or require further information, please call. Sincerely, NELSON GEOTECHNICAL ASSOCIATES, INC. of wash; o c `lk ti 'OQ 2883 �Sed Ge°O� LEE S. BELLAH Lee S. Bellah, LG Project Geologist Khaled M. Shawish, PE Principal LSB:KMS:sg Eight Figures Attached NELSON GEOTECHNICAL ASSOCIATES, INC. VICINITY MAP N Not to Scale f Lunds Gulch ,�.a�t�'•'T• Project Site ol Reich J es D m 's L � f4 1>0 a 10 t 164th St SW y� N m ` a2 C1 Z C L < 2 n rn I ON E 7 u ' N Meadowdale D n D s Lynnwood Playfields < m` b',41stf ^rater Treatment MeadoWda/ee 168th St SW 168t ® V Pack Meadowdale o` C 3 Rry Middle School �G o 6- D _ t Edmonds, WA Project Number 999017 Larsen Property Short Plat NELSON GEOTECHNICAL NGA ASSOCIATES INC. No. Date Revision By CK d t 8/10„7 Original oPN LSB Vicinity Map GEOTECHNICAL ENGINEERS & GEOLOGISTS 4 z Z Figure 1 Woodinville Office East Wenatchee Office 17311-135th Ave. NE, A-5W 5526 Industry lane, 62 Woedin ,11., WA 98072 East Wenatchee, WA 98602 (425) 48&1669/ Fax: 481-2510 n w.nelsongeetech.com (509) 665-7696/ Fax: 665-7692 Z anon mat ano una I dlc a 10 rI Z N 107� 1 I 1u l I U T C§rrIO 0 a � r S�J I I 200� � 3 I I 10$ IMENT IN CAS T BENCHMARK DISC & PUNCH 104 SITED 04/201F ^ �- SSM RIM=101.24 :TR CH=89.12 10 V N.E-OUT S) 29"E 14.88.11 S HANDRAIL 1 I ENT IN CASE 3C & PUNC EO 04/201 AH W/FLOW 0 TO NOR 6 RIMIM=99.7 a 12" PVC 12" PVC / 1/2 CAP) A �CL 4 i \ CB T RIM-96.90' IE=92.06'(12" IE=92.56'(12' L ". 0.5' 1 EAST r Existing Western Garage Existing Western Residence N) 0051530(0' I S) J IE= Approximate Location of Proposed Garage Approximate Top of Slope Approximate Toe of Slope Approximate 60-foot Steep Slope Setback I-7 .0 1 FOUND FEBAR & CAP 0. J 0 T � \ a' \� cA m \No d+ tf v\\6743 \ B-1 Z� SS -l--11 SS I - N_X9 T T RIM=102.48' FOUND REBAR & CAP-� IE=97.57'(12" Chip W)'*It 16916' IE-98.46'(12' CMP E) 0.69' N X 0.25' W 55MH FOUND BARE REBAR 0.06' N X 0.06' E CB T_t RIM=131.77* TR CH=126.47' .CB T-1 4 RIM- 128.48' IE=125.40'(18" PVC E) E,NE-OJT W� RIM-96.51' IE=125.20'(12" CMP W) IE=91.89'(12- PVC S) IE-W.23'(4' PVC S) IE=92.39'(12' PVC NW) IE-92.71'(4" PVC E) LEGEND • — Property line B-1 Number and approximate location of boring A Al Approximate location L� of cross-section AG P E G Reference: Site Plan based on an undated, untitled plan provided by CG Engineering. CB 1-2 RIM-140.47' IE=135.67'(12" CHIP E) IE=136.67'(12" HDPE SE) 135,62'(18- PVC W) M1ii IR 1 "rdo ♦ v •fi/ �y Z OF PPE I IE=144&j4L CPPIEi�' ' c P q �M=140.95' 2 PORTION OF PIPE R CH-134.10' 1a' _i4 TICAL PIPE ABOVE GROUND 'll1 -OUT W) IE=139.46'(12" S) IE=139.86'(12- E) - DRIP UNE TYP. 1 " VERTICAL PIPE IE=139.31'(12' W) Existing Central Residence LtIP 8' EL=261 f20,400 FDUNO 3 4' IRON PIPE W/"T" CB -2 •02' N X 1'24' W s1.2s' !E=21.4_ 2" HDPE W) SD SO SO 750 — .S' — in 25 MH �� RIM=252.42' SS4H CTR CH=241.18' RIM=247.12' (IN E-OU- W) CTR CH=241.87' (IN E„N.S-OUT W) I 0 50 100 Scale: 1 inch = 50 feet n. tCD I T U in Z''� z O X r Q _ N U FOUND RE .. w EMERSOr U 3.25' W ( W 0' W < w U ZVr.M Z 0 Z I U) z w w pd513�0 Z Q M U ce T-, ') z 2" PV Z *IE45.2' PV+ PVC rU6" V w F 0 C M=24246.57' (AI 1=243.29'02` I' rR V I=243.27'(18" i 1 / i1=243.87(12 I. L CB T-1 RIM=247.38' IE=245.1C'(12- CW W=245.03'(12' Ck1i^ rx 7 C Q) M J a) E N z o U C Q) d) I L O f1 Southwest 250 200 100 50 0 B-3 B-4 A' East 0 50 160 150 260 250 360 350 400 450 560 550 600 Distance (feet) Exploration Boring Designation —> B-1 23 23 <— SPT N-value Groundwater Level 23 During Exploration z3 23 Geologic Contact (approximate) Reference: Cross Section is based on field measurements using a hand-held clinometer and 100-ft tape measure. NOTES: 1) Stratigraphic conditions are interpolated between the explorations. Actual conditions may vary. 2) Elevations are approximate. 250 Q F N U 4 s "U O T W � U os y ^ W � W W t� UU d y d C r W Ev 200 Q }. di g W Q a U W Z p W 0 to z z J Z 150 W W J Z N N U N ao� Wma 2 ZQm x U f °' W >� E- o100 row 'W v � 50 a� E c� Z rn 73 U 0) 2) a) 0')LL O L UNIFIED SOIL CLASSIFICATION SYSTEM GROUP MAJOR DIVISIONS GROUP NAME SYMBOL CLEAN GW WELL -GRADED, FINE TO COARSE GRAVEL COARSE- GRAVEL GRAVEL GP POORLY -GRADED GRAVEL GRAINED MORE THAN 50 % GRAVEL GM SILTY GRAVEL OF COARSE FRACTION RETAINED ON SOILS NO.4 SIEVE WITH FINES GC CLAYEY GRAVEL SAND CLEAN SW WELL -GRADED SAND, FINE TO COARSE SAND SAND SP POORLY GRADED SAND MORE THAN 50 % RETAINED ON IN MORE THAN 50 NO. SIEVE OF COARSE FRACTION SAND SM SILTY SAND PASSES NO. 4 SIEVE SC CLAYEY SAND WITH FINES FINE - SILT AND CLAY ML SILT INORGANIC CL CLAY GRAINED LIQUID LIMIT LESS THAN 50 % ORGANIC OL ORGANIC SILT, ORGANIC CLAY SOILS SILT AND CLAY MH SILT OF HIGH PLASTICITY, ELASTIC SILT INORGANIC MORE THAN 50 % PASSES LIQUID LIMIT CH CLAY OF HIGH PLASTICITY, FAT CLAY NO. 200 SIEVE 50 % OR MORE ORGANIC OH ORGANIC CLAY, ORGANIC SILT HIGHLY ORGANIC SOILS PT PEAT NOTES: 1) Field classification is based on visual SOIL MOISTURE MODIFIERS: examination of soil in general accordance with ASTM D 2488-93. Dry - Absence of moisture, dusty, dry to the touch 2) Soil classification using laboratory tests Moist -Damp, but no visible water. is based on ASTM D 2488-93. 3) Descriptions of soil density or Wet - Visible free water or saturated, consistency are based on usually soil is obtained from interpretation of blowcount data, below water table visual appearance of soils, and/or test data. Project Number NELSON GEOTECHNICAL No. Date Revision By CK 999017 Larsen Property Short Plat N A ASSOCIATES, INC. 1 8/10/17 Original DPN LS8 Soil Classification Chart GEOTECHNICAL ENGINEERS & GEOLOGISTS wooam lue°rn<e East Wenatcheemeoe Figure 4 17311-135th Ave. NE, A-500 5526 Industry Lane, i2 WoodinLl,, WA 12 Ea9 Wenatchee, WA98802 (4., 466-1669/Fax:451-2510 vnnvnelsongeotech— (509)665-7696/Fax: 665-7692 BORING LOG B-1 Approximate Ground Surface Elevation: ?? Soil Profile Sample Data Penetration Resistance (Blows/foot - •) 10 20 30 40 50 50+ a' 1°' Z 0 �+ 0 m J Piezometer Installation - Ground Water Data (Depth in Feet) Description L CL 0 m J 0 fl. 0 n .0 o (D U) o 0 m U o fl » m- t0n 0 -o Moisture Content (Percent - N) 10 20 30 40 50 50+ Gray -brown, silty fine to medium sand with gravel, iron -oxide staining, and trace roots (very loose, moist) ; 3 41 -becomes dark brown 3 5 ................................................... 5 — -becomes dark brown to gray -brown, loose 5 -no organics:.. -becomes gray, moist to wet ;• .; .. 10 ................................................... 10 — -becomes gray -brown 9 SM 15 ............................. 15 —,.: .. -becomes gray, medium dense, wet 17 20 ................................................... 20 -becomes loose with trace organics, charcoal 5 25 ................................................... 25 LEGEND Solid PVC Pipe Concrete M Moisture Content Slotted PVC Pipe Bentonite A Atterberg Limits Depth Driven and Amount Recovered G Grain -size Analysis with 2-inch O.D. Split -Spoon Sam ler Monument/ Cap Yz4 Native Soil DS Direct Shear P r ��., to Piezometer PP Pocket Penetrometer Readings, tons/ft Depth Driven and Amount Recovered * Liquid Limit Silica Sand P Sample Pushed with 3-inch Shelby Tube Sampler + Plastic Limit V Water Level T Triaxial NOTE: Subsurface conditions depicted represent our observations at the time and location of this exploratory hole, modified by engineering tests, analysis and judgement. They are not necessarily representative of other times and locations. We cannot accept responsibility for the use or interpretation by others of information presented on this log. Project Number 999017 Larsen Property Short Plat NELSON GEOTECHNICAL N A ASSOCIATES, INC. No. Date Revision By CK 1 8„0/17 Original DPN L3B Boring Log GEOTECHNICAL ENGINEERS & GEOLOGISTS Woodinville Offce East Wenatchee o lce 17311-1351h Ave. NE, A-500 5526IMustry Lane, S2 V&cdmifl., WA A 98802 1425)46&48 1669/ ax: 4872510 vrvnv.neisogeotech.wm (509)72 t6GS7a696/ Ftchee,� 665-7692 Figure 5 Page 1 of 2 I BORING LOG B-1 (cont.) Soil Profile Sam le Data p Penetration Resistance (Blows/foot - 0) C' N Piezometer 10 20 30 40 50 50+ Installation - Ground Water Description a rn Q.a 3 � o Moisture Content to Data m o O m v E m - (Percent - ■) (Depth in Feet) (D (D U) U) 10 20 30 40 50 50+ m d) J Orange -brown to gray -brown, silty fine to medium sand . • 2 with gravel, trace organics, and charcoal (very loose, wet) •_ -becomes gray to brown, with trace iron -oxide staining, 35 ••••..•. ••• •••• ......•• ......•• ••••••.•• •••.•••. 35 - medium dense 17 ' — -no organics or charcoal SM -becomes gray, with trace gravel , . ' ,. 40 •..••••• •• ......•.......••• ......••• •••...• 40 -no iron -oxide staining 20 ' y -with trace iron -oxide staining = 19 Boring terminated below existing grade at 46.5 feet on 8/7/17. Groundwater seepage was not encountered during drilling. 50 .................................................... 50 55 .................................................... 55 LEGEND Solid PVC Pipe >° Concrete M Moisture Content Slotted PVC Pipe Bentonite A Atterberg Limits Depth Driven and Amount Recovered G Grain -size Analysis with 2-inch O.D. Split -Spoon Sampler Monument/ Cap Native Soil DS Direct Shear to Piezometer PP Pocket Penetrometer Readings, tons/ft Depth Driven and Amount Recovered * Liquid Limit 0 Silica Sand p Sample Pushed with 3-inch Shelby Tube Sampler + Plastic Limit Water Level T Triaxial NOTE: Subsurface conditions depicted represent our observations at the time and location of this exploratory hole, modified by engineering tests, analysis and judgement. They are not necessarily representative of other times and locations. We cannot accept responsibility for the use or interpretation by others of information presented on this log. Project Number NELSON GEOTECHNICAL No. Date Revision By CK 999017 Larsen Property Short Plat ASSOCIATES, INC. , 8110/11 Original DPN LSB Boring Log GEOTECHNICAL ENGINEERS & GEOLOGISTS East Wenatchee Office Figure 5 Woodinville office 17311-135th Ave. NE, A-500 5526 lMnstry Lane• ®2 (425)45ra1669/ ax:4t 812510 vrcrcr.nelsongeotechcorn (509)6657a6961 Fax: 66655-70692 Page 2 of 2 AA n a rn 0 0 J BORING LOG B-2 Approximate Ground Surface Elevation: ?? Soil Profile Sample Data Penetration Resistance (Blows/foot - •) Piezometer 10 20 30 40 50 50+ t Installation - m Z Ground Water Moisture Content Description a Cz o Qm Datao (D _ L (Percent - ®) Co (Depth in Feet)O C� ) U cn 10 20 30 40 50 50+ J Gray -brown, silty fine to medium sand with gravel and roots (very loose to loose, moist) 4 -becomes gray with iron -oxide staining, loose 5 5 ................................................... 5 -no roots -becomes gray to orange -brown 6 ' �, ;• ., -becomes gray with trace gravel and roots — -becomes gray -brown 8 ' , -becomes medium dense SM 15 .................................................... 15 ; . :' -no roots -becomes wet, with gravel 13 -becomes gray, with trace gravel T. 13 ' Z5 ................................................... 25 ' • . LEGEND Solid PVC Pipe ;t °. Concrete M Moisture Content Slotted PVC Pipe Bentonite A Atterberg Limits Depth Driven and Amount Recovered G Grain -size Analysis with 2-inch O.D. Split -Spoon Sampler Monument/ Cap ( Native Soil DS Direct Shear r, to Piezometer PP Pocket Penetrometer Readings, tons/ft Depth Driven and Amount Recovered 0 Silica Sand P * Liquid Limit P Sample Pushed with 3-inch Shelby Tube Sampler + Plastic Limit Water Level T Triaxial NOTE: Subsurface conditions depicted represent our observations at the time and location of this exploratory hole, modified by engineering tests, analysis and judgement- They are not necessarily representative of other times and locations. We cannot accept responsibility for the use or interpretation by others of information presented on this log. Project Number NELSON GEOTECHNICAL No. Date Revision By CK 999017 Larsen Property Short Plat N A ASSOCIATES, INC. 1 anom Original DPN LSa Boring Log GEOTECHNICAL ENGINEERS & GEOLOGISTS East Wenatchee Office Figure 6 Woodinville Office 17311-135th Ave. NE, A-5W 55261nduslry Lane, 52 Wcodinvifle,1 (425)4B61669/ az:4Bi 2510 v v.nelaongeotechcom (509)6657696/Fa 7665 692 Page 1 of 2 r 0 N r 00 t= 0 m Q a rn 0 0 J BORING LOG B-2 (cont.) Penetration Resistance E' Soil Profile Sample Data (Blows/foot - ®) Piezometer 10 20 30 40 50 50+ t°' Installation - ;, Ground Water Moisture Content Description n a Q a 3 o y m Data J o E o m U m- (Percent ■) Q (Depth in Feet) 0 (D U) U) _ 10 20 30 40 50 50+ m a —1 Gray, silty fine to medium sand with gravel, iron -oxide .: 8 staining, and trace charcoal (loose, wet) becomes medium dense SM 24 35 ................................................. 35 �- _ -no charcoal 17 40 'Gray ................................................... 40 -blue silt with fine sand (very stiff, dry) ML 29 45 ................................................... 45 50 ................................................... 50 30 Boring terminated below existing grade at 51.5 feet on 8/7/17. Groundwater seepage was not encountered during drilling. 55 .................................................... 55 LEGEND Solid PVC Pipe sK. Concrete M Moisture Content Slotted PVC Pipe Bentonite A Atterberg Limits Depth Driven and Amount Recovered G Grain -size Analysis with 2-inch O.D. Split -Spoon Sampler Im Monument/ Cap Native Soil DS Direct Shear to Piezometer PP Pocket Penetrometer Readings, tons/ft Depth Driven and Amount Recovered Q Silica Sand p * Liquid Limit P Sample Pushed with 3-inch Shelby Tube Sampler + Plastic Limit Water Level T Triaxial NOTE: Subsurface conditions depicted represent our observations at the time and location of this exploratory hole, modified by engineering tests, analysis and judgement. They are not necessarily representative of other times and locations. We cannot accept responsibility for the use or interpretation by others of information presented on this log. Project Number NELSON GEOTECHNICAL No. Date Revision By CK 999017 Larsen Property Short Plat A ASSOCIATES, INC. 1 8110/17 Original DPN Lse Boring Log GEOTECHNICAL ENGINEERS & GEOLOGISTS East Wenatchee Olf ce Figure 6 Woodinville Office 17311.135th Ave. NE, M00 5526 Industry lane, l2 A 98802 (425)466-16691 Fax: 48t 1-2510 wvnv.nelsorgeotech.min (509)66576961 Fax: 665-67 Page 2 of 2 r 0 N 00 20 Q a » J ( c BORING LOG i B-3 Approximate Ground Surface Elevation: ?? Penetration Resistance O' Soil Profile Sample Data (Blows/foot - ®) ti Piezometer 10 20 30 40 50 50+ H Installation - Ground Water Description E o C n o o Moisture Content (Percent - E) m Data Feet) m c� J o f O o m U E to 010 20 30 40 50 50+ o (Depth in c� cn Gray -brown to orange -brown, silty fine to medium sand with gravel, iron -oxide weathering, and trace metal/plastic (dense, moist) FILL 41 —------------ Gray — to orange-bro—wn, silty fine to—me—dium sand with — 5 ....•••• ••••••••• .................. ••••••••• ••••••• 5 :,•;.�.•.' gravel, iron -oxide weathering, and trace roots (very ' 54 � — dense, moist) -becomes orange -brown to gray -no roots 50-6.1 ;• •> •. -becomes gray, medium dense 23 — SM -15 -no iron -oxide staining :;— 29 -becomes gray -brown with iron -oxide staining, 32 20 ................................................... 20 ;::.. . dense, wet Gray silt with fine sand (very stiff, moist) 31 ' 25 ................................................... 25 ML LEGEND Solid PVC Pipe R Concrete M Moisture Content Slotted PVC Pipe Bentonite A Atterberg Limits Depth Driven and Amount Recovered G Grain -size Analysis with 2-inch O.D. Split -Spoon Sampler Monument/ Cap; Native Soil DS Direct Shear to Piezometer PP Pocket Penetrometer Readings, tons/ft Q Silica Sand Depth Driven and Amount Recovered p �t Liquid Limit P Sample Pushed with 3-inch Shelby Tube Sampler + Plastic Limit Water Level T Triaxial NOTE: Subsurface conditions depicted represent our observations at the time and location of this exploratory hole, modified by engineering tests, analysis and judgement. They are not necessarily representative of other times and locations. We cannot accept responsibility for the use or interpretation by others of information presented on this log. Project Number NELSON GEOTECHNICAL No. Date Revision By CK 999017 Larsen Property Short Plat ASSOCIATES INC. 1 8110117 Original DPN use Boring Log GEOTECHNICAL ENGINEERS & GEOLOGISTS East Wenatchee Office Figure 7 Woodlnvllie Office 173tl-135th Ave. NE, A500 5526Industry Lane. ®2 Woodinville, WA M072 (425)486.16691 Fax: 4481-2510 w ,Wwngmuch.min (509)66 -76961 FaW Page 1 of 2 I I BORING LOG B-3 (cont.) Soil Profile Sample Data Penetration Resistance (Blows/foot - ®) a' Piezometer 10 20 30 40 50 50+ f°' Installation - Z Ground Water Description Q o �� 3: Q.0 Moisture Content m Data m _J o E � a m U ca - to (Percent �) o (Depth in Feet) c� J 10 20 30 40 50 50+ m 30 Gray -brown, silty fine to medium sand with iron -oxide staining (medium dense to dense, moist) SM 25 35 .................................................... 35 • ' Gray silt with fine sand (very stiff, moist) ML Gray, silty fine sand with iron -oxide staining (dense, moist)' 45 SM .. ..•. — 48 Boring terminated below existing grade at 46.5 feet on 8/7/17. Groundwater seepage was not encountered during drilling. 50 .................................................... 50 55 .................................................... 55 LEGEND Solid PVC Pipe >':9` Concrete M Moisture Content Slotted PVC Pipe Bentonite A Atterberg Limits Depth Driven and Amount Recovered G Grain -size Analysis with 2-inch O.D. Split -Spoon Sampler Monument/ Cap k, Native Soil DS Direct Shear to Piezometer PP Pocket Penetrometer Readings, tons/ft Silica Sand Depth Driven and Amount Recovered p * Liquid Limit P Sample Pushed with 3-inch Shelby Tube Sampler + Plastic Limit Water Level T Triaxial NOTE: Subsurface conditions depicted represent our observations at the time and location of this exploratory hole, modified by engineering tests, analysis and judgement. They are not necessarily representative of other times and locations. We cannot accept responsibility for the use or interpretation by others of information presented on this log. Project Number NELSON GEOTECHNICAL NO. Date Revision By CK 999017 Larsen Property Short Plat N A ASSOCIATES, INC. 1 8170/77 Original DPN LSa Boring Log GEOTECHNICAL ENGINEERS & GEOLOGISTS Woodinville Office East Wenatchee Office Figure 7 17311-135th Ave. NE, A-9W 5526 Industry lane, R2 t (425)) 48616WI Fax:: 461-2510 mrn.nalscngectech.com (509)6567a�6961 Fax: 76E5 692 Page 2 of 2 D V K1 0 Y n t c�, I i BORING LOG i B-4 Approximate Ground Surface Elevation: ?? Penetration Resistance Soil Profile Sample Data (Blows/foot - ®) Piezometer 10 20 30 40 50 50+ t�' Installation - S Ground Water Description o a o -0 o w a» _ Moisture Content o m Data is � o f _o o cu- (Percent - ®) o (Depth in Feet) cn _ m 10 20 30 40 50 50+ a —I Gray -brown, silty fine to medium sand with gravel, , iron -oxide staining, and trace organics (loose, moist) ; .. SM 7 — — — — — — — — — — — — — — — No recovery — 2 5 a ................................................... 5 — --- Gray, silty fine to medium sand with gravel and iron -oxide 41 weathering (dense, moist) :• •> -becomes medium dense — 25 SM -becomes gray -brown ,• ::— 28 35 ' 20 ................................................... 20 Gray silt with fine sand (hard, moist) -interbedded with gray, silty fine to medium sand with 45 ' 25 """" """'" '."""""""" "" ""' """• 25 gravel ML LEGEND Solid PVC Pipe Concrete M Moisture Content Slotted PVC Pipe Bentonite A Atterberg Limits Depth Driven and Amount Recovered G Grain -size Analysis with 2-inch O.D. Split -Spoon Sampler Monument/ Cap, Native Soil DS Direct Shear to Piezometer Silica Sand PP Pocket Penetrometer Readings, tons/ft Depth Driven and Amount Recovered * Liquid Limit P Sample Pushed with 3-inch Shelby Tube Sampler + Plastic Limit Water Level T Triaxial NOTE: Subsurface conditions depicted represent our observations at the time and location of this exploratory hole, modified by engineering tests, analysis and judgement. They are not necessarily representative of other times and locations. We cannot accept responsibility for the use or interpretation by others of information presented on this log. Project Number NELSON GEOTECHNICAL No. Date Revision By CK 999017 Larsen Property Short Plat N A ASSOCIATES, INC. 1 8/10/17 Original DPN Lsa Boring Log GEOTECHNICAL ENGINEERS & GEOLOGISTS East Wenatchee Office Figure 8 Woodinville office 17311-135th Ave. NE, A500 55261ndmtry Lane, A2 A 98802 (425 Woodinville,66- Wenatchee, `/ 1669/ Fax: 4812510 v n,nelw,geotech.mm (509)665-7696/ Fax665-7692 Page 1 of( 2 I BORING LOG B-4 (cont.) Penetration Resistance Profile Soil Pro Sample Data p (Blows/foot - ®) � Piezometer 10 20 30 40 50 50+ t_ Installation - m Ground Water Description sz 0 C. 0 � -0 0 o m 0 n w - Moisture Content (Percent - ®) 0 Data m _1 C� to 0 m U L m in � m 10 20 30 40 50 50+ 0 m (Depth in Feet) J 43 Gray silt with fine sand (very stiff to hard, moist) 32 49 ' 35 ................................................... 35 -interbedded with gray, silty fine to medium sand ML 53 ' 40 ................................................... 40 45 ................................................... 45 52 Boring terminated below existing grade at 46.5 feet on 8/7/17. Groundwater seepage was not encountered during drilling. 50 .................................................... 50 55 .................................................... 55 LEGEND Solid PVC Pipe >ai Concrete M Moisture Content Slotted PVC Pipe Bentonite A Atterberg Limits Depth Driven and Amount Recovered G Grain -size Analysis with 2-inch O.D. Split -Spoon Sampler Monument/ Cap Native Soil DS Direct Shear d��' to Piezometer Silica Sand PP Pocket Penetrometer Readings, tons/ft Depth Driven and Amount Recovered * Liquid Limit P Sample Pushed with 3-inch Shelby Tube Sampler + Plastic Limit Water Level T Triaxial NOTE: Subsurface conditions depicted represent our observations at the time and location of this exploratory hole, modified by engineering tests, analysis and judgement. They are not necessarily representative of other times and locations. We cannot accept responsibility for the use or interpretation by others of information presented on this log. Project Number NELSON GEOTECHNICAL No. Date Revision By CK 999017 Larsen Property Short Plat -"'ONGA ASSOCIATES, INC. 1 8/10117 Original DPN LSB Boring Log GEOTECHNICAL ENGINEERS & GEOLOGISTS East Wenatchee Office Figure 8 Woodinville Office 17311-1 Min Ave. NE, A-500 55261ndostry Lane, @2 Woodinvifte, WA 9W72 East (425)486-1669/ avx:4612510 w.nelsongectech.ccm (509)a 66S7696/Fax: 665tcbee, WA 7692 Page 2 of 2 y DO 0 Y M n a 0 rn 01 0 J USDA United States A product of the National Custom Soil Resource Department of Agriculture Cooperative Soil Survey, a joint effort of the United Report for n II\ � RCS States Department of Agriculture and other Snohomish County v Federal agencies, State Natural Resources agencies including the Agricultural Experiment Area, Washington Conservation Stations, and local Service participants Larsen Residence 1 April 29, 2020 ry Custom Soil Resource Report 3 Soil Map 549920 549930 549940 549950 549960 549970 5499W 549990 550000 550010 550020 }}�� 47° 51' 21" N � � fir# 47° 51' 21" N o•� ; y r fir, 1 .._ - • f- - r. o_ 91 F 4•t c ti �* T .99 vow ;-h 4 dr ' � I - ,i i , o + 9%• Z *} Soil M. rPyn; li r- 1 jV �ji ,p 47- 51' 19" N l I 47° 51' 19" N 549920 549930 549940 549950 549960 549970 549980 549990 550000 550010 550020 3 3 Map Scale: 1:519 if printed on A landsope (ll" x 8.5') gieet N Meters ~ 1 0 5 10 20 30 11 Feet '■�\V1 0 25 50 100 150 Map projection: Web Mercator comer coordinates: WGS84 Edge tics: UTM Zone ION WGS84 LAI MAP LEGEND Area of Interest (AOI) Area of Interest (AOI) Soils 0 Soil Map Unit Polygons rwr Soil Map Unit Lines Soil Map Unit Points Special Point Features Uo Blowout ® Borrow Pit Clay Spot Closed Depression Gravel Pit Gravelly Spot Landfill Lava Flow Marsh or swamp + Mine or Quarry Miscellaneous Water Perennial Water Rock Outcrop Saline Spot d Sandy Spot Severely Eroded Spot Sinkhole Slide or Slip oa Sodic Spot Custom Soil Resource Report MAP INFORMATION Spoil Area The soil surveys that comprise your AOI were mapped at 1:24,000. Stony Spot Very Stony Spot Warning: Soil Map may not be valid at this scale. Wet Spot Enlargement of maps beyond the scale of mapping can cause Other misunderstanding of the detail of mapping and accuracy of soil .- Special Line Features line placement. The maps do not show the small areas of contrasting soils that could have been shown at a more detailed Water Features scale. Streams and Canals Transportation Please rely on the bar scale on each map sheet for map 1 1 F Rails measurements. Interstate Highways Source of Map: Natural Resources Conservation Service r.x US Routes Web Soil Survey URL: Coordinate System: Web Mercator (EPSG:3857) Major Roads Local Roads Maps from the Web Soil Survey are based on the Web Mercator projection, which preserves direction and shape but distorts Background distance and area. A projection that preserves area, such as the Aerial Photography Albers equal-area conic projection, should be used if more accurate calculations of distance or area are required. This product is generated from the USDA-NRCS certified data as of the version date(s) listed below. Soil Survey Area: Snohomish County Area, Washington Survey Area Data: Version 21, Sep 16, 2019 Soil map units are labeled (as space allows) for map scales 1:50,000 or larger. Date(s) aerial images were photographed: Sep 2, 2018—Sep 25, 2018 The orthophoto or other base map on which the soil lines were compiled and digitized probably differs from the background imagery displayed on these maps. As a result, some minor shifting of map unit boundaries may be evident. Custom Soil Resource Report Map Unit Legend Map Unit Symbol Map Unit Name Acres in AOI Percent of AOI 4 Alderwood-Everett gravelly 0.7 100.0% sandy loams, 25 to 70 percent slopes Totals for Area of Interest 0.7 100.0% Map Unit Descriptions The map units delineated on the detailed soil maps in a soil survey represent the soils or miscellaneous areas in the survey area. The map unit descriptions, along with the maps, can be used to determine the composition and properties of a unit. A map unit delineation on a soil map represents an area dominated by one or more major kinds of soil or miscellaneous areas. A map unit is identified and named according to the taxonomic classification of the dominant soils. Within a taxonomic class there are precisely defined limits for the properties of the soils. On the landscape, however, the soils are natural phenomena, and they have the characteristic variability of all natural phenomena. Thus, the range of some observed properties may extend beyond the limits defined for a taxonomic class. Areas of soils of a single taxonomic class rarely, if ever, can be mapped without including areas of other taxonomic classes. Consequently, every map unit is made up of the soils or miscellaneous areas for which it is named and some minor components that belong to taxonomic classes other than those of the major soils. Most minor soils have properties similar to those of the dominant soil or soils in the map unit, and thus they do not affect use and management. These are called noncontrasting, or similar, components. They may or may not be mentioned in a particular map unit description. Other minor components, however, have properties and behavioral characteristics divergent enough to affect use or to require different management. These are called contrasting, or dissimilar, components. They generally are in small areas and could not be mapped separately because of the scale used. Some small areas of strongly contrasting soils or miscellaneous areas are identified by a special symbol on the maps. If included in the database for a given area, the contrasting minor components are identified in the map unit descriptions along with some characteristics of each. A few areas of minor components may not have been observed, and consequently they are not mentioned in the descriptions, especially where the pattern was so complex that it was impractical to make enough observations to identify all the soils and miscellaneous areas on the landscape. The presence of minor components in a map unit in no way diminishes the usefulness or accuracy of the data. The objective of mapping is not to delineate pure taxonomic classes but rather to separate the landscape into landforms or landform segments that have similar use and management requirements. The delineation of such segments on the map provides sufficient information for the development of resource plans. If intensive use of small areas is planned, however, Custom Soil Resource Report onsite investigation is needed to define and locate the soils and miscellaneous areas. An identifying symbol precedes the map unit name in the map unit descriptions. Each description includes general facts about the unit and gives important soil properties and qualities. Soils that have profiles that are almost alike make up a soil series. Except for differences in texture of the surface layer, all the soils of a series have major horizons that are similar in composition, thickness, and arrangement. Soils of one series can differ in texture of the surface layer, slope, stoniness, salinity, degree of erosion, and other characteristics that affect their use. On the basis of such differences, a soil series is divided into soil phases. Most of the areas shown on the detailed soil maps are phases of soil series. The name of a soil phase commonly indicates a feature that affects use or management. For example, Alpha silt loam, 0 to 2 percent slopes, is a phase of the Alpha series. Some map units are made up of two or more major soils or miscellaneous areas. These map units are complexes, associations, or undifferentiated groups. A complex consists of two or more soils or miscellaneous areas in such an intricate pattern or in such small areas that they cannot be shown separately on the maps. The pattern and proportion of the soils or miscellaneous areas are somewhat similar in all areas. Alpha -Beta complex, 0 to 6 percent slopes, is an example. An association is made up of two or more geographically associated soils or miscellaneous areas that are shown as one unit on the maps. Because of present or anticipated uses of the map units in the survey area, it was not considered practical or necessary to map the soils or miscellaneous areas separately. The pattern and relative proportion of the soils or miscellaneous areas are somewhat similar. Alpha -Beta association, 0 to 2 percent slopes, is an example. An undifferentiated group is made up of two or more soils or miscellaneous areas that could be mapped individually but are mapped as one unit because similar interpretations can be made for use and management. The pattern and proportion of the soils or miscellaneous areas in a mapped area are not uniform. An area can be made up of only one of the major soils or miscellaneous areas, or it can be made up of all of them. Alpha and Beta soils, 0 to 2 percent slopes, is an example. Some surveys include miscellaneous areas. Such areas have little or no soil material and support little or no vegetation. Rock outcrop is an example. Custom Soil Resource Report Snohomish County Area, Washington 4—Alderwood-Everett gravelly sandy loams, 25 to 70 percent slopes Map Unit Setting National map unit symbol: 2hyy Elevation: 50 to 800 feet Mean annual precipitation: 25 to 60 inches Mean annual air temperature: 48 to 52 degrees F Frost -free period: 180 to 220 days Farmland classification: Not prime farmland Map Unit Composition Alderwood and similar soils: 60 percent Everett and similar soils: 25 percent Minor components: 15 percent Estimates are based on observations, descriptions, and transects of the mapunit. Description of Alderwood Setting Landform: Till plains Parent material: Basal till Typical profile H1 - 0 to 7 inches: gravelly ashy sandy loam H2 - 7 to 35 inches: very gravelly ashy sandy loam H3 - 35 to 60 inches: gravelly sandy loam Properties and qualities Slope: 25 to 70 percent Depth to restrictive feature: 20 to 40 inches to densic material Natural drainage class: Moderately well drained Capacity of the most limiting layer to transmit water (Ksat): Very low to moderately low (0.00 to 0.06 in/hr) Depth to water table: About 18 to 36 inches Frequency of flooding: None Frequency of ponding: None Available water storage in profile: Low (about 3.0 inches) Interpretive groups Land capability classification (irrigated): None specified Land capability classification (nonirrigated): 7e Hydrologic Soil Group: B Hydric soil rating: No Description of Everett Setting Landform: Terraces, plains Parent material: Glacial outwash Typical profile H1 - 0 to 6 inches: gravelly ashy sandy loam H2 - 6 to 18 inches: very gravelly ashy sandy loam H3 - 18 to 60 inches: extremely gravelly sand Custom Soil Resource Report Properties and qualities Slope: 25 to 70 percent Depth to restrictive feature: 14 to 20 inches to strongly contrasting textural stratification Natural drainage class: Somewhat excessively drained Capacity of the most limiting layer to transmit water (Ksat): High (1.98 to 5.95 in/hr) Depth to water table: More than 80 inches Frequency of flooding: None Frequency of ponding: None Available water storage in profile: Very low (about 2.1 inches) Interpretive groups Land capability classification (irrigated): None specified Land capability classification (nonirrigated): 7e Hydrologic Soil Group: A Hydric soil rating: No Minor Components Mckenna Percent of map unit. 5 percent Landform: Depressions Hydric soil rating: Yes Norma, undrained Percent of map unit. 5 percent Landform: Depressions Hydric soil rating: Yes Terric medisaprists, undrained Percent of map unit. 5 percent Landform: Depressions Hydric soil rating: Yes Larsen Residence - CG #20039.20 Drainage Report June 30, 2020 Section VII, Page 1 Section VII — Other Permits Section VII Summary: Narrative This project will not require permits beyond those required by the City of Edmonds. C GM ENGINEERING 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Larsen Residence - CG #20039.20 June 30, 2020 Drainage Report Section VIII, Page 1 Section VIII — Bond Quantities and Operation and Maintenance Manual Section VIII Summarv: Narrative A bond quantity worksheet is a separate document that can also be submitted at the City's request. The Operation and Maintenance Manual is a standalone document that will be given to the homeowner following the construction of the project. The contractor will be responsible for the maintenance and operation of all stormwater structures and BMPs requiring maintenance during construction and, after construction, responsibility will pass to the homeowner. Upon request by the City of Edmonds, it shall be made available for their inspection. It is generally expected that few to none of these defects will be present upon the yearly inspection of each facility. C GM ENGINEERING 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Larsen Residence - CG #20039.20 Drainage Report June 30, 2020 Section VIII, Page 2 Larsen Residence 7329 15811 St SW Edmonds, WA 98026 [a] "A -All V le] i, I_V 01 & f_l l ih"0 Y_L L" W_L 14_H Date: May 2020 C CM ENGINEERING 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Larsen Residence - CG #20039.20 Drainage Report June 30, 2020 Section VIII, Page 3 Operation and Maintenance Manual This Operation and Maintenance Manual has been created for Larsen Residence, which consists of the addition of approximately 3,360 sf of new/replaced hard surfaces to an existing house on a 0.61 acre lot located at 7329 158th St SW, Edmonds, WA 98026. The proposed stormwater management system consists of conveyance pipes and catch basins that collect runoff and route it to a downstream conveyance system in 1581h St SW. Included in this Operation and Maintenance Manual is an 11" x 17" grading and drainage plan sheet showing the locations of the drains and catch basins. Please note that this map is generated during the design phase and may not reflect all changes made in permitting and construction. CG Engineering may be contacted for an updated copy of this map once the as -built drawings are completed for the site. The contractor will be responsible for the maintenance and operation of all stormwater structures and BMPs requiring maintenance during construction and, after construction, responsibility will pass to the homeowner. Included in this manual are maintenance sheets taken from the 2014 Stormwater Management Manual for Western Washington. Maintenance sheets are included for the following facilities/activities: Catch Basins: Concrete structures with steel grates that collect stormwater runoff from the site and act as junctions for storm conveyance pipes. See "No. 5" for maintenance. Vegetation Management: Landscaping can include grading, soil transfer, vegetation removal, pesticide and fertilizer applications, and watering. Stormwater contaminants include toxic organic compounds, heavy metals, oils, total suspended solids, coliform bacteria, fertilizers, and pesticides. Facilities shall be inspected for defects listed in the following facility sheets. Most maintenance tasks are generally reactionary to a defect being found, rather than a matter of constant upkeep. It is generally expected that few to none of these defects will be present upon the yearly inspection of each facility. The facility sheets list the potential conditions warranting maintenance and the expected result following any maintenance. Several engineer's notes for specific tasks are provided within the facility sheets. Unless otherwise noted on the facility sheets the maintenance tasks should be performed on an "as needed" basis: (a) When the described defect is visible to whomever performs the yearly inspection, (b) Should any defect become apparent between inspections. C GM ENGINEERING 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com Larsen Residence - CG #20039.20 Drainage Report June 30, 2020 Section VIII, Page 4 SAMPLE ACTIVITY LOG DATE FACILITY MAINTENANCE PERFORMED RESULTS/ NOTES C CM ENGINEERING 250 4th Avenue South, Suite 200 Edmonds, WA 98020 ph.425.778.8500 1 f.425.778.5536 www.cgengineering.com IL J 4l0C, I z5 I III — I �W I 20' V A� 210' r3 i �I I I CB T-1 RIM=96.90' IE-92.06'(12" PVC N) IE-92.56'(12" PVC S) 0 1" o513300002502 LOT 2, SECTION 5, TOWNSHIP 27 NORTH, RANGE 4 EAST, W.M. - NATIVE VEGEIA^IONcRET^N^vONcARTA _c o IE=125.40'(18" PVC4E� (,'IN CH=IZti.4/�� IE=135. �7'(R2M' CMP4E) CTRNCE-OUTIECO BLbCIK�p!'ALL' ' "'-A"'III 11 I II'\ I CB T-1 E,NE-OUT W 5' PORTION OF I 0• 18" VERTICAL PIPE ABOVE GROUT RIM=96.51' T IE=125.20' 12" CMP W IE=136.67' 12" HDPE SE W)�Eo=1=3194'31'(12" P5.51' IE=91.89'(12" PVC S) IE=127.23'(4" PVC S) 135.62'(18" PVC W) =139.46'(12" S) a E=92.39'(12" PVC NW) el KEYSTONE WALL =139.6'(12" E) I E=92.71'(4" PVC E) �� I I 18" VERTICAL PIPE W) a II TOP-145.53'�� IE-140.08'(12" N) GRADING AND DRAINAGE PLAN 1 SCALE: 1"=20'-0" GRADING AND DRAINAGE PLAN NOTES: 20 0 30 20 40 1. A MINIMUM OF 3' HORIZONTAL SEPARATION AND V VERTICAL SEPARATION IS REQUIRED BETWEEN DRY UTILITIES (POWER, GAS, PHONE, CABLE, ETC.) AND SEWER, WATER AND STORM, AND A MINIMUM OF 5' HORIZONTAL PAVING LEGEND GRADING QUANTITIES SEPARATION AND V VERTICAL SEPARATION FROM ANY CITY -OWNED LINES. - NEW ASPHALT TOTAL EXCAVATION (CUT) - 5 CU YDS 2. DRIVEWAYS SHALL NOT EXCEED I4%SLOPE. TOTALEMBANKMENT(FILL)- 5 CU YDS OVERLAY EXISTING ASPHALT/UTILITY SAWCUT TOTAL 10 CU YDS 3. ALL DISTURBED AREAS ON AND OFF -SITE SHALL BE COMPOST AMENDED PER THE REQUIREMENTS OF BMPT5.13 IN THE STORMWATER MAN UAL VOLUME V, CHAPTER 5. NEWGRAVEL THE QUANTITIES SHOWN ABOVE ARE FOR THE PERMIT PROCESS 4. CONTRACTOR SHALL VERIFY THE EXISTENCE OF ROOF AND FOOTING DRAINS AROUND THE HOUSE AND ADD OR ONLY. THESE VALUES ARE APPROXIMATE NOT USE FOR REPLACE PER DETAIL2IF NEEDED. BIDDING, PAYMENT, OR ESTIMATING PURPOO SES. 5. NEW/REPLACED HARD SURFACES: 3,360SF - 2,421 SF ROOFAREA. RUNOFF ROUTEDTHROUGH NEW ROOF DRAINSAND TIED TO EXISTING HOUSE DRAINS. - 464 SF GRAVEL DRIVEWAY. RUNOFF COLLECTED IN NEW CATCH BASIN AND TIED TO NEW ROOF DRAINS. - 475 SF ASPHALT DRIVEWAY. RUNOFF COLLECTED IN NEW CATCH BASIN AND TIED TO NEW ROOF DRAINS. 4" DIA PERFORATED PVC PIPE WITH 6" OF 1" MINUS GRAVEL 8" MIN FROM WOOD. ALL AROUND, WRAPPED IN SEE STRUCTURAL DRAWINGS NON -WOVEN GEOTEXTILE FABRIC, SLOPE AT 0.5%MIN. TURN DOWN PERFORATIONS AS SHOWN 6" DOWNSPOUTTIGHTUNETO FINISHEDGRADE CONVEYANCE SYSTEM @0.5% -� MIN. PLACE NE%TTO FOOTING DRAINORAS SHOWNON =_ GRADING & DRAINAGE PLAN (CONTRACTOR MAY LOCATE ON EITHER SIDE OF FOOTING DRAIN) 00 ° O - 00600 e 0,0° c °0� ,O Oo0 LINE OF MAX EXCAVATION. ^ IF SOIL'S OVEREXCAVATED, O D° O oO O n d REPLACE WITH LEAN MIX O o0 0 0 - CONCRETE �� FOOTING 1 / 1 FOOTING AND ROOF DRAIN SECTION 2 SCALE: NTS 5/8" MINUS CRUSHED SURFACING TOP COURSE PER WSDOT STD SPEC 9-03.9(3) 11/4" MINUS CRUSHED SURFACING BASE COURSE PER WSDOT STD SPEC 9-03.9(3) COMPACTED SUBGRADE 90% FOR NATIVE MATERIAL AND 95% FOR IMPORTED © GRAVEL PAVING DETAIL APPROVED FOR CONSTRUCTION CITY OF EDMONDS DATE: B,, CITY ENGINEERING DIVISION C � ENGINEERING 250 4TH AVE. S.. SUITE 200 EDMONDS, WASHINGTON 98020 PHONE (425) 778-8500 FAX (425) 778-5536 DESIGN: TAF DRAWN: ATD CHECK: JPU JOB NO: 20039.20 DATE: 05/05/2020 W Q Z Q v ' cV F_ W 0 U 0 o0 r) Ld W I.- rn Z 0 N Q Q Q LEI H I N Ur Z Z Q LJ Z C Z 0 Q Q M O J I� W J < (7 w SHEET C3.1 No. 5 — Catch Basins Maintenance Defect Conditions When Maintenance is Needed Results Expected When Component Maintenance is performed General Trash & Trash or debris which is located immediately No Trash or debris located Debris in front of the catch basin opening or is immediately in front of blocking inletting capacity of the basin by catch basin or on grate more than 10%. opening. Trash or debris (in the basin) that exceeds 60 No trash or debris in the percent of the sump depth as measured from catch basin. the bottom of basin to invert of the lowest pipe into or out of the basin, but in no case less than a minimum of six inches clearance from the debris surface to the invert of the lowest pipe. Trash or debris in any inlet or outlet pipe Inlet and outlet pipes free blocking more than 1/3 of its height. of trash or debris. Dead animals or vegetation that could No dead animals or generate odors that could cause complaints vegetation present within or dangerous gases (e.g., methane). the catch basin. Sediment Sediment (in the basin) that exceeds 60 No sediment in the catch percent of the sump depth as measured from basin the bottom of basin to invert of the lowest pipe into or out of the basin, but in no case less than a minimum of 6 inches clearance from the sediment surface to the invert of the lowest pipe. Structure Top slab has holes larger than 2 square Top slab is free of holes Damage to inches or cracks wider than 1/4 inch and cracks. Frame and/or Top Slab (Intent is to make sure no material is running into basin). Frame not sitting flush on top slab, i.e., Frame is sitting flush on separation of more than 3/4 inch of the frame the riser rings or top slab from the top slab. Frame not securely and firmly attached. attached Fractures or Maintenance person judges that structure is Basin replaced or repaired Cracks in unsound. to design standards. Basin Walls/ Bottom Grout fillet has separated or cracked wider Pipe is regrouted and than 1/2 inch and longer than 1 foot at the secure at basin wall. joint of any inlet/outlet pipe or any evidence of soil particles entering catch basin through cracks. Settlement/ If failure of basin has created a safety, Basin replaced or repaired Misalignment function, or design problem. to design standards. Vegetation Vegetation growing across and blocking more No vegetation blocking than 10% of the basin opening. opening to basin. Vegetation growing in inlet/outlet pipe joints No vegetation or root that is more than six inches tall and less than growth present. six inches apart. Contamination See "Detention Ponds" (No. 1). No pollution present. and Pollution Volume V — Runoff Treatment BMPs — December 2014 4-38 No. 5 — Catch Basins Maintenance Defect Conditions When Maintenance is Needed Results Expected When Component Maintenance is performed Catch Basin Cover Not in Cover is missing or only partially in place. Catch basin cover is Cover Place Any open catch basin requires maintenance. closed Locking Mechanism cannot be opened by one Mechanism opens with Mechanism maintenance person with proper tools. Bolts proper tools. Not Working into frame have less than 1/2 inch of thread. Cover Difficult One maintenance person cannot remove lid Cover can be removed by to Remove after applying normal lifting pressure. one maintenance person. (Intent is keep cover from sealing off access to maintenance.) Ladder Ladder Rungs Ladder is unsafe due to missing rungs, not Ladder meets design Unsafe securely attached to basin wall, standards and allows misalignment, rust, cracks, or sharp edges. maintenance person safe access. Metal Grates Grate opening Grate with opening wider than 7/8 inch. Grate opening meets (If Applicable) Unsafe design standards. Trash and Trash and debris that is blocking more than Grate free of trash and Debris 20% of grate surface inletting capacity. debris. Damaged or Grate missing or broken member(s) of the Grate is in place and Missing. grate. meets design standards. ai ena a '• Defect on do he ai na �/7p ed en C po nts Xisl N de is P rfor ed Ge ral Trash acid ' Tra or /eo/ingpnthVarri rier ear e to d ign w :•1)ebrfs t n 2 0 of Xapa y. M I p'amag2d/!/�Baare ent t of ape ore an rs i lac ith be s m e Missing . han /4 i Baas. mi ing enti bar ' r asn Bar in pl e a ordi to ign. % / mi ars a to e a rus ca ing % ��e r ace or r aire ode rior n t ny rt of arrie . an ds. Inlet/Outtet eb ' bar ' r m' ing no tac d to B ier f ly ach to p' e Pipe pi Volume V — Runoff Treatment BMPs — December 2014 4-39 No. 18 — Catchbasin Inserts Maintenance Defect Conditions When Maintenance is Results Expected When Component Needed Maintenance is Performed General Sediment When sediment forms a cap over the No sediment cap on the insert Accumulation insert media of the insert and/or unit. media and its unit. Trash and Trash and debris accumulates on insert Trash and debris removed Debris unit creating a blockage/restriction. from insert unit. Runoff freely Accumulation flows into catch basin. Media Insert Not Effluent water from media insert has a Effluent water from media Removing Oil visible sheen. insert is free of oils and has no visible sheen. Media Insert Catch basin insert is saturated with water Remove and replace media Water Saturated and no longer has the capacity to insert absorb. Media Insert -Oil Media oil saturated due to petroleum spill Remove and replace media Saturated that drains into catch basin. insert. Media Insert Use Media has been used beyond the typical Remove and replace media at Beyond Normal average life of media insert product. regular intervals, depending on Product Life insert product. Volume V — Runoff Treatment BMPs — December 2014 4-51 pp ' ab O at' al M • El' in u e to asadi ar s t tose er, oui ;. at 'o rfa w er.: / onv u e to isc rg to an ry we fa we,.d`by;the to se er tho y, to e pp e ea en • Ob in ro iat tat nd oca e is th e d' cha es. e m en d dit' nal pe do al P . At in erc' an du ial cil' 'es, nd t a ry of ast at dis ar co ec ' ns to o dra' s a to rf e w er -e- to ws• • on ct iel su y ui in , pa cu y der it gs nd o er i us al as to to rai Er b ' di a pa d Sur es of h e t e j 'n t pu is s rm rai ). ur' g n -st er nd' on 'ns t e s rm ai or n- '' s ter isc rge . Re rd e 1 ati s o 11 n-s rm ater is ar I ud 11 rm' ed 'sch ge us 19 ep e a ap ea ar . S w th a he o % to atio of o ew s, s it se ers nd er ed d / np i d ch es er- p os ay us 1. e re rds s ch pi ' g s e I ics o f of n ns* es er on do an ho the o e ap. on ' er in in e, e, o the ica al s t s t et co ec 'ons etw en o c ve nc ys s (e , p ces a a sto w er). de rab , c duc TV sp do of e s rm rai an ec t fo ge v• eot e. in re e o se d 1 ati s o co ect' ns ith e ' o do o e ap d r is e ap co n . N e s pe co ect ns at e i on ten it e eld ry ent' a co ect' ns st er or su ace at an tak `th cti s s cif d a ve a is e $•VIPs S411 BMPs for Landscaping and Lawn/ Vegetation Management Description of Pollutant Sources: Landscaping can include grading, soil transfer, vegetation removal, pesticide and fertilizer applications, and watering. Stormwater contaminants include toxic organic compounds, heavy metals, oils, total suspended solids, coliform bacteria, fertilizers, and pesticides. Lawn and vegetation management can include control of objectionable weeds, insects, mold, bacteria, and other pests with pesticides. Examples include weed control on golf course lawns, access roads, and utility corridors and during landscaping; sap stain and insect control on lumber and logs; rooftop moss removal; killing nuisance rodents; fungicide application to patio decks, and residential lawn/plant care. It is possible to Volume IV - Souree Control BMPs — Deeember 2014 2-21 release toxic pesticides such as pentachlorophenol, carbamates, and organometallics to the environment by leaching and dripping from treated parts, container leaks, product misuse, and outside storage of pesticide contaminated materials and equipment. Poor management of the vegetation and poor application of pesticides or fertilizers can cause appreciable stormwater contamination. Pollutant Control Approach: Control of fertilizer and pesticide applications, soil erosion, and site debris to prevent contamination of stormwater. Develop and implement an Integrated Pest Management Plan (IPM) and use pesticides only as a last resort. Carefully apply pesticides/ herbicides, in accordance with label instructions. Maintain appropriate vegetation, with proper fertilizer application where practicable, to control erosion and the discharge of stormwater pollutants. Where practicable grow plant species appropriate for the site, or adjust the soil properties of the subject site to grow desired plant species. Applicable Operational BMPs for Landscaping: • Install engineered soil/landscape systems to improve the infiltration and regulation of stormwater in landscaped areas. Do not dispose of collected vegetation into waterways or storm sewer systems. Recommended Additional Operational BMPs for Landscaping: • Conduct mulch -mowing whenever practicable • Dispose of grass clippings, leaves, sticks, or other collected vegetation, by composting, if feasible. • Use mulch or other erosion control measures on soils exposed for more than one week during the dry season or two days during the rainy season. • Store and maintain appropriate oil and chemical spill cleanup materials in readily accessible locations when using oil or other chemicals. Ensure that employees are familiar with proper spill cleanup procedures. • Till fertilizers into the soil rather than dumping or broadcasting onto the surface. Determine the proper fertilizer application rate for the types of soil and vegetation encountered. • Till a topsoil mix or composted organic material into the soil to create a well -mixed transition layer that encourages deeper root systems and drought -resistant plants. • Use manual and/or mechanical methods of vegetation removal rather than applying herbicides, where practical. Volume IV - Source Control BMPs December 2014 2-22 Applicable Operational BMPs for the Use of Pesticides: • Develop and implement an IPM (See section on IPM in ,4pplicable Operational BM for Vegetation Management) and use pesticides only as a last resort. • Implement a pesticide -use plan and include at a minimum: a list of selected pesticides and their specific uses; brands, formulations, application methods and quantities to be used; equipment use and maintenance procedures; safety, storage, and disposal methods; and monitoring, record keeping, and public notice procedures. All procedures shall conform to the requirements of Chapter 17.21 RCW and Chapter 16-228 WAC (Appendix IV-D R.7). • Choose the least toxic pesticide available that is capable of reducing the infestation to acceptable levels. The pesticide should readily degrade in the environment and/or have properties that strongly bind it to the soil. Conduct any pest control activity at the life stage when the pest is most vulnerable. For example, if it is necessary to use a Bacillus thurin _ ig ens application to control tent caterpillars, apply it to the material before the caterpillars cocoon or it will be ineffective. Any method used should be site -specific and not used wholesale over a wide area. • Apply the pesticide according to label directions. Do not apply pesticides in quantities that exceed manufacturer's instructions. • Mix the pesticides and clean the application equipment in an area where accidental spills will not enter surface or ground waters, and will not contaminate the soil. • Store pesticides in enclosed areas or in covered impervious containment. Do not discharge pesticide contaminated stormwater or spills/leaks of pesticides to storm sewers. Do not hose down the paved areas to a storm sewer or conveyance ditch. Store and maintain appropriate spill cleanup materials in a location known to all near the storage area. • Clean up any spilled pesticides. Keep pesticide contaminated waste materials in designated covered and contained areas. • The pesticide application equipment must be capable of immediate shutoff in the event of an emergency. • Spraying pesticides within 100 feet of open waters including wetlands, ponds, and rivers, streams, creeks, sloughs and any drainage ditch or channel that leads to open water may have additional regulatory requirements beyond just following the pesticide product label. Additional requirements may include: • Obtaining a discharge permit from Ecology. • Obtaining a permit from the local jurisdiction. • Using an aquatic labeled pesticide. Volume IV - Source Control BMPs — December 2014 2-23 Flag all sensitive areas including wells, creeks, and wetlands prior to spraying. • Post notices and delineate the spray area prior to the application, as required by the local jurisdiction or by Ecology. Conduct spray applications during weather conditions as specified in the label direction and applicable local and state regulations. Do not apply during rain or immediately before expected rain. Recommended Additional Operational BMPs for the use of pesticides: Consider alternatives to the use of pesticides such as covering or harvesting weeds, substitute vegetative growth, and manual weed control/moss removal. • Consider the use of soil amendments, such as compost, that are known to control some common diseases in plants, such as Pythium root rot, ashy stem blight, and parasitic nematodes. The following are three possible mechanisms for disease control by compost addition (USEPA Publication 530-F-9-044): 1. Successful competition for nutrients by antibiotic production; 2. Successful predation against pathogens by beneficial microorganism; and 3. Activation of disease -resistant genes in plants by composts. Installing an amended soil/landscape system can preserve both the plant system and the soil system more effectively. This type of approach provides a soil/landscape system with adequate depth, permeability, and organic matter to sustain itself and continue working as an effective stormwater infiltration system and a sustainable nutrient cycle. • Once a pesticide is applied, evaluate its effectiveness for possible improvement. Records should be kept showing the effectiveness of the pesticides considered. Develop an annual evaluation procedure including a review of the effectiveness of pesticide applications, impact on buffers and sensitive areas (including potable wells), public concerns, and recent toxicological information on pesticides used/proposed for use. If individual or public potable wells are located in the proximity of commercial pesticide applications, contact the regional Ecology hydrogeologist to determine if additional pesticide application control measures are necessary. • Rinseate from equipment cleaning and/or triple -rinsing of pesticide containers should be used as product or recycled into product. For more information, contact the Washington State University (WSU) Extension Home -Assist Program, (253) 445-4556, or Bio-Integral Resource Center (BIRC), P.O. Box 7414, Berkeley, CA.94707, or EPA to Volume IV - Source Control BMPs — December 2014 2-24 obtain a publication entitled "Suspended, Canceled, and Restricted Pesticides " which lists all restricted pesticides and the specific uses that are allowed. Applicable Operational BMPs for Vegetation Management: Use at least an eight -inch "topsoil' layer with at least 8 percent organic matter to provide a sufficient vegetation -growing medium. Amending existing landscapes and turf systems by increasing the percent organic matter and depth of topsoil can substantially improve the permeability of the soil, the disease and drought resistance of the vegetation, and reduce fertilizer demand. This reduces the demand for fertilizers, herbicides, and pesticides. Organic matter is the least water-soluble form of nutrients that can be added to the soil. Composted organic matter generally releases only between 2 and 10 percent of its total nitrogen annually, and this release corresponds closely to the plant growth cycle. Return natural plant debris and mulch to the soil, to continue recycling nutrients indefinitely. Select the appropriate turfgrass mixture for the climate and soil type. Certain tall fescues and rye grasses resist insect attack because the symbiotic endophytic fungi found naturally in their tissues repel or kill common leaf and stem -eating lawn insects. However, they do not, repel root -feeding lawn pests such as Crane Fly larvae, and are toxic to ruminants such as cattle and sheep. The fungus causes no known adverse effects to the host plant or to humans. Endophytic grasses are commercially available; use them in areas such as parks or golf courses where grazing does not occur. Local agricultural or gardening resources such as Washington State University Extension office can offer advice on which types of grass are best suited to the area and soil type. • Use the following seeding and planting BMPs, or equivalent BMPs to obtain information on grass mixtures, temporary and permanent seeding procedures, maintenance of a recently planted area, and fertilizer application rates: Temporary and Permanent Seeding, Mulching, Plastic Covering, and Sodding as described in Volume II. Adjusting the soil properties of the subject site can assist in selection of desired plant species. For example, design a constructed wetland to resist the invasion of reed canary grass by layering specific strata of organic matters (e.g., composted forest product residuals) and creating a mildly acidic pH and carbon -rich soil medium. Consult a soil restoration specialist for site -specific conditions. • Aerate lawns regularly in areas of heavy use where the soil tends to become compacted. Conduct aeration while the grasses in the lawn are growing most vigorously. Remove layers of thatch greater than 3/4-inch deep. Volume IV - Source Control BMPs — December 2014 2-25 Mowing is a stress -creating activity for turfgrass. Grass decreases its productivity when mown too short and there is less growth of roots and rhizomes. The turf becomes less tolerant of environmental stresses, more disease prone and more reliant on outside means such as pesticides, fertilizers, and irrigation to remain healthy. Set the mowing height at the highest acceptable level and mow at times and intervals designed to minimize stress on the turf. Generally mowing only 1/3 of the grass blade height will prevent stressing the turf. Irrigation: The depth from which a plant normally extracts water depends on the rooting depth of the plant. Appropriately irrigated lawn grasses normally root in the top 6 to 12 inches of soil; lawns irrigated on a daily basis often root only in the top 1 inch of soil. Improper irrigation can encourage pest problems, leach nutrients, and make a lawn completely dependent on artificial watering. The amount of water applied depends on the normal rooting depth of the turfgrass species used, the available water holding capacity of the soil, and the efficiency of the irrigation system. Consult with the local water utility, Conservation District, or Cooperative Extension office to help determine optimum irrigation practices. Fertilizer Management: Turfgrass is most responsive to nitrogen fertilization, followed by potassium and phosphorus. Fertilization needs vary by site depending on plant, soil, and climatic conditions. Evaluation of soil nutrient levels through regular testing ensures the best possible efficiency and economy of fertilization. For details on soils testing, contact the local Conservation District, a soils testing professional, or a Washington State University Extension office. Apply fertilizers in amounts appropriate for the target vegetation and at the time of year that minimizes losses to surface and ground waters. Do not fertilize when the soil is dry. Alternatively, do not apply fertilizers within three days prior to predicted rainfall. The longer the period between fertilizer application and either rainfall or irrigation, the less fertilizer runoff occurs. Use slow release fertilizers such as methylene urea, IDBU, or resin coated fertilizers when appropriate, generally in the spring. Use of slow release fertilizers is especially important in areas with sandy or gravelly soils. Time the fertilizer application to periods of maximum plant uptake. Ecology generally recommends application in the fall and spring, although Washington State University turf specialists recommend four fertilizer applications per year. Volume IV - Source Control BMPs — December 2014 2-26 • Properly trained persons should apply all fertilizers. Apply no fertilizer at commercial and industrial facilities, to grass swales, filter strips, or buffer areas that drain to sensitive water bodies unless approved by the local jurisdiction. Integrated Pest Management An IPM program might consist of the following steps: Step 1: Correctly identify problem pests and understand their life cycle Step 2: Establish tolerance thresholds for pests. Step 3: Monitor to detect and prevent pest problems. Step 4: Modify the maintenance program to promote healthy plants and discourage pests. Step 5: Use cultural, physical, mechanical or biological controls first if pests exceed the tolerance thresholds. Step 6: Evaluate and record the effectiveness of the control and modify maintenance practices to support lawn or landscape recovery and prevent recurrence. For an elaboration of these steps, refer to Appendix IV-F. 5412 `BM.Ps for Loading a U.nloadin Ar s r L• ui or oli ri D cri io f P ut t ur s: era rs is y c du oa ' g/ oa g li d a s d ers a 'nd ria nd c incia aci ' ies s pi an ec ' in , ou de ora in are , et . ria tra fe d c in de ro cts aw ate Is e di pr uc , w e ter' s, is, cra et s, e . L s d sp' so Uel ,oil , po er or ics ea in als alt aci , al lis c. rin ra er ay us to a co a ati . SFils om h ra c li br s e a in on obl a oa g cks Po to C tr Ap oa ver nd nt ' th oa 'ng/ to in ea er nec sar to eve ru on St w er d of f: c a at sto w er. A is a er io B s•.` At L di / U oa ' g ea Sig ' is a un f d ris an cu lat at o si , un ve d o ng/ to in re a . S ep ese rf es qu tly re ove ose at ial at c ld ont in e st ter. we ar s to or ily ve d a r r ov of e c ta' ers ogs r o er at al ver' g t gr nd ace rip ans or o er pr ria to or co al en ev' e, a to do w re 1 s sp' S y o ur ch ho co ec 'onS ose eel nd le oz s. wa us dri a wh m in nd Volume IV - Source Control BMPs — December 2014 2-27