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APPROVED STM RESUB1 BLD2022-0287+Storm_Drainage_Report+7.13.2022_4.01.55_PM+2988187MFE) INSIGHT ENGINEERING CO. STORMWATER SITE PLAN For 92ND LOT BLD2022-0287 Prepared for The City of Edmonds RESUB 1215thAve N Jul 13 2022 Edmonds, WA 98020 CITY OF EDMONDS 425-771-0220 DEVELOPMENT SERVICES DEPARTMENT Project Site Location: 21322 92" d Pl W Edmonds, WA -98020 Applicant: Contact: Echelbarger Homes Inc IECO 4001 198th ST SW, #2 P.O. Box 1478 (425) 673-1100 Everett, WA98206 todd@echelbarger.com 425-303-9363 Tax Id:00479300000600 IECO Project: 21-1173 Certified Erosion and Sedimentation Control Lead: To be named by contractor Stormwater Site Plan Prepared By: Shilpa Xavier, E. I. T. Stormwater Site Plan Preparation Date: March 4th, 2022 Date Revised July 12, 2022 Approximate Construction Date: September 1st, 2022 P.O. Box 1478 *Everett, WA 98206• P: 425.303.9363 • info@insightengineering.net 0711212022 COMPLIES WITH APPLICABLE CITY STORMWATER CODE 6� 08/31 /2022 TABLE OF CONTENTS 1.0 Executive Summary................................................................................................................3 1.1 Minimum Requirements Summary..............................................................................................7 2.0 Existing Conditions.................................................................................................................8 3.0 Offsite Analysis.....................................................................................................................11 3.1 Upstream Analysis.....................................................................................................................12 3.2 Downstream Analysis................................................................................................................12 4.0 Developed Conditions..........................................................................................................15 5.0 Site Hydraulic Analysis.........................................................................................................16 5.1 Existing Basin Summary............................................................................................................16 5.2 Developed Basin Summary........................................................................................................16 6.0 Appendix...............................................................................................................................17 Figures Figure1 - Vicinity Map.................................................................................................................6 Figure2 - Soil Map........................................................................................................................9 Figure3 — Downstream Analysis Map-1....................................................................................13 Figure 4 - Downstream Analysis Map-2....................................................................................14 Insight Engineering Co. - Stormwater Site Plan 07/12/22 -1- Acronyms and Abbreviations BLA Boundary Line Adjustment BMP Best Management Practices DOE Department of Ecology ECDC Edmonds Community Development Code EDDS Engineering Design and Development Standards ESC Erosion and Sediment Control IECO Insight Engineering Company MR Minimum Requirement SWPPP Stormwater Pollution Prevention Plan SWMMWW Stormwater Management Manual for Western Washington TESC Temporary Erosion and Sediment Control WWHM Western Washington Hydrology Model Insight Engineering Co. - Stormwater Site Plan 07/12/22 -2- 1.0 Executive Summary The proposed project 92"d Lot is located at 21322 92nd Pl W, Edmonds, Washington. More generally, the site is in Section 25, Township 27 North, and Range 3 East of the Willamette Meridian in Snohomish County, Washington. Please refer to the Vicinity Map attached later in the section. This project is subject to the City of Edmond's 2017 requirements and the requirements defined in DOE's 2014 SWMMWW. The site area is 0.19 Acres. The site is currently partly developed with two sheds. The remainder of the site exists as lawn and residential landscaping. The site contains one drainage basin that slopes to the west. Please refer to the downstream analysis map for more details. The site is in an erosion hazard area. Per NRCS survey of Snohomish County, the project site contains Alderwood soils that have a hydrologic classification of Type "C". Please refer to the soils map and descriptions attached later in this report for more details. The proposal is to construct a single-family home with associated utilities. The access to the home will be from 92nd Pl W. The total new and replaced hard surface area will be 3,898 SF. Per Figure 3.1, (Flow Chart for Determining Requirements for Development) from the 2017 Edmonds Stormwater Addendum, Minimum requirements # 1 through 5 shall apply for this project. See section 1.1 for Minimum Requirements Summary included later in this report. Per ECDC 18.30.060.D.5.d Minimum Requirement #5: On -site Stormwater Management, the following roof BMP's must be considered in the following order: Full Dispersion or Full Infiltration, Rain Gardens or Bioretention, Downspout Dispersion Systems, Perforated Stub -out Connections, and Detention Vaults or Pipes. The runoff from the roof will be connected to the detention pipe (32 LF-36" CMP) located within the open area. The overflow from this will be connected to the proposed storm drainage along 92nd Pl W. The runoff from the footing drain will be directed to a gravel trench. Per ECDC 18.30.060.D.5.d Minimum Requirement #5: On - site Stormwater Management, the BMP's for other hard surfaces must be considered in the Insight Engineering Co. - Stormwater Site Plan 07/12/22 -3- following order: Full Dispersion, Permeable Pavement or Rain Gardens, Sheet Flow Dispersion, Detention Vault or Pipe. The proposed driveway will sheet flow to a French drain to be connected to the detention pipe. The overflow will be connected to proposed drainage system along 92nd Pl W. Please refer to Appendix A for tables detailing BMP feasibility. Post - Construction Soil Quality and Depth BMP T.5.13 is proposed to provide onsite stormwater management for the pervious areas of the site. Insight Engineering Co. - Stormwater Site Plan 07/12/22 -4- Flow Chart for Determining Requirements for Development. Ducs the project result in 2,000 square feet, or greater, of new plus replaced hard surface area? OR Dees the land disturbing activity total 7,000 square feet or greater? Yes No Minimum requirements No. I through 5 apply Minimum Requiremcni No. 2 applies Next Questian Dues the project add 5,000 square feet or more of new plus replaced hard surfaces? OR Convert 0.75 acres or more of vegetation to lawn or landscaped areas? OR Convert 2.5 acres or more of native vegetat ion to pasture? No Yes Is this a road related project` All Minimum Requirements apply to the new and replaced hard surfaces and converted vegetation areas_ All Minimum Requirements apply to the new hard surfaces and converted vegetation areas. Yes Insight Engineering Co. - Stormwater Site Plan Yes Does the project add 5,000 square feet or H more of new hard surfaces' Yes Dc) new hard surfaces add 50% or more to the existing hard surtaccs within the project limits? No No No additional requirements. 07/12/22 -5- FIGURE]. VICINITYMAP �' �OwdOjn v waY 8°waOin �aY 8 r °ldO+nWey x c CD � � a � B°h'dOjR �aY a � n d C O N a n N n n m SITE N d Figure 1 -Vicinity May 92nd Lot Edmonds, Washington INSIGHT ENGINEERING CO. SCALE: DATE 7/12/22 JOB#: 21-1173 P.O. Box 1478, Everett, WA 98206 NTS 425-303-9363 Info@insightengineering.net BY FILE NAME: SX 21-1173/doe/Stormwater Site Plan Insight Engineering Co. - Stormwater Site Plan 07/12/22 -6- 1.1 Minimum Requirements Summary MR : Minimum Requirement SWPPP : Stormwater Pollution Prevention Plan MR #1 Stormwater Site Plan Narrative: The Stormwater Site Plan preparation follows the City of Edmonds 2017 requirements and in accordance with DOE's 2014 SWMMWW. Refer to the executive summary within Section 1.0. MR #2 SWPPP Narrative: A SWPPP has been included in the Appendix A under Section 6. MR #3 Water Pollution Source Control for New Development: Source control BMP S411- BMPs for Landscaping and Lawn/Vegetation Management per SWMMWW Volume IV will apply to the site. MR #4 Preservation of Natural Drainage Systems and Outfalls: The runoff from the developed site will be connected to the proposed drainage system along 92nd Pl W. MR #5 Onsite Stormwater Management: Per ECDC 18.30.060.D.5.d Minimum Requirement #5: On -site Stormwater Management, the following roof BMP's must be considered in the following order: Full Dispersion or Full Infiltration, Rain Gardens or Bioretention, Downspout Dispersion Systems, Perforated Stub -out Connections, and Detention Vaults or Pipes. The runoff from the roof will be connected to the detention pipe (32 LF-36" CMP) located within the open area. The overflow from this will be connected to the proposed storm drainage along 92nd PI W. The runoff from the footing drain will be directed to a gravel trench. Per ECDC 18.30.060.D.5.d Minimum Requirement #5: On -site Stormwater Management, the BMP's for other hard surfaces must be considered in the following order: Full Dispersion, Permeable Pavement or Rain Gardens, Sheet Flow Dispersion, Detention Vault or Pipe. The proposed driveway will sheet flow to a French drain to be connected to the detention pipe. The overflow will be connected to proposed drainage system along 92nd PI W. Please refer to Appendix A for tables detailing BMP Insight Engineering Co. - Stormwater Site Plan 07/12/22 -7- feasibility. Post -Construction Soil Quality and Depth BMP T.5.13 is proposed to provide onsite stormwater management for the pervious areas of the site. 2.0 Existing Conditions The proposed project 92nd Lot is located at 21322 92nd Pl W, Edmonds, Washington. More generally, the site is in Section 25, Township 27 North, and Range 3 East of the Willamette Meridian in Snohomish County, Washington. The site area is 0.19 Acres. The site is currently partly developed with two sheds. The remainder of the site exists as lawn and residential landscaping. The site contains one drainage basin that slopes to the west. Please refer to the downstream analysis map for more details. The site is in an erosion hazard area. Per NRCS survey of Snohomish County, the project site contains Alderwood soils that have a hydrologic classification of Type "C". Please refer to the soils map and descriptions attached later in this report for more details. Insight Engineering Co. - Stormwater Site Plan 07/12/22 -8- FIGURE 2. SOIL MAP SOILS LEGEND 6— Alderwood-Urban land complex, 8 to 15 percent slopes IEC INSIGHT ENGINEERING t SCALE: NONE P.O. Box 1478, Everett, WA 98206 425-303-9363 Info@insightengineering.net BY: SX Figure 2 - Soil May 92nd Lot Edmonds, Washington DATE: 7/12/22 1 JOB #: 21-1173 FILE NAME: 21-1173/ doc/Stormwater Site Plan Insight Engineering Co. - Stormwater Site Plan 07/12/22 -9- Snohomish County Area, Washington 6—Alderwood-Urban land complex, 8 to 15 percent slopes Map Unit Setting • National map unit symbol: 2hzn • 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 • Urban land: 25 percent • Minor components: 5 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 • HI - 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: 8 to 15 percent • Depth to restrictive feature: 20 to 40 inches to densic material • 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 supply, 0 to 60 inches: Low (about 3.0 inches) Interpretive groups • Land capability classification (irrigated): None specified • Land capability classification (nonirrigated): 4s • Hydrologic Soil Group: B • Ecological site: F002XA004WA - Puget Lowlands Forest • Forage suitability group: Limited Depth Soils (G002XN302WA) • Other vegetative classification: Limited Depth Soils (G002XN302WA) • Hydric soil rating: No Insight Engineering Co. - Stormwater Site Plan 07/12/22 -10- Minor Components Norma, undrained • Percent of map unit: 5 percent • Landform: Drainageways • Other vegetative classification: Wet Soils (G002XN102WA) • Hydric soil rating: Yes Insight Engineering Co. - Stormwater Site Plan 07/12/22 -11- 3.0 Offsite Analysis A site reconnaissance was performed by Ryan Korte of Insight Engineering on January 19th, 2022 to verify the downstream flow paths and observe any drainage problems downstream of the site. The sky was cloudy with a temperature of 49 degrees. The site area is 0.19 Acres. The site is currently partly developed with two sheds. The remainder of the site exists as lawn and residential landscaping. The site contains one drainage basin that slopes to the west. 3.1 Upstream Analysis Based on the site reconnaissance and the topographic survey of the site, the upstream flows appear to be minimal. Refer to the Downstream Analysis Map attached in the next page for more details. 3.2 Downstream Analysis The runoff from the site flows to the west through the yard and enters the existing drainage system along 93rd Pl W. These flows travel north for 200-ft and combines with 12" CMP drainage system along Bowdoin Way. After flowing northwest for 1,770-ft, it discharges into Shell Creek. This creek meanders northwest in an unrestricted manner before discharging into Puget Sound. This is where the downstream analysis was concluded. Insight Engineering Co. - Stormwater Site Plan 07/12/22 -12- 1 FIGURE 3. DOWNSTREAM ANALYSIS MAP 1 it IEC INSIGHT ENGINEERING CO. P.O. Box 1478, Everett, WA 98206 425-303-9363 Info@insightengineering.net Map taken from City of Edmonds GIs Figure 3 - Downstream Analysis Map-1 92nd Lot Edmonds, Washington SC DATE: 7/12/22 I JOB #: 21-1173 NONE I I FILE NAME: BY: SX 21-1173/doc/Stormwater Site Plan 0 Insight Engineering Co. - Stormwater Site Plan 07/12/22 -13- FIGURE 4. DOWNSTREAM ANALYSIS MAP -2 IEC INSIGHT ENGINEERING CO. P.O. Box 1478, Everett, WA 98206 425-303-9363 Info@insightengineering.net Figure 4 - Downstream Analysis Map-2 92nd Lot Edmonds, Washington SC DATE: 7/12/22 I JOB #: 21-1173 NONE I I FILE NAME: BY: SX 21-1173/doc/Stormwater Site Plan Insight Engineering Co. - Stormwater Site Plan 07/12/22 -14- 4.0 Developed Conditions The proposed project 92nd Lot is located at 21322 92nd PI W, Edmonds, Washington. More generally, the site is in Section 25, Township 27 North, and Range 3 East of the Willamette Meridian in Snohomish County, Washington. Per NRCS survey of Snohomish County, the project site contains Alderwood soils that have a hydrologic classification of Type "C". The proposal is to construct a single-family home with associated utilities. The access to the home will be from 92nd PI W. The total new and replaced hard surface area will be 3,898 SF. Per ECDC 18.30.060.D.5.d Minimum Requirement #5: On -site Stormwater Management, the following roof BMP's must be considered in the following order: Full Dispersion or Full Infiltration, Rain Gardens or Bioretention, Downspout Dispersion Systems, Perforated Stub -out Connections, and Detention Vaults or Pipes. The runoff from the roof will be connected to the detention pipe (32 LF-36" CMP) located within the open area. The overflow from this will be connected to the proposed storm drainage along 92nd PI W. The runoff from the footing drain will be directed to a gravel trench. Per ECDC 18.30.060.D.5.d Minimum Requirement #5: On - site Stormwater Management, the BMP's for other hard surfaces must be considered in the following order: Full Dispersion, Permeable Pavement or Rain Gardens, Sheet Flow Dispersion, Detention Vault or Pipe. The proposed driveway will sheet flow to a French drain to be connected to the detention pipe. The overflow will be connected to proposed drainage system along 92nd PI W. Please refer to Appendix A for tables detailing BMP feasibility. Post - Construction Soil Quality and Depth BMP T.5.13 is proposed to provide onsite stormwater management for the pervious areas of the site. Insight Engineering Co. - Stormwater Site Plan 07/12/22 -15- 5.0 Site Hydraulic Conditions to Determine Flow Control Constraints Per NRCS survey of Snohomish County, the project site contains Alderwood soils that have a hydrologic classification of Type "C". Site Area= 0.19 Acres (8,291 SF) Clearing Area= 0.19 Acres (8,100 SF) 5.1 Existing Basin Summary Study Area = 0.19 Acres Total Area included in the Analysis = 0.19 Acres 5.2 Developed Basin Summary Study Area = 0.19 Acres Total Area included in the Analysis = 0.19 Acres Impervious: Driveway and walkway = 0.02 Acres (1,083 SF) Roof = 0.07 Acres (2,815 SF) Total Impervious = 0.09 Acres (3,898 SF) Pervious: (Lawn) = 0.10 Acres DETENTION PIPE SIZING CALCULATIONS Per the City of Edmonds Stormwater Manual Addendum dated June 2017, detention pipes being used for MR5 that are not subject to MR7 may use a simplified sizing method found in section 6.3.1. For 36-inch pipes: L = 0.008*A A= Total Impervious = 3,898 SF L = 0.008*3,898 SF L = 31.18-feet A 32-foot pipe will be used. where A= contributing area Insight Engineering Co. - Stormwater Site Plan 07/12/22 -16- 8 \ V \ °LACE 145001 I FOUND REBAR W/CAP STAMPED '22969" S1- SEpyER I 7- — , I/ 0.1 '(W) & 0.1 '(S) OF CORNER. FENCE l FOUND 1-1/4" IRON COR. 0.12'(N) & 0.5'(W) OF CORNER I; / PIPE AT CORNER I I I loi 3' 0. 1 I— — I N 90'00'00" W 126.00' 1 I \ I \ \ \,, _ \ \\ \ 10 P I I \ I I I \ I I I I LOT 6 I I 8,291 SF 00479J00000600 I I I \ I I I \ I I \ \ I I \ \ I I \ \ I I \ \ I I \ I I I \ I I I I I I I I j I j I I / I I I I I I I I I I I I I \\ I / / 10" �4. DEC N �84-38'4�0'E VIF UND 1-1/4" IRq N PIPE 0.1 '(N) OF CO ER. FENCE Of(W) AT CORNER IITII ITY 17AC9'Ad7NT PPP EX. SHED (MR) T 5.4' Ll _ SS EX. pp \ SHED / I R ��h�ti• / l EX. BASIN AREA l ll I (0.19 A / 1 / I I I / FOUND REBAR W/CAP l , STAMPED '22969" I I w 0.2'(S) OF CORNER \ I I o I 50 \ I I I I I c c I I I \ 5 " COMMON TRENCH F01 POWER, CABLE, P 0A 1 II I \ I \ , I I \ I I \\ I \ I \ 96.42' I \ \ I I \\ \ FOUND REBAR W/ILLEGIBLE 7 I y CAP 0.1 (N) OF CORN LOT 5 HULL & ANDERSON ADDITION 1/nl 10 011 Al POWER BE i SCALE: 1" = 10' 0 5 10 EXISTING BASIN MAP FOUND REBAR W/CAP STAMPED '22969" SS C SEWER I 0.1 '(W) /& 0.1 '(S) OF CORNER. FENCE I FOUND 1-1/4" IRON COR. 0.12'(N) & 0.5'(W) OF CORNER 1 / � PIPE AT CORNER I / I 0.3' 0. N 90'00'00" W 126.00' SS i \ \ I m SS 0% ---lep 71 \ I 45.0' / l a a 6 loll P EX. ppp �� \ a \ I I , PROPOSED DRIVE W' Y 115. BSBL I \ I II l� l ° DV BASINAREA II I I I II l l (0.19 AQ LOT 6 / ° , ° \ 8,291 SF 00479300000600 / / PROPOSED 1-1/2" \ I I II \ POLY WATERLINE PROPOSED HOUSE WITH TRACER WIRE (2,815 SF ROOF) F / /l °LACE 1 I I \ PROPOSED 1" \ \ / a WATER METER I \ I 145001 I W W W— I \ UFF=395.00 I / FOUND REBAR W/CAP I I I o I LFF=385.50 I o I � � / v STAMPED '22969" o I z GA GAS GAS GAS I I ¢ I w I I w 0.2'(S) OF CORNER � w I I I o �00 50 I \ I c I c 00 18.0' 5 " COMMON TRENCH FOI z UTILITY EASEMENT / \ I \\ , + �s POWER, CABLE, P 0A / / I 25.0' BSBL \ I p II P \ P P EX. SHED I I \ \ / �i 1 D' 1 4.0, I r \ I M 96.42 I \ I IR DEC I \ FOUND REBAR W/ILLEGIBLE N g4'38'40" E I 8' 7 1y CAP 0.1'(N) OF CORN PO BSCALE: 1 " = 10, i 0 5 10 I F UND 1-1/4" IR N PIPE 0.1 '(N) OF T 4' LOT 5 CO ER. FENCE 0+(W) AT CORNER HULL & ANDERSON ADDITION DE VEL OPED BA SIN MA P I � I I ITII ITV RdCFMI-NT PFR / vni 10 nn Al 6.0 Appendix A. Minimum Requirement #5 Bmp Feasibility B. Stormwater Pollution Prevention Plan C. Geotech Report Insight Engineering Co. - Stormwater Site Plan 07/12/22 -17- A. MINIMUM REQUIREMENT #5 BMP FEASIBILITY Roof BMPs (List #1) Per ECDC 18.30.060.D.5.d Minimum Requirement #5 BMP Feasibility BMP Feasibility Conditions T5.30 Full Dispersion Infeasible Full Dispersion is infeasible because of the required flow path and preserved vegetation area could not be achieved. T5. l0A Downspout Full Infeasible Downspout Full Infiltration is infeasible as it is in Infiltration an erosion hazard area and infiltration is not recommended per Geotech report attached under Section 6 Appendix C. According to the Geotech Report infiltration is infeasible due to the presence of fill, restrictive layers and variable slopes that would direct runoff to the west and potentially into the adjacent residences. T5.14A Rain Gardens Infeasible According to the Geotech Report attached under Section 6 Appendix C infiltration is not recommended due to the presence of fill, restrictive layers and variable slopes that would direct runoff to the west and potentially into the adjacent residences. T5.1 OB Downspout Infeasible The minimum recommended flow path could not Dispersion be achieved. T5.1 OC Perforated Stub- Infeasible Per design criteria under City Addendum checklist - out Connections 15," The location of the connection allows the maximum amount of runoff to infiltrate into the ground (ideally a dry, relatively well -drained location). " is required. But infiltration is not recommended by the Geotech. Detention Pipe or Vault Feasible Insight Engineering Co. - Stormwater Site Plan 07/12/22 -18- Other Hard Surface BMPs (List #1) : Proposed Driveway Per ECDC 18.30.060.D.5.d Minimum Requirement #5 BMP Feasibility BMP Feasibility Conditions T5.30 Full Dispersion Infeasible Full Dispersion is infeasible because of the lack of preserved vegetated could not be achieved for this MR zoned project. Permeable Pavement or Infeasible Infiltration is infeasible as it is in an erosion hazard Rain Gardens area and infiltration is not recommended per Geotech report attached under Section 6 Appendix C. According to the Geotech Report infiltration is infeasible due to the presence of fill, restrictive layers and variable slopes that would direct runoff to the west and potentially into the adjacent residences. Also it could compromise the daylight basement floor of the proposed residence. T5.12 Sheet Flow Infeasible The minimum recommended flow path could not Dispersion be achieved. Detention Pipe or Vault Feasible Insight Engineering Co. - Stormwater Site Plan 07/12/22 -19- B. STORMWATER POLLUTION PREVENTION PLAN This Stormwater Pollution Prevention Plan (SWPPP) has been prepared as part of the Construction stormwater permit requirements for the project 92nd Lot in located at 21322 92nd Pl W, Edmonds, Washington. It is located in Section 25, Township 27 North, and Range 3 East of the Willamette Meridian in Snohomish County, Washington. The site area is 0.19 Acres. The site is currently partly developed with two sheds. The remainder of the site exists as lawn and residential landscaping. The site contains one drainage basin that slopes to the west. Please refer to the downstream analysis map for more details. The site is in an erosion hazard area. Per NRCS survey of Snohomish County, the project site contains Alderwood soils that have a hydrologic classification of Type "C". Please refer to the soils map and descriptions attached later in this report for more details. The proposal is to construct a single-family home with associated utilities. The access to the home will be from 92nd Pl W. The total new and replaced hard surface area will be 3,898 SF. Per Figure 3.1, (Flow Chart for Determining Requirements for Development) from the 2017 Edmonds Stormwater Addendum, Minimum requirements #1 through 5 shall apply for this project. See section 1.1 for Minimum Requirements Summary included later in this report. Per ECDC 18.30.060.D.5.d Minimum Requirement #5: On -site Stormwater Management, the following roof BMP's must be considered in the following order: Full Dispersion or Full Infiltration, Rain Gardens or Bioretention, Downspout Dispersion Systems, Perforated Stub -out Connections, and Detention Vaults or Pipes. The runoff from the roof will be connected to the detention pipe (32 LF-36" CMP) located within the open area. The overflow from this will be connected to the proposed storm drainage along 92nd PI W. The runoff from the footing drain will be directed to a gravel trench. Per ECDC 18.30.060.D.5.d Minimum Requirement #5: On - site Stormwater Management, the BMP's for other hard surfaces must be considered in the following order: Full Dispersion, Permeable Pavement or Rain Gardens, Sheet Flow Dispersion, Detention Vault or Pipe. The proposed driveway will sheet flow to a French drain to be connected to the detention pipe. The overflow will be connected to proposed drainage system Insight Engineering Co. - Stormwater Site Plan 07/12/22 -20- along 92nd Pl W. Please refer to Appendix A for tables detailing BMP feasibility. Post - Construction Soil Quality and Depth BMP T.5.13 is proposed to provide onsite stormwater management for the pervious areas of the site. The purpose of this SWPPP is to describe the proposed construction activities and all temporary and permanent erosion and sediment control (TESC) measures, pollution prevention measures, inspection/monitoring activities, and recordkeeping that will be implemented during the proposed construction project. The objectives of the SWPPP are to: 1. Implement Best Management Practices (BMPs) to prevent erosion and sedimentation, and to identify, reduce, eliminate or prevent stormwater contamination and water pollution from construction activity. 2. Prevent violations of surface water quality, ground water quality, or sediment management standards. 3. Prevent, during the construction phase, adverse water quality impacts including impacts on beneficial uses of the receiving water by controlling peak flow rates and volumes of stormwater runoff at the Permittee's outfalls and downstream of the outfalls. This SWPPP was prepared using the Ecology SWPPP Template. This SWPPP was prepared based on the requirements set forth in the Construction Stormwater General Permit and in the Stormwater Management Manual for Western Washington (SWMMWW 2005). Insight Engineering Co. - Stormwater Site Plan 07/12/22 -21- The 13 BMP Elements 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. 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, and wheel washing, street sweeping, and street cleaning shall be employed to prevent sediment from entering state waters. Install the temporary construction entrance, according to the approved construction plans, prior to any clearing or grading activities. Maintain until the access road is paved. Element #3 — Control Flow Rates In order to protect the properties and waterways downstream of the project site, stormwater discharges from the site will be controlled. In general, discharge rates of stormwater from the site will be controlled where increases in impervious area or soil compaction during construction could lead to downstream erosion, or where necessary to meet local agency stormwater discharge requirements. Element #4 — Install Sediment Controls Install silt fencing, according to the approved plans, prior to any clearing or grading activities. Maintain until all construction activities are completed. Install catch basin filters, according to the approved construction plans, as catch basins become operable. Maintain until all construction activities are completed. Element #5 — Stabilize Soils Exposed and un-worked soils shall be stabilized with the application of effective BMPs to prevent erosion throughout the life of the project. Apply temporary hydro -seed to exposed and un-worked soils, according to the approved construction plans, as needed to prevent erosion during site grading. Apply permanent hydro - seed to areas at final grade as site grading is completed. Apply mulching to exposed and un-worked soils, according to the approved construction plans, as needed to prevent erosion during site grading. Maintain until site grading is completed and permanent hydro -seed is applied. Cover stockpiles with plastic sheeting, according to the approved construction plans, as needed to prevent erosion during site grading. Maintain until stockpiles are removed from site. Element #6 — Protect Slopes All cut and fill slopes will be designed, constructed, and protected in a manner than minimizes erosion. Slopes will be protected by plastic covering, mulching, and existing vegetation. Insight Engineering Co. - Stormwater Site Plan 07/12/22 -22- Element #7 — Protect Drain Inlets All storm drain inlets and culverts made operable during construction shall be protected to prevent unfiltered or untreated water from entering the drainage conveyance system. However, the priority is to keep all access roads clean of sediment and keep street wash water separate from entering storm drains until treatment can be provided. Inlet protection is proposed for all catch basins with open grates. Element #8 — Stabilize Channels and Outlets Where site runoff is to be conveyed in channels or discharged to a stream or some other natural drainage point, efforts will be taken to prevent downstream erosion. No channels are proposed, and no outfalls are proposed that would require stabilization. All on -site flows will be directed to a closed conveyance system Element #9 — Control Pollutants All pollutants, including waste materials and demolition debris, that occur onsite shall be handled and disposed of in a manner that does not cause contamination of stormwater. Good housekeeping and preventative measures will be taken to ensure that the site will be kept clean, well -organized, and free of debris. Element #10 — Control Dewatering There will be no dewatering expected as part of this proposal. If dewatering occurs, Baker tanks will be used for dewatering. Element #11— Maintain BMPs All temporary and permanent erosion and sediment control BMPs shall be maintained and repaired as needed to assure continued performance of their intended function. Maintenance and repair shall be conducted in accordance with each particular BMP's specifications. Visual monitoring of the BMPs will be conducted at least once every calendar week and within 24 hours of any rainfall event that causes a discharge from the site. If the site becomes inactive, and is temporarily stabilized, the inspection frequency will be reduced to once every month. All temporary erosion and sediment control BMPs shall be removed within 30 days after the final site stabilization is achieved or after the temporary BMPs are no longer needed. Trapped sediment shall be removed or stabilized on site. Disturbed soil resulting from removal of BMPs or vegetation shall be permanently stabilized. Insight Engineering Co. - Stormwater Site Plan 07/12/22 -23- Element #12 — Manage the Project Erosion and sediment control BMPs for this project have been designed based on the following principles: Design the project to fit the existing topography, soils, and drainage patterns; Emphasize erosion control rather than sediment control; Minimize the extent and duration of the area exposed; Keep runoff velocities low; Retain sediment on site; Thoroughly monitor site and maintain all ESC measures and Schedule major earthwork during the dry season. 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 On -site Stormwater Management BMPs for Runoff from Roofs and Other Hard Surfaces Protect onsite stormwater bmp's from roofs and other hard surfaces: stockpile existing topsoil per soil amendment calculation per BMP T5.13. Cover stockpile with plastic covering to prevent erosion. High visibility fence shall be placed around the area proposed for gravel trench to prevent compaction of the soil. (BMP C 103) The area of the gravel trench shall be over excavated and obtain geotechnical approval of the subgrade condition and infiltration rate prior to installation. Insight Engineering Co. - Stormwater Site Plan 07/12/22 -24- C. GEOTECH REPORT Insight Engineering Co. - Stormwater Site Plan 07/12/22 -25- COBALT G E 0 S C I E N C E S March 2, 2022 Todd Echelbarger toddoechelbarger.com RE: Limited Geotechnical Evaluation Proposed Development 21322 92nd Place West Lynnwood, Washington Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, Washington 98028 In accordance with your authorization, Cobalt Geosciences, LLC has prepared this letter to discuss the results of our geotechnical evaluation at the referenced site. The purpose of our evaluation was to provide recommendations for foundation design, grading, and earthwork. Site Description The site is located at 21322 92nd Place West in Edmonds, Washington. The site consists of one irregularly shaped parcel (No. 00479300000600) with a total area of o.19 acres. The property is undeveloped and vegetated with grasses, bushes, understory, and sparse trees. The site slopes downward from east to west at magnitudes of 5 to 12 percent and relief of about 15 feet. There is a short fill slope near the west property line. This slope is 40 percent in magnitude and about 6 feet in height. The site contains erosion hazard areas per City of Edmonds mapping. The site is bordered to the north, south, and west by residences, and to the east by 92nd Place West. There is a sewer line near the west property line. The proposed development includes construction of a new residence with likely daylight basement and driveway. Foundation loads are expected to be light and grading may include cuts of 8 feet or less. We have reviewed the civil plans that show that stormwater will be routed to City infrastructure. Area Geology The Geologic Map of the Edmonds East Quadrangle, indicates that the site is underlain by Vashon Glacial Till. Vashon Glacial Till includes mixtures of silt, sand, gravel, and clay in a nonsorted matrix or diamict. These deposits are typically dense below a weathered zone. These materials are typically impermeable. Soil & Groundwater Conditions As part of our evaluation, we excavated two test pits within the property, where accessible. The explorations encountered approximately 6 inches of grass and topsoil underlain by approximately 2.5 to 5 feet of loose to medium dense, silty -fine to medium grained sand trace to with gravel (Fill). This layer was underlain by 1.5 to 4.5 feet of loose to medium dense, silty -fine www.cobaltgeo.com (2o6) 331-1097 March 2, 2022 Page 2 of 12 Geotechnical Evaluation to medium grained sand with gravel (Weathered Glacial Till). These materials were underlain by dense to very dense, silty -fine to medium grained sand trace to with gravel (Vachon Glacial Till), which continued to the termination depths of the explorations. Groundwater was not encountered in the test pits during the exploration work. Cemented soils were observed at depth in TP-2 and we anticipate that perched groundwater will develop on the denser till at shallow depths during the wet season. Water table elevations often fluctuate over time. The groundwater level will depend on a variety of factors that may include seasonal precipitation, irrigation, land use, climatic conditions and soil permeability. Water levels at the time of the field investigation may be different from those encountered during the construction phase of the project. Erosion Hazard The Natural Resources Conservation Services (NRCS) maps for Snohomish County indicate that the property is underlain by Alderwood-Urban land complex (8 to 15 percent slopes). These soils can have a slight to moderate erosion potential when exposed. It is our opinion that soil erosion potential at this project site can be reduced through landscaping and surface water runoff control. Typically, erosion of exposed soils will be most noticeable during periods of rainfall and may be controlled by the use of normal temporary erosion control measures, such as silt fences, hay bales, mulching, control ditches and diversion trenches. The typical wet weather season, with regard to site grading, is from October 31st to April 1st. Erosion control measures should be in place before the onset of wet weather. The site may be mapped within an erosion hazard area per City of Edmonds GIS. Relevant code items with discussion for erosion hazards are as follows: 23.80.o6o Development standards — General requirements. A. Alterations of geologically hazardous areas or associated buffers may only occur for activities that: 1. Will not increase the threat of the geological hazard to adjacent properties beyond predevelopment conditions; 2. Will not adversely impact other critical areas; 3. Are designed so that the hazard to the project is eliminated or mitigated to a level equal to or less than predevelopment conditions; and 4. Are certified as safe as designed and under anticipated conditions by a qualified engineer or geologist, licensed in the state of Washington. The proposed residence will not increase the threat of geologic hazards on adjacent properties, will not impact other critical areas, and are safe as designed under anticipated conditions. The level of risk due to the proposed development will be the same or lower than what is currently present provided temporary and permanent runoff is fully controlled. 23.80.07o Development standards — Specific hazards. A. Erosion and Landslide Hazard Areas. Activities on sites containing erosion or landslide hazards shall meet the requirements of ECDC 23.80.o6o, Development standards — General requirements, and the specific following requirements: www.cobaltgeo.com (2o6) 331-1097 March 2, 2022 Page 3 of 12 Geotechnical Evaluation 1. Minimum Building Setback. The minimum setback shall be the distance required to ensure the proposed structure will not be at risk from landslides for the life of the structure, considered to be 120 years, and will not cause an increased risk of landslides taking place on or off the site. A setback shall be established from all edges of landslide hazard areas. The size of the setback shall be determined by the director consistent with recommendations provided in the geotechnical report to eliminate or minimize the risk of property damage, death, or injury resulting from landslides caused in whole or part by the development, based upon review of and concurrence with a critical areas report prepared by a qualified professional; No building setback is warranted from the erosion hazard areas. These areas have very low risk and can be managed with proper TESC and BMPs. 2. Buffer Requirements. A buffer may be established with specific requirements and limitations, including but not limited to, drainage, grading, irrigation, and vegetation. Buffer requirements shall be determined by the director consistent with recommendations provided in the geotechnical report to eliminate or minimize the risk of property damage, death, or injury resulting from landslides caused in whole or part by activities within the buffer area, based upon review of and concurrence with a critical areas report prepared by a qualified professional; No buffer is required from the erosion hazard areas. We recommend that all site soils be vegetated or protected from erosion through placement of surface landscaping materials/hardscapes. 3. Alterations. Alterations of an erosion or landslide hazard area, minimum building setback and/or buffer may only occur for activities for which a hazards analysis is submitted and certifies that: a. The alteration will not increase surface water discharge or sedimentation to adjacent properties beyond predevelopment conditions; b. The alteration will not decrease slope stability on adjacent properties; and c. Such alterations will not adversely impact other critical areas; Provided earthwork activities are performed in accordance with the approved plans, all runoff is fully control, and periodic geotechnical oversight is performed, the development will not decrease slope stability on adjacent properties, will not increase surface water discharge or sedimentation beyond current levels, and will not impact other critical areas. Temporary and permanent erosion and sediment control devices should be in at all times during construction. 4. Design Standards within Erosion and Landslide Hazard Areas. Development within an erosion or landslide hazard area and/or buffer shall be designed to meet the following basic requirements unless it can be demonstrated that an alternative design that deviates from one or more of these standards provides greater long-term slope stability while meeting all other provisions of this title. The requirement for long-term slope stability shall exclude designs that require regular and periodic maintenance to maintain their level of function. The basic development design standards are: a. The proposed development shall not decrease the factor of safety for landslide occurrences below the limits of 1.5 for static conditions and 1.2 for dynamic conditions. If stability at the proposed development site is below these limits, the proposed development shall provide practicable approaches to reduce risk to human safety and improve the factor of safety for landsliding. In no case shall the existing factor of safety be reduced for the subject property or adjacent properties; www.cobaltgeo.com (2o6) 331-1097 March 2, 2022 Page 4 of 12 Geotechnical Evaluation b. Structures and improvements shall be clustered to avoid geologically hazardous areas and other critical areas; c. Structures and improvements shall minimize alterations to the natural contour of the slope, and foundations shall be tiered where possible to conform to existing topography; d. Structures and improvements shall be located to preserve the most critical portion of the site and its natural landforms and vegetation; e. The proposed development shall not result in greater risk or a need for increased buffers on neighboring properties; f. The use of retaining walls that allow the maintenance of existing natural slope area is preferred over graded artificial slopes; and g. Development shall be designed to minimize impervious lot coverage; The construction will not require grading or disturbance of any slope/landslide hazard areas: however, the work may occur within the locally mapped erosion hazard areas. This work can be completed without affecting the hazard provided work is completed in a timely manner and proper erosion control measures are in place during construction and permanent landscaping is in place upon completion of the construction. The construction does not result in greater risk to the property or adjacent areas. Performed with proper erosion control and engineering controls, the proposed development does not pose a risk to critical areas or the need to increase buffers on adjacent properties. 5. Vegetation Retention. Unless otherwise provided or as part of an approved alteration, removal of vegetation from an erosion or landslide hazard area or related buffer shall be prohibited; 6. Seasonal Restriction. Clearing shall be allowed only from May 1st to October ist of each year; provided, that the director may extend or shorten the dry season on a case -by -case basis depending on actual weather conditions, except that timber harvest, not including brush clearing or stump removal, may be allowed pursuant to an approved forest practice permit issued by the city of Edmonds or the Washington State Department of Natural Resources; 7. Point Discharges. Point discharges from surface water facilities and roof drains onto or upstream from an erosion or landslide hazard area shall be prohibited except as follows: a. Conveyed via continuous storm pipe downslope to a point where there are no erosion hazard areas downstream from the discharge; b. Discharged at flow durations matching predeveloped conditions, with adequate energy dissipation, into existing channels that previously conveyed storm water runoff in the predeveloped state; or c. Dispersed discharge upslope of the steep slope onto a low -gradient, undisturbed buffer demonstrated to be adequate to infiltrate all surface and storm water runoff, and where it can be demonstrated that such discharge will not increase the saturation of the slope; and We concur with the above code items and have no additional comments at this time. www.cobaltgeo.com (2o6) 331-1097 March 2, 2022 Page 5 of 12 Geotechnical Evaluation Seismic Hazard The overall subsurface profile corresponds to a Site Class D as defined by Table 1613.5.2 of the International Building Code (IBC). A Site Class D applies to an overall profile consisting of stiff/medium dense soils within the upper ioo feet. We referenced the U.S. Geological Survey (USGS) Earthquake Hazards Program Website to obtain values for Ss, S,, FQ, and F,,. The USGS website includes the most updated published data on seismic conditions. The following tables provide seismic parameters from the USGS web site with referenced parameters from ASCE 7-io and 7-16. Seismic Design Parameters (ASCE 7-10) Site Spectral Spectral Site Design Spectral Design Class Acceleration Acceleration Coefficients Response Parameters PGA at 0.2 sec. (g) at i.o sec. (g) Fa Fv SDS SD, D 1.269 0.496 1.0 1.504 o.846 0.498 0.514 Seismic Design Parameters (ASCE 7-16) Site Spectral Spectral Site Design Spectral Design Class Acceleration Acceleration Coefficients Response Parameters PGA at 0.2 sec. (g) at l.o sec. (g) Fa Fv SDS SD, D 1.283 0.451 1.0 Null o.856 Null 0.546 Additional seismic considerations include liquefaction potential and amplification of ground motions by soft/loose soil deposits. The liquefaction potential is highest for loose sand with a high groundwater table. The site has a low likelihood of liquefaction. For items listed as "Null' see Section 11.4.8 of the ASCE. Conclusions and Recommendations General The site is underlain by fill and weathered and unweathered Vashon Glacial Till. The proposed residential structure may be supported on a shallow foundation system bearing on medium dense or firmer native soils or on structural fill placed on the native soils. Local overexcavation or recompaction of loose weathered native soils may be necessary depending on the proposed elevations and locations of the new footings. Overexcavation of any fill is required. Note that the fill is mostly present near the west property line and likely decreases to nil to the east, where the residence will be located. www.cobaltgeo.com (2o6) 331-1097 March 2, 2022 Page 6 of 12 Geotechnical Evaluation Infiltration is not feasible due to the presence of fill, restrictive layers, and variable slopes that would direct runoff to the west and potentially into the adjacent residences. We anticipate that direct or perforated connection to City infrastructure will be utilized. We can provide additional information if other systems are being considered or proposed. Note a drywell for footing drains west of the residence is suitable. Site Preparation Trees, shrubs and other vegetation should be removed prior to stripping of surficial organic -rich soil and fill. Based on observations from the site investigation program, it is anticipated that the stripping depth will be 6 to 12 inches. Deeper excavations will be necessary in areas of existing foundation systems and in any areas underlain by undocumented fill. The native soils consist of silty -sand with gravel. Most of the native soils may be used as structural fill provided they achieve compaction requirements and are within 3 percent of the optimum moisture. Some of these soils may only be suitable for use as fill during the summer months, as they will be above the optimum moisture levels in their current state. These soils are variably moisture sensitive and may degrade during periods of wet weather and under equipment traffic. Imported structural fill should consist of a sand and gravel mixture with a maximum grain size of 3 inches and less than 5 percent fines (material passing the U.S. Standard No. 200 Sieve). Structural fill should be placed in maximum lift thicknesses of 12 inches and should be compacted to a minimum of 95 percent of the modified proctor maximum dry density, as determined by the ASTM D 1557 test method. Temporary Excavations Based on our understanding of the project, we anticipate that the grading could include local cuts on the order of approximately 8 feet or less for foundation and utility placement. Temporary excavations should be sloped no steeper than 1.5H:1V (Horizontal:Vertical) in loose native soils and fill, 1H:1V in medium dense native soils and 3/4H:1V in dense to very dense native soils if encountered in deeper excavations. If an excavation is subject to heavy vibration or surcharge loads, we recommend that the excavations be sloped no steeper than 2H:1V, where room permits. Temporary cuts should be in accordance with the Washington Administrative Code (WAC) Part N, Excavation, Trenching, and Shoring. Temporary slopes should be visually inspected daily by a qualified person during construction activities and the inspections should be documented in daily reports. The contractor is responsible for maintaining the stability of the temporary cut slopes and reducing slope erosion during construction. Temporary cut slopes should be covered with visqueen to help reduce erosion during wet weather, and the slopes should be closely monitored until the permanent retaining systems or slope configurations are complete. Materials should not be stored or equipment operated within 10 feet of the top of any temporary cut slope. Soil conditions may not be completely known from the geotechnical investigation. In the case of temporary cuts, the existing soil conditions may not be completely revealed until the excavation work exposes the soil. Typically, as excavation work progresses the maximum inclination of temporary slopes will need to be re-evaluated by the geotechnical engineer so that supplemental recommendations can be made. Soil and groundwater conditions can be highly variable. www.cobaltgeo.com (2o6) 331-1097 March 2, 2022 Page 7 of 12 Geotechnical Evaluation Scheduling for soil work will need to be adjustable, to deal with unanticipated conditions, so that the project can proceed and required deadlines can be met. If any variations or undesirable conditions are encountered during construction, we should be notified so that supplemental recommendations can be made. If room constraints or groundwater conditions do not permit temporary slopes to be cut to the maximum angles allowed by the WAC, temporary shoring systems may be required. The contractor should be responsible for developing temporary shoring systems, if needed. We recommend that Cobalt Geosciences and the project structural engineer review temporary shoring designs prior to installation, to verify the suitability of the proposed systems. Foundation Design The proposed residence may be supported on a shallow spread footing foundation system bearing on undisturbed dense or firmer native soils or on properly compacted structural fill placed on the suitable native soils. Any undocumented fill and/or loose native soils should be removed and replaced with structural fill below foundation elements. Structural fill below footings should consist of clean angular rock 5/8 to 4 inches in size. It may be feasible to recompact loose outwash soils in lieu of overexcavation and replacement. We should verify soil conditions during foundation excavation work. For shallow foundation support, we recommend widths of at least 16 and 24 inches, respectively, for continuous wall and isolated column footings supporting the proposed structure. Provided that the footings are supported as recommended above, a net allowable bearing pressure of 2,500 pounds per square foot (psf) may be used for design. A 1/3 increase in the above value may be used for short duration loads, such as those imposed by wind and seismic events. Structural fill placed on bearing, native subgrade should be compacted to at least 95 percent of the maximum dry density based on ASTM Test Method D1557. Footing excavations should be inspected to verify that the foundations will bear on suitable material. Exterior footings should have a minimum depth of 18 inches below pad subgrade (soil grade) or adjacent exterior grade, whichever is lower. Interior footings should have a minimum depth of 12 inches below pad subgrade (soil grade) or adjacent exterior grade, whichever is lower. If constructed as recommended, the total foundation settlement is not expected to exceed 1 inch. Differential settlement, along a 25-foot exterior wall footing, or between adjoining column footings, should be less than 1/2 inch. This translates to an angular distortion of 0.002. Most settlement is expected to occur during construction, as the loads are applied. However, additional post -construction settlement may occur if the foundation soils are flooded or saturated. All footing excavations should be observed by a qualified geotechnical consultant. Resistance to lateral footing displacement can be determined using an allowable friction factor of 0.4o acting between the base of foundations and the supporting subgrades. Lateral resistance for footings can also be developed using an allowable equivalent fluid passive pressure of 225 pounds per cubic foot (pcf) acting against the appropriate vertical footing faces (neglect the upper 12 inches below grade in exterior areas). The frictional and passive resistance of the soil may be combined without reduction in determining the total lateral resistance. Care should be taken to prevent wetting or drying of the bearing materials during construction. Any extremely wet or dry materials, or any loose or disturbed materials at the bottom of the footing excavations, should be removed prior to placing concrete. The potential for wetting or drying of the bearing materials can be reduced by pouring concrete as soon as possible after www.cobaltgeo.com (2o6) 331-1097 March 2, 2022 Page 8 of 12 Geotechnical Evaluation completing the footing excavation and evaluating the bearing surface by the geotechnical engineer or his representative. Concrete Retaining Walls The following table, titled Wall Design Criteria, presents the recommended soil related design parameters for retaining walls with a level backslope. Contact Cobalt if an alternate retaining wall system is used. This has been included for basement walls. Wall Design Criteria "At -rest" Conditions (Lateral Earth Pressure — EFD+) 55 pcf (Equivalent Fluid Density) "Active" Conditions (Lateral Earth Pressure — EFD+) 35 pcf (Equivalent Fluid Density) Seismic Increase for "At -rest" Conditions (Lateral Earth Pressure) 21H* (Uniform Distribution) 1 in 2,500 year event Seismic Increase for "At -rest" Conditions (Lateral Earth Pressure) 14H* (Uniform Distribution) 1 in 500 year event Seismic Increase for "Active" Conditions (Lateral Earth Pressure) 7H* (Uniform Distribution) Passive Earth Pressure on Low Side of Wall (Allowable, includes F.S. = 1.5) Neglect upper 2 feet, then 250 pcf EFD+ Soil -Footing Coefficient of Sliding Friction (Allowable; includes F.S. = 1.5) 0.40 *H is the height of the wall; Increase based on one in 500 year seismic event (10 percent probability of being exceeded in 50 years), +EFD — Equivalent Fluid Density The stated lateral earth pressures do not include the effects of hydrostatic pressure generated by water accumulation behind the retaining walls. Uniform horizontal lateral active and at -rest pressures on the retaining walls from vertical surcharges behind the wall may be calculated using active and at -rest lateral earth pressure coefficients of 0.3 and 0.5, respectively. A soil unit weight Of 125 pcf may be used to calculate vertical earth surcharges. To reduce the potential for the buildup of water pressure against the walls, continuous footing drains (with cleanouts) should be provided at the bases of the walls. The footing drains should consist of a minimum 4-inch diameter perforated pipe, sloped to drain, with perforations placed down and enveloped by a minimum 6 inches of pea gravel in all directions. The backfill adjacent to and extending a lateral distance behind the walls at least 2 feet should consist of free -draining granular material. All free draining backfill should contain less than 3 percent fines (passing the U.S. Standard No. 200 Sieve) based upon the fraction passing the U.S. Standard No. 4 Sieve with at least 30 percent of the material being retained on the U.S. Standard No. 4 Sieve. The primary purpose of the free -draining material is the reduction of hydrostatic pressure. Some potential for the moisture to contact the back face of the wall may exist, even with treatment, which may require that more extensive waterproofing be specified for walls, which require interior moisture sensitive finishes. www.cobaltgeo.com (2o6) 331-1097 March 2, 2022 Page 9 of 12 Geotechnical Evaluation We recommend that the backfill be compacted to at least go percent of the maximum dry density based on ASTM Test Method D1557. In place density tests should be performed to verify adequate compaction. Soil compactors place transient surcharges on the backfill. Consequently, only light hand operated equipment is recommended within 3 feet of walls so that excessive stress is not imposed on the walls. Slab -on -Grade We recommend that the upper 18 inches of the existing native soils within slab areas be re - compacted to at least 95 percent of the modified proctor (ASTM D1557 Test Method). Often, a vapor barrier is considered below concrete slab areas. However, the usage of a vapor barrier could result in curling of the concrete slab at joints. Floor covers sensitive to moisture typically requires the usage of a vapor barrier. A materials or structural engineer should be consulted regarding the detailing of the vapor barrier below concrete slabs. Exterior slabs typically do not utilize vapor barriers. The American Concrete Institutes ACI 36oR-o6 Design of Slabs on Grade and ACI 302.1R-04 Guide for Concrete Floor and Slab Construction are recommended references for vapor barrier selection and floor slab detailing. Slabs on grade may be designed using a coefficient of subgrade reaction of 210 pounds per cubic inch (pci) assuming the slab -on -grade base course is underlain by structural fill placed and compacted as outlined above. A 4- to 6-inch-thick capillary break layer should be placed over the prepared subgrade. This material should consist of pea gravel or 5/8 inch clean angular rock. A perimeter drainage system is recommended unless interior slab areas are elevated a minimum Of 12 inches above adjacent exterior grades. If installed, a perimeter drainage system should consist of a 4-inch diameter perforated drain pipe surrounded by a minimum 6 inches of drain rock wrapped in a non -woven geosynthetic filter fabric to reduce migration of soil particles into the drainage system. The perimeter drainage system should discharge by gravity flow to a suitable stormwater system. Exterior grades surrounding buildings should be sloped at a minimum of one percent to facilitate surface water flow away from the building and preferably with a relatively impermeable surface cover immediately adjacent to the building. Stormwater Management Feasibility The site is underlain by fill and thin weathered glacial till over dense glacial till. The dense glacial till acts as an aquitard, reducing vertical infiltration of stormwater to near zero. Infiltration is not feasible due to the presence of fill, restrictive layers, and variable slopes that would direct runoff to the west and potentially into the adjacent residences. We understand that roof runoff will be directed into City infrastructure. Note that a drywell for footing drains west of the residence is suitable. Erosion and Sediment Control Erosion and sediment control (ESC) is used to reduce the transportation of eroded sediment to wetlands, streams, lakes, drainage systems, and adjacent properties. Erosion and sediment control measures should be implemented, and these measures should be in general accordance www.cobaltgeo.com (2o6) 331-1097 March 2, 2022 Page io of 12 Geotechnical Evaluation with local regulations. At a minimum, the following basic recommendations should be incorporated into the design of the erosion and sediment control features for the site: • Schedule the soil, foundation, utility, and other work requiring excavation or the disturbance of the site soils, to take place during the dry season (generally May through September). However, provided precautions are taken using Best Management Practices (BMP's), grading activities can be completed during the wet season (generally October through April). • All site work should be completed and stabilized as quickly as possible. • Additional perimeter erosion and sediment control features may be required to reduce the possibility of sediment entering the surface water. This may include additional silt fences, silt fences with a higher Apparent Opening Size (AOS), construction of a berm, or other filtration systems. • Any runoff generated by dewatering discharge should be treated through construction of a sediment trap if there is sufficient space. If space is limited other filtration methods will need to be incorporated. Utilities Utility trenches should be excavated according to accepted engineering practices following OSHA (Occupational Safety and Health Administration) standards, by a contractor experienced in such work. The contractor is responsible for the safety of open trenches. Traffic and vibration adjacent to trench walls should be reduced; cyclic wetting and drying of excavation side slopes should be avoided. Depending upon the location and depth of some utility trenches, groundwater flow into open excavations could be experienced, especially during or shortly following periods of precipitation. In general, silty and sandy soils were encountered at shallow depths in the explorations at this site. These soils have low cohesion and density and will have a tendency to cave or slough in excavations. Shoring or sloping back trench sidewalls is required within these soils in excavations greater than 4 feet deep. All utility trench backfill should consist of imported structural fill or suitable on site soils. Utility trench backfill placed in or adjacent to buildings and exterior slabs should be compacted to at least 95 percent of the maximum dry density based on ASTM Test Method D1557. The upper 5 feet of utility trench backfill placed in pavement areas should be compacted to at least 95 percent of the maximum dry density based on ASTM Test Method D1557. Below 5 feet, utility trench backfill in pavement areas should be compacted to at least 90 percent of the maximum dry density based on ASTM Test Method D1557. Pipe bedding should be in accordance with the pipe manufacturer's recommendations. The contractor is responsible for removing all water -sensitive soils from the trenches regardless of the backfill location and compaction requirements. Depending on the depth and location of the proposed utilities, we anticipate the need to re -compact existing fill soils below the utility structures and pipes. The contractor should use appropriate equipment and methods to avoid damage to the utilities and/or structures during fill placement and compaction procedures. www.cobaltgeo.com (2o6) 331-1097 March 2, 2022 Page ii of 12 Geotechnical Evaluation CLOSURE This report was prepared for the exclusive use of Todd Echelbarger and his appointed consultants. Any use of this report or the material contained herein by third parties, or for other than the intended purpose, should first be approved in writing by Cobalt Geosciences, LLC. The recommendations contained in this report are based on assumed continuity of soils with those of our test holes, and assumed structural loads. Cobalt Geosciences should be provided with final architectural and civil drawings when they become available in order that we may review our design recommendations and advise of any revisions, if necessary. Use of this report is subject to the Statement of General Conditions provided in Appendix A. It is the responsibility of Todd Echelbarger who is identified as "the Client" within the Statement of General Conditions, and its agents to review the conditions and to notify Cobalt Geosciences should any of these not be satisfied. Sincerely, Cobalt Geosciences, LLC 54896 3/2/2022 Phil Haberman, PE, LG, LEG Principal 2513 Q Sed Gea PHIL P HABEIW,�J www.cobaltgeo.com (2o6) 331-1097 March 2, 2022 Page 12 of 12 Geotechnical Evaluation Statement of General Conditions USE OF THIS REPORT: This report has been prepared for the sole benefit of the Client or its agent and may not be used by any third party without the express written consent of Cobalt Geosciences and the Client. Any use which a third party makes of this report is the responsibility of such third parry. BASIS OF THE REPORT: The information, opinions, and/or recommendations made in this report are in accordance with Cobalt Geosciences present understanding of the site specific project as described by the Client. The applicability of these is restricted to the site conditions encountered at the time of the investigation or study. If the proposed site specific project differs or is modified from what is described in this report or if the site conditions are altered, this report is no longer valid unless Cobalt Geosciences is requested by the Client to review and revise the report to reflect the differing or modified project specifics and/or the altered site conditions. STANDARD OF CARE: Preparation of this report, and all associated work, was carried out in accordance with the normally accepted standard of care in the state of execution for the specific professional service provided to the Client. No other warranty is made. INTERPRETATION OF SITE CONDITIONS: Soil, rock, or other material descriptions, and statements regarding their condition, made in this report are based on site conditions encountered by Cobalt Geosciences at the time of the work and at the specific testing and/or sampling locations. Classifications and statements of condition have been made in accordance with normally accepted practices which are judgmental in nature; no specific description should be considered exact, but rather reflective of the anticipated material behavior. Extrapolation of in situ conditions can only be made to some limited extent beyond the sampling or test points. The extent depends on variability of the soil, rock and groundwater conditions as influenced by geological processes, construction activity, and site use. VARYING OR UNEXPECTED CONDITIONS: Should any site or subsurface conditions be encountered that are different from those described in this report or encountered at the test locations, Cobalt Geosciences must be notified immediately to assess if the varying or unexpected conditions are substantial and if reassessments of the report conclusions or recommendations are required. Cobalt Geosciences will not be responsible to any party for damages incurred as a result of failing to notify Cobalt Geosciences that differing site or sub -surface conditions are present upon becoming aware of such conditions. PLANNING, DESIGN, OR CONSTRUCTION: Development or design plans and specifications should be reviewed by Cobalt Geosciences, sufficiently ahead of initiating the next project stage (property acquisition, tender, construction, etc), to confirm that this report completely addresses the elaborated project specifics and that the contents of this report have been properly interpreted. Specialty quality assurance services (field observations and testing) during construction are a necessary part of the evaluation of sub -subsurface conditions and site preparation works. Site work relating to the recommendations included in this report should only be carried out in the presence of a qualified geotechnical engineer; Cobalt Geosciences cannot be responsible for site work carried out without being present. www.cobaltgeo.com (2o6) 331-1097 Approximate TP_ Test Pit Location Sno. Co GIs Map Proposed Residence 21322 92nd Place West Edmonds, Washington SITE MAP FIGURE i N A Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, WA 98028 (206) 331-1097 www.cobaltgeo.com cobaltgeoPgmail.com Slab on Grade Basement or Shallow Foundation Wall 12" Free Draining Backfill and/or Drainage Mat Attached to Wall Backfill Soils Compacted per Geotechnical Report 4" Diameter Perforated Pipe --�� H Native Soils Benched as Required Filter Fabric Over Rock (Mirafi 14oN) 3//4" Washed Rock or Clean Angular Rock Not to Scale Cobalt Geosciences, LLC PO Box 1792 Typical Foundation Drain Detail North Bend, WA 9So45 COBALTAttachment (2o6) 331-1097 GEOSCIENCLI-Swww.cobaltgeo.com philocobaltgeo.com Unified Soil Classification System (USCS) MAJOR DIVISIONS SYMBOL TYPICAL DESCRIPTION GW Clean Gravels Well -graded gravels, gravels, gravel -sand mixtures, little or no fines Gravels (more than 50% (less than 5% _ GP fines) Poorly graded gravels, gravel -sand mixtures, little or no fines COARSE GRAINED SOILS of coarse fraction retained on No. 4 sieve) GM Gravels with Fines (more than 12% GC fines) Silty gravels, gravel -sand -silt mixtures Clayey gravels, gravel -sand -clay mixtures (more than 50% retained on ;°e sw Clean Sands Well -graded sands, gravelly sands, little or no fines No. 200 sieve) Sands (50% or more of coarse fraction (less than 5% SP fines) Poorly graded sand, gravelly sands, little or no fines passes the No. 4 sieve) sM Sands with Fines Silty sands, sand -silt mixtures (more than 12% sc fines) Clayey sands, sand -clay mixtures ML Inorganic silts of low to medium plasticity, sandy silts, gravelly silts, FINE GRAINED SOILS (50% or more Silts and Clays (liquid limit less than 50) Inorganic cL or clayey silts with slight plasticity Inorganic clays of low to medium plasticity, gravelly clays, sandy clays silty clays, lean clays oL Organic Organic silts and organic silty clays of low plasticity passes the MH Inorganic silts, micaceous or diatomaceous fine sands or silty soils, No. 200 sieve) Silts and Clays (liquid limit 50 or more) Inorganic CH elastic silt Inorganic clays of medium to high plasticity, sandy fat clay, or gravelly fat clay OH Organic Organic clays of medium to high plasticity, organic silts HIGHLY ORGANIC SOILS primarily organic matter, dark in color, PT and organic odor Peat, humus, swamp soils with high organic content (ASTM D4427) 1 Classification of Soil Constituents 1 MAJOR constituents compose more than 50 percent, by weight, of the soil. Major constituents are capitalized (i.e., SAND). Minor constituents compose 12 to 50 percent of the soil and precede the major constituents (i.e., silty SAND). Minor constituents preceded by "slightly" compose 5 to 12 percent of the soil (i.e., slightly silty SAND). Trace constituents compose o to 5 percent of the soil (i.e., slightly silty SAND, trace gravel). Relative Density (Coarse Grained Soils) Consistency (Fine Grained Soils) N, SPT, Relative N, SPT, Relative Blows/FT Density Blows/FT Consistency 0-4 Very loose Under 2 Very soft 4 -10 Loose 2-4 Soft 10 - 30 Medium dense 4-8 Medium stiff 30 - 50 Dense 8 -15 Stiff Over 50 Very dense 15 - 30 Very stiff Over 3o Hard Grain Size Definitions Description Sieve Number and/or Size Fines <#200 (0.08 mm) Sand -Fine #200 to #40 (0.08 to 0.4 mm) -Medium #40 to #10 (0.4 to 2 MM) -Coarse #10 to #4 (2 to 5 mm) Gravel -Fine #4 to 3/4 inch (5 to 19 mm) -Coarse 3/4 to 3 inches (19 to 76 mm) Cobbles 3 to 12 inches (75 to 305 mm) Boulders >12 inches (305 mm) Moisture Content Definitions Dry Absence of moisture, dusty, dry to the touch Moist Damp but no visible water Wet Visible free water, from below water table Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, WA98028 Soil Classification Chart Figure C1 (206) 331-1097 _ www.cobalt eg o.com cobaltgeo(d-) gmail.com Test Pit TP-1 Date: November 3, 2021 Depth: 10' Groundwater: None Contractor: Client provided Elevation: Logged By: PH Checked By: SC -- o> o Moisture Content (%) N JO -0 Plastic 1 Liquid U E Limit Limit L � L Material Description o ? o DCP Equivalent N-Value G 0 10 20 30 40 50 R __ To.P oilation ________________________________ 1 SM Loose to medium dense, silty -fine to medium grained sand with gravel l dark yellowish brown to grayish brown, moist. 2 (Fill) 3 4 5 ------ 6 ---- X. --- SM --------------------------------------------- Medium dense to dense, silty -fine to medium grained sand with grave , grayish brown, moist. (Glacial Till) 8 9 18 End of Test Pit 10' Test Pit TP-2 Date: November 3, 2021 Depth: 8' Groundwater: None Contractor: Client provided Elevation: Logged By: PH Checked By: SC — 0 0) o Moisture Content (%) JO U -0 T U Plastic 1 Liquid Limit Limit Material Description DCP Equivalent N-Value o 6 ? o G 0 10 20 30 40 5 -- Topsoilv_egetation -------------------------------- SM 1 Loose to medium dense, silty -fine to medium grained sand with gravel dark yellowish brown to grayish brown, moist. 2 (Fill) —.3 ----------- •:.'• -- --------------------------------------------- SM Medium dense, silty -fine to medium grained sand with gravel, 4 yellowish brown to grayish brown, moist. (Weathered Glacial Till) 5 --6 ----------- --- --------------------------------------------- SM Dense to very dense, silty -fine to medium grained sand with gravel, 7 grayish brown, moist. (Glacial Till) End of Test Pit 8' 9 10 Cobalt Geosciences, LLC Proposed Residence P.O. Box 82243 COBALT21322 92nd Place West Test Pit Kenmore, WA 98028 (2o6) 331-1097 F Edmonds, Washington Logs www.cobaltgeo.com cobaltgeoPgmail.com