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Shen-Federman Residence_Drainage Report_Civil Engineering Solutions_Dated 03.29.2023 (1)CIVIL ENGINEERING MR 1-5 Storm Drainage Summary Shen/Federman Residence 23310 Humber Lane Edmonds, WA 98020 Parcel I D : 00 571300101000 4,455 sf new & replaced impervious March 29, 2023 SOILS ....................glacial till STORM DISCHARGE ......... basic dispersion Coauthored by Stephenie Seawall, civil engineer Duffy Ellis, P.E. Page 1 of 9 MR1-5 Drainage Report 23310 Humber Lane Edmonds, WA 98020 3/29/2023 CES #2070 S O L U T 1 0 N S General: This site's proposed impervious area is below 5,000 sf, site is subject to minimum requirements MR1-5 identified below. Reference attached Edmonds Addendum Figure 3.1. MR1 = Preparation of Storm Water Site See C2.0 Drainage Plan Plans MR2 = Construction Storm Water Pollution See CSWPPP included with this Prevention Plan submittal. Also see sheet C1.0 TESCP in the civil plan set. MR3 = Source Control of Pollution See C1.0 for Temporary Erosion & Sediment Control. Also see the CSWPPP document prepared for this project. MR4 = Preservation of Natural Drainage This suburban lot and surrounding Systems and Outfalls area have a moderate topographic tilt toward the west. There are no natural drainage systems. City storm is available across the street, but would require a pump system to make this solution work. It is unknown what this existing house does with stormwater. We discuss MR4 in more depth on page 6. MR5 = On -site Stormwater Management Basic dispersion is proposed for the roof and driveway areas. Infiltration is not recommended into the glacial till soil. See specific MR5 discussion of each BMP on pages 6 and 7 of this report. See the Infiltration report in the appendix. MR6 = Runoff Treatment N/A, project under 5,000 sf MR7 = Flow Control N/A, project under 5,000 sf MR8 = Wetlands Protection N/A, project under 5,000 sf MR9 = Operations and Maintenance N/A, project under 5,000 sf Page 2 of 9 MR1-5 Drainage Report 23310 Humber Lane Edmonds, WA 98020 3/29/2023 CES #2070 CIVIL FNGINFFRING Street View Vicinity Map � I i rr� - +Southwest County Park ' J iget Sound ��Jrj f l 99; Edmonds L _ 4n Shen/Federman /'_ip Wood I — r Mountlak S -� Terrace I Lake B+Jli.yn Page 3 of 9 MR1-5 Drainage Report 23310 Humber Lane Edmonds, WA 98020 3/29/2023 CES #2070 SO_LU_T10NS Background: A new single-family two-story house is proposed on this lot adjacent to the east border of Woodway. Subject lot slopes down to the west at an average grade of 7%. Soil is glacial till and not great for infiltration. See soil summary below for full explanation. This lot falls within the Deer Creek drainage basin which is tributary to Lake Washington. The total new or replaced impervious area is under 5,000 sf including all roof area and on -site driveway. See exact areas in the appendix. Soils and Infiltration Feasibility: Conclusion sourced from the geotech report (see appendix for report)... Soil is glacial till. During our January 18, 2023 site visit, we assessed the subsurface conditions in two test holes located... Published geologic maps indicate that the site and surrounding area are underlain by glacial till, a glacially -compressed mixture of gravel, silt, and fine-grained sand. The natural soil conditions encountered in the test holes consist of topsoil overlying weathered, slightly gravelly, silty sand. Beneath this thin weathered layer is dense, slightly gravelly, silty sand that has been glacially compressed, and which is referred to as glacial till. This is consistent with the mapped geology for the area. A layer of fill was encountered over the native soils in the western test hole (Test Pit 2). We expect that fill may be encountered elsewhere around the existing house, resulting from the excavation and grading that occurred during the original site development. The test holes were conducted following several months of wet weather. Shallow seepage perched on top of the glacial till, which is relatively impervious, was observed. This is a common occurrence in the Pacific Northwest on properties where glacial till or other impervious soils lie close to the ground surface... Considering the observed soil conditions and the presence of seasonal shallow perched groundwater, it is our professional opinion that infiltration of concentrated storm water runoff is infeasible for this site. Attempting to infiltrate or disperse stormwater on the site would only increase the potential for surface and subsurface drainage problems on neighboring properties. Page 4 of 9 MR1-5 Drainage Report 23310 Humber Lane Edmonds, WA 98020 3/29/2023 CES #2070 CIVIL ENGINEERING Existing Drainage Pattern narrative: It is unknown what the existing house does with its stormwater. (Normally houses built in this era splash -blocked their water.) See downstream map below, assuming water is not pumped up to the existing city storm (which would also end up in Deer Creek). Washi Existing Impervious area: 3,800 sf Proposed Impervious area: < 5,000 sf See impervious spreadsheet in appendix for breakdown Page 5 of 9 MR1-5 Drainage Report 23310 Humber Lane Edmonds, WA 98020 3/29/2023 CES #2070 S O L U T I O N S MR 4 Preservation of Natural Drainage Systems and Outfalls MR#4 Definition 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 downgradient properties. All outfalls require energy dissipation. (ref. DOE Manual, 1-2.5.4) Response: There are no well-defined natural outfalls and natural drainage systems to release to on this urban lot. The "natural" place to discharge stormwater for this lot is basic dispersion, which is proposed for the roof and driveway. See the map on previous page. MR#4 Objective To preserve and utilize natural drainage systems to the fullest extent because of the multiple stormwater benefits these systems provide; and to prevent erosion at and downstream of the discharge location. (ref. DOE Manual, 1-2.5.4) Response: Basic Dispersion is proposed for the roof and driveway. See the maps on previous page. MR#4 Supplemental Guidelines Creating new drainage patterns results in more site disturbance and more potential for erosion and sedimentation during and after construction. Creating new discharge points can create significant stream channel erosion problems as the receiving water body typically must adjust to the new flows. Diversions can cause greater impacts than would otherwise occur by discharging runoff at the natural location. (ref. DOE Manual, 1-2.5.4) Response: This project should in no way cause adverse problems mentioned above which are more potential issues related to larger projects as opposed to urban single family. Page 6 of 9 MR1-5 Drainage Report 23310 Humber Lane Edmonds, WA 98020 3/29/2023 CES #2070 S_ O L _U_ T_I O N_S_ MR 5 Proposed On -site Stormwater management: The List Approach (List #1) selection process was applied to site: Lawn and Landscaped Areas: • Post -Construction Soil Quality and Depth in accordance with BMP T5.13 in Chapter 5 of Volume V of the DOE Manual. Compost -Amended Soil is required and proposed. Roofs: • Full Dispersion: Not feasible: A minimum native vegetative flowpath length of 100 lineal feet is not achievable. • Downspout Full Infiltration: Full Infiltration is not recommended. Dense glacial till soil. Project geotech, Marc McGinnis, PE, does not advise infiltration due to the till soil with shallow groundwater. • Bio-retention / Raingarden: Not advised in light of the dense till native soils. There would be a concern about chronic standing water problem especially during the rainy season. • Downspout Dispersion: Proposed for roof and driveway, using the calculation 10 LF / 700 sf tributary area. The tributary area is around 3,300 sf (= 3,094 sf roof — 400 sf splash -blocked + 582 sf exposed driveway), so we used the maximum allowed 3,500 sf --> 50 LF trench. • PTC No connection to city storm proposed. Other Hard Surfaces (i.e. Driveway): • Full Dispersion serving driveway: Not feasible: a minimum native vegetative flowpath length of 100 lineal feet is not even close to being achievable. • Permeable Pavement Driveway Infiltration is not recommended. Dense glacial till soil. Project geotech, Marc McGinnis, PE, does not advise infiltration due to the till soil with shallow groundwater. In this case the driveway is tilting down toward the house, another reason not to attempt driveway infiltration. Page 7 of 9 MR1-5 Drainage Report 23310 Humber Lane Edmonds, WA 98020 3/29/2023 CES #2070 CIVIL ENGINEERING • Bioretention On this driveway it is impossible to site a rain garden due to the south -to -north slope of the front yard, and the underground utilities. Sheet Flow Dispersion serving driveway Impossible to ever use this BMP on urban lots. You can't achieve the minimum 25' flowpath required between trench and ROW; reference DOE Figure V-5.3.2. MR6 Runoff Treatment N/A — Project under 5,000 sf MR7 Flow Control N/A — Project under 5,000 sf Page 8 of 9 MR1-5 Drainage Report 23310 Humber Lane Edmonds, WA 98020 3/29/2023 CES #2070 CIVIL ENGINEERING APPENDIX • Impervious Area Spreadsheet • Figure 3.1 — Flow Chart for Determining Requirements for Development • Geotechnical Report, Geotech Consultants, Inc., January 2023 Attached separately • CSWPPP (sourced from Bellevue, WA) Page 9 of 9 MR1-5 Drainage Report 23310 Humber Lane Edmonds, WA 98020 3/29/2023 CES #2070 Impervious Area Spreadsheet (Stormwater) Shen/Federman Residence - 23310 Humber Lane, Edmonds, WA 98020 Site Area 12,600 sf 0.289 acres Existing impervious area Existing imprevious area to be demo'ed 3,595 sf Existing imprevious area to remain 205 sf total existing impervious = 3,800 sf Existing vegetated area 8,800 sf Proposed Impervious Area (on -site) Roof 3,094 sf Exposed driveway, on -site 582 sf Exposed front/side walkway 779 sf total new + replaced impervious = 4,455 sf Existing imprevious area to remain 205 sf Total proposed impervious = 4,660 sf Proposed vegetated area 7,940 sf Does the project result in 2,000 square feet, or greater, of new plus replaced hard surface area? OR Does the land disturbing activity total 7,000 square feet or greater? Yes No Minimum Requirements No. 1 through 5 apply I Minimum Requirement No. 2 applies Next Question IF Does 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 vegetation 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 No Yes Does the project add 5,000 square feet or No more of new hard surfaces? Yes Do new hard surfaces add 50% or more to the existing hard surfaces within the project limits? No Figure 3.1. Flow Chart for Determining Requirements for Development. No additional requirements. 8 3. APPLICABILITY OF THE MINIMUM REQUIREMENTS OEOTECH CONSULTANTS, INC_ Bernard Shen & Catherine Federman 417 N.W. 32"d Street Seattle, Washington 98177 via email: bernard8shen(aMotmail.cam & cefer8(a-hotmail.com Subject: Geotechnical Report Proposed New Residence 23310 Humber Lane Edmonds, Washington Greetings: 2401 1 Oth Ave E Seattle, Washington 98102 (425)747-5618 January 19, 2023 J N 23011 This report presents our geotechnical conclusions related to the planned redevelopment of the subject property. On January 18, 2023, the undersigned principal geotechnical engineer visited the subject site to assess both the surface and subsurface conditions. In addition to this work, we have reviewed published geologic maps, as well as the mapping by the Natural Resources Conservation Service (NRCS). Based on the information provided by Patricia Brennan Architects, the existing residence will be demolished and be replaced with a new home having a larger footprint. The west wall of the new house will be close to the western extent of the existing residence, and the new house's footprint will extend eastward from the existing. The new home will be two stories in height. No basement or deep below -grade spaces will be included. SITE CONDITIONS The existing house is one story in height and is centrally located on the lot. A paved driveway extends from Humber Lane through the southern portion of the property to the garage located in the south end of the current residence. The remainder of the property is covered primarily with grass, with some landscape beds. There are two small detached sheds located in the southwestern portion of the lot. There are no steep slopes on, or near, the subject property. Consistent with the neighboring properties, the natural ground surface slopes only gently downward toward the west from the edge of Humber Lane. It appears that a storm sewer and associated catch basins are located along the western edge of paved portion of Humber Lane. The neighboring properties are similarly developed with one-story homes. During our January 18, 2023 site visit, we assessed the subsurface conditions in two test holes located as indicated on the attached Site Exploration Plan. Logs of the test pits are also attached. Published geologic maps indicate that the site and surrounding area are underlain by glacial till, a glacially -compressed mixture of gravel, silt, and fine-grained sand. The natural soil conditions encountered in the test holes consist of topsoil overlying weathered, slightly gravelly, silty sand. GEOTECH CONSULTANTS, INC. Shen & Federman A 23011 January 19, 2023 Page 2 Beneath this thin weathered layer is dense, slightly gravelly, silty sand that has been glacially compressed, and which is referred to as glacial till. This is consistent with the mapped geology for the area. A layer of fill was encountered over the native soils in the western test hole (Test Pit 2). We expect that fill may be encountered elsewhere around the existing house, resulting from the excavation and grading that occurred during the original site development. The test holes were conducted following several months of wet weather. Shallow seepage perched on top of the glacial till, which is relatively impervious, was observed. This is a common occurrence in the Pacific Northwest on properties where glacial till or other impervious soils lie close to the ground surface. CONCLUSIONS AND RECOMMENDATIONS Based on the conditions observed in the test holes, it is our professional opinion that conventional foundations can be utilized for the proposed residence. All footing areas will have to be excavated down to the dense, native, glacially -compressed soil. It is possible that this may require overexcavation below the planned footing grades. We recommend that the footings be excavated using a smooth bucket, in order to prevent the subgrade disturbance that can result from the teeth on an excavator's bucket. Where overexcavation below the planned footing grades is necessary, the additional excavation can be backfilled to the planned footing grade using compacted quarry spalls or railroad ballast rock. In wet conditions, the footing subgrades should be protected with a layer of clean crushed gravel, in order to prevent disturbance and softening of the bearing soils during the placement of foundation forms and rebar. There are no geologically critical areas on, or near, the site. The potential for future slope movement is nonexistent and the glacially -compressed soils do not present a seismic hazard. No significant retaining walls are expected for the new project. The erosion control measures needed during the site development will depend heavily on the weather conditions that are encountered. We anticipate that a silt fence will be needed around the downslope sides of any cleared areas until house excavation is are lowered below the surrounding grades. Cut slopes and soil stockpiles should be covered with plastic during wet weather. Following rough grading, it may be necessary to mulch or hydroseed bare areas that will not be immediately covered with landscaping or an impervious surface. As with most projects, it may be necessary to implement additional erosion control measures to address site and weather conditions during the earthwork and construction. If crawl spaces or interior slab floors are lower than the surrounding grade, footing drains should be provided. A typical footing drain detail is attached. Given the impermeable nature of the dense soils, it is not uncommon for shallow seepage to enter crawl spaces. As a result, it would be prudent to provide a layer of gravel and perforated pipes under the vapor retarder/barrier, in addition to the perimeter footing drains. SEISMIC CONSIDERATIONS In accordance with the International Building Code (IBC), the site class within 100 feet of the ground surface is best represented by Site Class Type D (stiff soil). GEOTECH CONSULTANTS, INC. Shen & Federman JN 23011 January 19, 2023 Page 3 The IBC and ASCE 7 require that the potential for liquefaction (soil strength loss) be evaluated for the peak ground acceleration of the Maximum Considered Earthquake (MCE), which has a probability of occurring once in 2,475 years (2 percent probability of occurring in a 50-year period). The dense, glacially -compressed soil that will support the foundations is not susceptible to seismic liquefaction under the ground motions of the MCE. CONVENTIONAL FOUNDATIONS The proposed residence can be supported on conventional continuous and spread footings bearing on undisturbed, dense soil, or on compacted crushed rock structural fill placed above this competent native soil. Prior to placing structural fill beneath foundations, the excavation should be observed by the geotechnical engineer or building inspector to document that adequate bearing soils have been exposed. We recommend that continuous and individual spread footings have minimum widths of 12 and 16 inches, respectively. Exterior footings should also be bottomed at least 18 inches below the lowest adjacent finish ground surface for protection against frost and erosion. The local building codes should be reviewed to determine if different footing widths or embedment depths are required. Footing subgrades must be cleaned of loose or disturbed soil prior to pouring concrete. Depending upon site and equipment constraints, this may require removing the disturbed soil by hand. Depending on the encountered soil conditions, overexcavation may be required below the footings to expose competent native soil. Unless lean concrete is used to fill an overexcavated hole, the overexcavation must be at least as wide at the bottom as the sum of the depth of the overexcavation and the footing width. For example, an overexcavation extending 2 feet below the bottom of a 2-foot-wide footing must be at least 4 feet wide at the base of the excavation. If lean concrete is used, the overexcavation need only extend 6 inches beyond the edges of the footing. An allowable bearing pressure of 2,500 pounds per square foot (psf) is appropriate for footings supported on competent native soil. A one-third increase in this design bearing pressure can be used when considering short-term wind or seismic loads. For the above design criteria, it is anticipated that the total post -construction settlement of footings founded on competent native soil will be less than one inch, with differential settlements on the order of one -half -inch in a distance of 25 feet along a continuous footing with a uniform load. Lateral loads due to wind or seismic forces may be resisted by friction between the foundation and the bearing soil, or by passive earth pressure acting on the vertical, embedded portions of the foundation. For the latter condition, the foundation must be either poured directly against relatively level, undisturbed soil or be surrounded by level, well -compacted fill. We recommend using the following ultimate values for the foundation's resistance to lateral loading: PARAMETER ULTIMATE VALUE Coefficient of Friction 0.40 Passive Earth Pressure 300 pcf Where: pcf is Pounds per Cubic Foot, and Passive Earth Pressure is computed using the Equivalent Fluid Density. GEOTECH CONSULTANTS, INC. Shen & Federman JN 23011 January 19, 2023 Page 4 If the ground in front of a foundation is loose or sloping, the passive earth pressure given above will not be appropriate. The above ultimate values for passive earth pressure and coefficient of friction do not include a safety factor. EVALUATION OF INFILTRATION FEASIBILITY The dense soil known to underlie this area, and which was observed in the test holes is glacially compressed. Our review of published geologic maps confirms that the near -surface geologic unit in this area is glacial till (aka hardpan), which is consistent with the observed soil conditions. There are no large or continuous pore spaces in the glacial till soils that can transmit water. This soil is essentially impermeable, preventing downward percolation of water, which often causes a perched water table to form following extended heavy rainfall. A 1997 study published by U.S. Geologic Survey (USGS) in cooperation with the Washington Department of Ecology (WDOE) determined the infiltration capacity of various Washington till soils to vary between 0.0005 and 0.005 inches/hour. We have found similar extremely low infiltration rates in Pilot Infiltration Tests (PITS) our firm has conducted in glacial till soils. Often, the impermeable nature of the glacial till causes a shallow seasonal perched water table to form where the ground surface is not covered by an impervious layer. This is a common problem in the wet season throughout the Pacific Northwest, and shallow seepage is evident in the test pits conducted on the site. Our review of the Web Soil Survey website run by the Natural Resources Conservation Service (NRCS) indicates that the soil mapping is Alderwood-Urban land complex. This is consistent with the soil conditions observed in the test pit. The following information is from the Web Soil Survey, and also indicates that the limiting layer has a very low permeability, far below what is allowed for design of infiltration systems. 5—Alderwood-Urban land complex, 2 to 8 percent slopes Map Unit Composition • Alderwood and similar soils: 60 percent • Urban land: 25 percent • Minor components: 15 percent • Estimates are based on observations, descriptions, and transects of the mapunit. Properties and qualities • Slope: 2 to 8 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) Considering the observed soil conditions and the presence of seasonal shallow perched groundwater, it is our professional opinion that infiltration of concentrated storm water runoff is infeasible for this site. Attempting to infiltrate or disperse stormwater on the site would only increase the potential for surface and subsurface drainage problems on neighboring properties. GEOTECH CONSULTANTS, INC. Shen & Federman January 19, 2023 Please contact us if you have any questions regarding this report. Respectfully submitted, GEOTECH CONSULTANTS, INC. ��-G R- McGI,�, e OF WASy� '1'lcP AL 1 /19/2023 Marc R. McGinnis, P.E. Principal Attachments: Vicinity Map, Site Exploration Plan, Test Pit Logs, Footing Drain Detail cc: Patricia Brennan Architects via email. office(a�.patriciabrennanarchitects.com MRM:kg J N 23011 Page 5 GEOTECH CONSULTANTS, INC. GEOTECH CONSULTANTS, INC. (Source: Microsoft MapPoint, 2013) VICINITY MAP 23310 Humber Lane Edmonds, Washington Job No: Date: Plate: 23011 Jan.2023 I 1 Legend: TP-1 Test Hole Location GEOTECH CONSULTANTS, INC. SITE EXPLORATION PLAN 23310 Humber Lane Edmonds, Washington Job No: Date: Plate: 23011 Jan. 2023 No Scale 1 2 TEST PIT 1 Depth feet Soil Description 0 — 1.0 Crushed Rock over Topsoil 1.0 — 2.0 Orangish-bro.°, n. slightly gravelly. silty SAND. fine-grained, very moist. loose 2.0 — 3.0 Grayish-bro�,,.rn, slightly gravelly, silty SAND, fine-grained, very moist, dense Glacial Till) Test Hole v,as terminated at a depth of 3.0 feet on January 17, 2023. Perched ground ,ater seepage ,%as observed at 2.0 feet. TEST PIT 2 Depth feet Soil Description 0-2.0 Bro-:,,m, slightly gravelly, silty SAND. fine-grained. very moist. loose i FILLI 2.0 — 2.5 Old Topsoil 2.5 — 3.5 Orangish-bro,.,:,n, slightly gravelly, silty SAND, fine-grained, very moist. loose 3.5 Grayish-bro,.^:,n, slightly gravelly, silty SAND, fine-grained, very moist, dense iGlacial Till) Test Hole ✓as terminated at a depth of 3.5 feet on January 18, 2023. Perched ground,• ater seepage was encountered at a depth of 3.5 feet. GEOTECH CONSULTANTS, INC. TEST PIT LOGS 23310 Humber Lane Edmonds, Washington Job No: Date: Plate: 23011 Jan.2023 1 1 3 Slope backfill away from foundation. Provide surface drains where necessary. 4" min. Tightline Roof Drain (Do not connect to footing drain) Backfill (See text for requirements) O c� Nonwoven Geotextile Filter Fabric O LL Possible Slab oo.a�-000.a� 000.a�0 .00a ° o°< 0 0 0 0 0 "Oo� oO oO oO oO oOo oo.00a 0'0� o O ° Q' aQ o 0 O L�IL)00(� �-o o� 000 4" Perforated Hard PVC Pipe (Invert at least 6 inches below slab or crawl space. Slope to drain to appropriate outfall. Place holes downward.) a.o.o 0 o.a.o-a o a.o:o-o 0 o.a:o-o 0 o.a 00° ` oO' 00 ` oO' 0o0 ` 00, 0000°° boa 0'o a�oa 0 0000�00 0 000�00 0 000�c o,00q °o ° Vapor Retarder/Barrier and Capillary Break/Drainage Layer (Refer to Report text) NOTES: (1) In crawl spaces, provide an outlet drain to prevent buildup of water that bypasses the perimeter footing drains. (2) Refer to report text for additional drainage, waterproofing, and slab considerations. GEOTECH CONSULTANTS, INC. TYPICAL FOOTING DRAIN 23310 Humber Lane Edmonds, Washington Job No: Date: Plate. 4 23011 Jan.2023