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APPROVED STM BLD2022-1324+Storm_Drainage_Report+9.1.2023_1.58.35_PM+3761397RESUB Sep 05 2023 BLD2022-1324 Drainage Summary CITY OF EDMONDS DEVELOPMENT SERVICES DEPARTMENT Project Name: 915 Brookmere Street BLD2022-1324 Project Address: 915 Brookmere Street Edmonds, WA 98020 Parcel No.: 00548900000302 Applicant: Ron and Maureen Haider Submittal Date: September 13, 2022 Resubmittal Date: August 30, 2023 Updates from previous submittal highlighted in grey. ii COMPLIES WITH APPLICABLE CITY STORM CODE 09/07/2023 Interlaken Engineering and Design, PLLC Seattle 1 (206) 470 — 9572 1 www.interlakenengineering.com Interlaken Engineering and Design, PLLC Project Overview The project site is located in Edmonds on a 10,500 sf (0.24 acre) lot zoned RS-12. The site contains an existing single-family residence (SFR) as well as a driveway and detached garage. The owner proposes to an addition to the existing residence. Access will be provided by a driveway entering the site from Brookmere St to the east of the existing driveway. WOODWAY }< �M XR�u r IYNPIWOO CD MONDc, Ai T, LTI , I A I I1- �_ * �_ I __J 11 1 Figure 1: Locus map with project site highlighted. Existing Conditions The project site is located in Edmonds on a 10,500 sf (0.24 acre) lot zoned RS-12 (Parcel No. 00548900000302) on the north side of Brookmere St. Access is provided by a driveway entering the lot from Brookmere St. The site is in the Edmonds Way watershed, which drains to the Puget Sound. The lot presently contains a single-family dwelling along with a driveway and garage. The site slopes downward from east to west at magnitudes of 0 to 10 percent with a total relief of approximately 9 feet. Under existing conditions there is approximately 2,773 sf of impervious surface on the project site. Presently runoff flows overland off -site to the west. Runoff that does not infiltrate or disperse is collected by the existing City of Edmonds storm drain near the intersection of Brookmere St and 9' Ave NE. Proposed Conditions The owner proposes to build an addition to the existing residence. Access will be provided by a new driveway entering the site from Brookmere St to the east of the existing driveway. The applicant proposes 2,437 sf of new roof, a new 622 sf of onsite driveway/parking area, 273 sf of offsite driveway area, and Interlaken Engineering and Design, PLLC total new walkway area of 512 s£ The new plus replaced impervious surface associated with this project is 3,843 sf. Table 1: Proposed Hard Surface Breakdown Roof Area Area (square feet) New 2437 Retrofit (25%) of remaining 276 Tota 1 2713 Other Hard Surfaces Driveway (on site) 622 Driveway (ROW) 273 Other ROW Improvements 0 Walkway 204 Patios/Landings/Stone Paths 307 Total 1406 Jurisdictional Requirements This project proposes greater than 2,000 sf but less than 5,000 sf of new plus replaced hard surface. According to 2022 Edmonds Stormwater Addendum Figure 3.1, the proposed project is subject to Minimum Requirements #1-5 (hereafter MR) as outlined below with a brief summary of how they will be met. MR #1: Preparation of Stormwater Site Stormwater Site Plans are included with this Plans submittal. Please see C3. MR #2: Construction Stormwater Pollution A Construction Stormwater Pollution Prevention Plan Prevention Plan (CSWPPP) has been prepared as part of the Stormwater Site Plan packet that is included with this submittal. Please see C2. MR #3: Source Control of Pollution A Temporary Erosion and Sediment Control Plan is included with this submittal. Please see C2. MR #4: Preservation of Natural Drainage The proposed development will not alter the existing Systems and Outfalls drainage pattern for this lot. Under existing conditions runoff flows overland to the west where flows that don't infiltrate or disperse are collected by the City of Edmonds storm drain and discharged toward the Puget Sound. Under proposed conditions runoff will be dispersed on site. This maintains the existing drainage pattern to the maximum extent practicable. MR #5: On -site Stormwater Management Given the site's drainage properties, dispersion trenches, rock pads, and permeable pavers are feasible BMPs for all runoff. As such, all stormwater runoff generated from new and replaced impervious surfaces will be dispersed via rock pad and dispersion trenches or locally infiltrated via permeable pavers/PICPs. Interlaken Engineering and Design, PLLC Infiltration Feasibility Assessment A Geotechnical Evaluation was prepared by Cobalt Geosciences LLC, dated August 7, 2022. Their explorations encountered the following: "Approximately 6 inches of vegetation and topsoil underlain by approximately 2 to 3 feet of loose to medium dense, silty -fine to medium grained sand with gravel (Weathered Glacial Till). These materials were underlain by medium dense to dense, silty -fine to medium grained sand trace gravel (Glacial Till), which continued to the termination depths of the explorations. The site is underlain by relatively dense glacial till. We observed mottled soils at shallow depths, indicating that perched groundwater may be present during the wet season. We also observed groundwater at relatively shallow depths and estimate a seasonal high groundwater table to be within 4 feet of the ground surface in most areas. Infiltration is not feasible due to the soil and anticipated groundwater conditions at the site. There is inadequate clearance for widespread infiltration systems above the restrictive layer and/or groundwater." Offsite Runoff Under existing conditions all flows from the site infiltrate or disperse on -site, or flow to the existing City of Edmonds public drain. This pattern will be maintained under proposed conditions. Hydrology/ Hydraulic Analysis Formal hydrology and pipe flow hydraulic calculations were not completed for the proposed project. Assuming a Manning's Roughness Coefficient of n--0.12 a 4" PVC pipe at a 2.0% slope will provide capacity for approximately 0.291 cfs. The Rational Method would yield a maximum flow for the 100-year storm from the entire roof area of approximately 0.34 cfs. The proposed 4" pipes provide adequate capacity. MR #5 On -Site Stormwater Management — List Approach Lawn and Landscaped Areas: Post -construction soil quality and depth will be in accordance with ECDC 18.30 and BMP T5.13 in Chapter 5 of Volume V of the DOE Manual. Compost -amended soil is required for all new lawn and landscaped areas. Roofs: New Roof Area: Full Dispersion is infeasible for this project as a 1001f vegetated flow path cannot be provided. Downspout Full Infiltration is infeasible for the proposed project. According to the Geotechnical Report prepared by Cobalt Geosciences dated August 7, 2022; "The site is underlain by relatively dense glacial till. We observed mottled soils at shallow depths, indicating that perched groundwater may be present during the wet season. We also observed groundwater at relatively shallow depths and estimate a seasonal high groundwater table to be within 4 feet of the ground surface in most areas. Infiltration is not feasible due to the soil and anticipated groundwater conditions at the site. There is inadequate clearance for widespread infiltration systems above the restrictive layer and/or groundwater." Interlaken Engineering and Design, PLLC Rain Garden/Bioretention is infeasible for the proposed project. According to the Edmonds Stormwater Appendix C; "Where the minimum vertical separation of 1 foot to the seasonal high groundwater or other impermeable layer would not be achieved below bioretention that would serve a drainage area less than the above thresholds." The project is unable to maintain the required vertical separation of 1 foot due to shallow groundwater, a sloping site, and the nature of the facilities requiring ponding depths. Downspout Dispersion Systems are proposed for new and replaced impervious surfaces in this project. According to SWMMWW BMP T5.IOB V-4.4, for roof areas larger than 700 square feet, a dispersion trench with notched grade board must provide at least 10 feet of trench length per 700 square feet of roof area. Four gravel filled dispersion trenches will be placed throughout the site, two to the north of the new section of SFR and two to the south. Two of the trenches will be 10 ft in length, one will be 10.5 ft in length, and one will be 13 ft in length. Each trench will disperse across a 25 ft vegetated flow path. Downspouts will route runoff from the roof to tightlines where flows will be directed to catch basins just before each of the dispersion trenches. Flows will enter the trenches from the catch basins and disperse over the vegetated flow paths. According to SWMMWW BMP T5.1 OB V-4.6, a maximum of 700 square feet of roof area may drain to a single splash block/ rock pad/ pop-up drainage emitter. A rock pad will be utilized in the northwestern corner of the site to mitigate 700 sf of roof area. Flows to the rock pad will be dispersed over a 50 ft vegetated flow path. Through the combination of dispersion trenches and a rock pad, 3,745 sf of mitigation is provided. This mitigates the entirety of the proposed new roof area. Retrofit Roof Area: Full Dispersion is infeasible for this project as a 1001f vegetated flow path cannot be provided. Downspout Full Infiltration is infeasible for the proposed project. According to the Geotechnical Report prepared by Cobalt Geosciences dated August 7, 2022; "The site is underlain by relatively dense glacial till. We observed mottled soils at shallow depths, indicating that perched groundwater may be present during the wet season. We also observed groundwater at relatively shallow depths and estimate a seasonal high groundwater table to be within 4 feet of the ground surface in most areas. Infiltration is not feasible due to the soil and anticipated groundwater conditions at the site. There is inadequate clearance for widespread infiltration systems above the restrictive layer and/or groundwater." Rain Garden/Bioretention is infeasible for the proposed project. According to the Edmonds Stormwater Appendix C; "Where the minimum vertical separation of 1 foot to the seasonal high groundwater or other impermeable layer would not be achieved below bioretention that would serve a drainage area less than the above thresholds." The project is unable to maintain the required vertical separation of 1 foot due to shallow groundwater, a sloping site, and the nature of the facilities requiring ponding depths. Interlaken Engineering and Design, PLLC Downspout Dispersion Systems are proposed for remaining impervious roof surfaces in this project. According to SWMMWW BMP T5.IOB V-4.4, for roof areas larger than 700 square feet, a dispersion trench with notched grade board must provide at least 10 feet of trench length per 700 square feet of roof area. Four gravel filled dispersion trenches will be placed throughout the site, two to the north of the new section of SFR and two to the south. Three of the trenches will be 10 ft in length, one will be 10.5 ft in length, and one will be 13 ft in length. Each trench will disperse across a 25 ft vegetated flow path. Downspouts will route runoff from the roof to tightlines where flows will be directed to catch basins just before each of the dispersion trenches. Flows will enter the trenches from the catch basins and disperse over the vegetated flow paths. Other Hard Surfaces: Walkways: Full Dispersion is infeasible for the project because a 1001f vegetated flow path cannot be provided. Permeable Pavers are feasible and proposed for most new and replaced walkways on the subject lot. Proposed walkways on -site will be constructed of permeable interlocking concrete pavements (PICP) in accordance with SWMMWW BMP T5.15, ECDC 18.30, and the requirements in the Edmonds Stormwater Addendum (Appendix C, Checklist 5, and Appendix D). PICPs are solid, precast, manufactured modular units. The solid pavers are (impervious) high -strength Portland cement concrete with pervious aggregate between slabs to ensure infiltration of runoff. Rain Garden/Bioretention is infeasible for the proposed project. According to the Edmonds Stormwater Appendix C; "Where the minimum vertical separation of 1 foot to the seasonal high groundwater or other impermeable layer would not be achieved below bioretention that would serve a drainage area less than the above thresholds." The project is unable to maintain the required vertical separation of 1 foot due to shallow groundwater, a sloping site, and the nature of the facilities requiring ponding depths. Sheet Flow Dispersion by way of a Downspout Dispersion System is proposed for the main entry walkway and front entry slab. This area accounts for 204 sf of new hard surface on the south side of the proposed addition. The runoff from this area will be collected with a trench drain and be routed to the 13 ft dispersion trench in the southwestern yard that also mitigates runoff from the adjacent portion of the new roof. As noted in the roof area sections, a total of 3745 sf of mitigated hard surface is accounted for. These trenches have the capacity for this walkway area, as depicted in the plans. On -site Driveway: Full Dispersion is infeasible for the project because a 1001f vegetated flow path cannot be provided. Permeable Pavers are feasible and proposed for the new on -site driveway area on the subject lot. The proposed driveway on -site will be constructed of Pervious Portland cement concrete in accordance with SWMMWW BMP T5.15, ECDC 18.30, and the requirements in the Edmonds Stormwater Addendum (Appendix C, Checklist 5, and Appendix D). Pervious Portland cement concrete is a rigid pavement similar to conventional concrete that uses a cementitious material to bind aggregate together to form spaces between the aggregate in the pavement surface and allow water to infiltrate. The proposed driveway within the subject lot slopes at 6% and check dams are proposed. It should be noted that a Interlaken Engineering and Design, PLLC trench drain is proposed at the garage entrance to ensure incidental runoff does not enter the garage. This trench drain will flow to the nearest dispersion trench. Off -site Driveway: Full Dispersion is infeasible for the project because a 1001f vegetated flow path cannot be provided. Permeable Pavers are infeasible as the city of Edmonds does not allow Permeable Pavers in the ROW. Runoff from the portion of the driveway within the right-of-way (273f sf) will sheet flow to the west. Footing Drains: Footing drains from the proposed foundation and walls will be routed to the nearest dispersion facility. Footing drain shall be perforated pipe only around the proposed house and along proposed retaining walls and solid walled pipe for conveyance. Footing drains and roof drain tightlines shall not tie together at the home. All connections between footing drains and roof drain tightlines shall be at least 10' from the residence. Appendix 1) Geotechnical Evaluation prepared by Cobalt Geosciences dated August 7, 2022. COBALT G E 0 S C I E N C E S August 7, 2022 Ron and Maureen Haider maureen (&haiderconstruction.com ron0haiderconstruction.com RE: Geotechnical Evaluation Proposed Additions 915 Brookmere Street Edmonds 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, stormwater management, grading, and earthwork. Site Description The site is located at 915 Brookmere Street in Edmonds, Washington. The site consists of one rectangular shaped parcel (No. 005489o0000302) with a total area of about 0.24 acres. The western portion of the site is developed with a residence, detached structure, and driveway. The site areas are vegetated with grasses, bushes and variable diameter trees. The site slopes downward from east to west at magnitudes of o to io percent and relief of about 9 feet. The site is bordered to the north, east, and west by residences, and to the south by Brookmere Street. The proposed development includes additions to the existing residence. Stormwater will include infiltration or other systems depending on feasibility. Site grading may include cuts and fills of 3 feet or less and foundation loads are expected to be light. We should be provided with the final plans to verify that our recommendations remain valid and do not require updating. Area Geology The Geologic Map of the Edmonds East Quadrangle, indicates that the site is near the contacts between Vashon Advance Outwash, Quaternary Till Undivided, and Transitional Beds. Glacial Till includes mixtures of silt, sand, clay, and gravel. These materials are usually impermeable and are typically dense to very dense below a weathered zone. Vashon Advance Outwash includes fine to medium grained sand with lesser gravel and silt/clay interbeds. Transitional Beds include silt and clay that underlie the outwash. These deposits are nearly impermeable. Soil & Groundwater Conditions As part of our evaluation, we excavated a test pit and hand boring within the property areas, where accessible. www.cobaltgeo.com (2o6) 331-1097 August 7, 2022 Page 2 of 10 Geotechnical Evaluation The explorations encountered approximately 6 inches of vegetation and topsoil underlain by approximately 2 to 3 feet of loose to medium dense, silty -fine to medium grained sand with gravel (Weathered Glacial Till). These materials were underlain by medium dense to dense, silty -fine to medium grained sand trace gravel (Glacial Till?), which continued to the termination depths of the explorations. Groundwater was observed at 7 feet below grade in TP-1. Groundwater was not encountered in the hand boring. Groundwater levels will likely be higher during the wet season based on our observations of mottling beginning about 3.5 feet below grade. 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 site is underlain by Alderwood-Urban land complex (2 to 8 percent slopes). These soils would have a slight to moderate erosion potential in a disturbed state depending on the slope magnitude. 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 ist. Erosion control measures should be in place before the onset of wet weather. 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 medium dense to very dense soils within the upper too feet. We referenced the U.S. Geological Survey (USGS) Earthquake Hazards Program Website to obtain values for Ss, Sl, F,, 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 7-16. 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 1.o sec. (g) Fa Fv SDs SD1 D 1.291 0.455 1.0 Null o.861 Null 0.55 www.cobaltgeo.com (2o6) 331-1097 August 7, 2022 Page 3 of 10 Geotechnical Evaluation 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 weathered and unweathered glacial till which becomes denser with depth. The proposed additions to the existing residential structure may be supported on shallow foundation systems bearing on medium dense or firmer native soils or on structural fill placed on the native soils. Local overexcavation or recompaction of fill and loose weathered native soils may be necessary depending on the proposed elevations and locations of the new footings. Widespread infiltration of runoff is not feasible based on the soil and groundwater conditions. There is inadequate clearance above the restrictive layer for infiltration trenches or drywells. We recommend connection of runoff devices to City infrastructure. If this is not possible, dispersion devices, curb cuts, splash blocks and potentially, rain gardens and permeable pavements may be feasible. The feasibility will depend on system locations and elevations along with proximity to new or existing building. We can provide additional recommendations once a civil plan has been prepared. 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 18 inches. Deeper excavations will be necessary below large trees 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 3 feet or less for foundation and most of the utility placement. Any deeper 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 above the groundwater table. www.cobaltgeo.com (2o6) 331-1097 August 7, 2022 Page 4 of 10 Geotechnical Evaluation 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. If groundwater is encountered, temporary excavations may need to be decreased to 1.51-1:1V or lower. 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. 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 additions to the existing residence may be supported on shallow spread footing foundation systems bearing on undisturbed medium 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. We should verify soil conditions during foundation excavation work. Local overexcavation may be required in areas where fill is present. 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 2,000 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. www.cobaltgeo.com (2o6) 331-1097 August 7, 2022 Page 5 of io Geotechnical Evaluation 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 0.40 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 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 new cast in place 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 275 pcf EFD+ Soil -Footing Coefficient of Sliding Friction (Allowable; includes F.S. = 1.5) 0.40 www.cobaltgeo.com (2o6) 331-1097 August 7, 2oaz Page 6 of io Geotechnical Evaluation `H is the height of the wall; Increase based on one in 500 year seismic event (io 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. We recommend that the backfill be compacted to at least 90 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. Stormwater Management Feasibility The site is underlain by relatively dense glacial till. We observed mottled soils at shallow depths, indicating that perched groundwater may be present during the wet season. We also observed groundwater at relatively shallow depths and estimate a seasonal high groundwater table to be within 4 feet of the ground surface in most areas. Infiltration is not feasible due to the soil and anticipated groundwater conditions at the site. There is inadequate clearance for widespread infiltration systems above the restrictive layer and/or groundwater. We recommend direct or perforated connection of runoff devices to City infrastructure or an approved conveyance. We can provide additional recommendations upon request and once civil plans have been prepared. Other options could include splash blocks, curb cuts, and dispersion devices. Permeable pavements for flow control may be feasible in some areas depending on the proposed depth of cuts for driveway or walkway areas. Rain gardens may be feasible depending on their location and elevations. The factored infiltration rate within the upper weathered soils would be 0.3 to 0.4 inches per hour. Near -surface systems could be feasible in areas that have very limited cuts (less than 1.5 feet from existing grades). We should be provided with final plans for review to determine if the intent of our recommendations has been incorporated or if additional modifications are needed. www.cobaltgeo.com (206) 331-1097 August 7, 2ozz Page 7 of io Geotechnical Evaluation Slab -on -Grade We recommend that the upper 12 inches of the native soils within slab areas be re -compacted to at least 95 percent of the modified proctor (ASTM D1557 Test Method). Any fill should be removed and replaced with compacted structural fill. 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 18o 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. 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 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. www.cobaltgeo.com (2o6) 331-1097 August 7, 2022 Page 8 of io Geotechnical Evaluation • 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. CONSTRUCTION FIELD REVIEWS Cobalt Geosciences should be retained to provide part time field review during construction in order to verify that the soil conditions encountered are consistent with our design assumptions and that the intent of our recommendations is being met. This will require field and engineering review to: ■ Monitor and test structural fill placement and soil compaction ■ Observe bearing capacity at foundation locations ■ Observe slab -on -grade preparation ■ Observe stormwater system placement ■ Monitor foundation drainage placement ■ Observe excavation stability Geotechnical design services should also be anticipated during the subsequent final design phase to support the structural design and address specific issues arising during this phase. Field and engineering review services will also be required during the construction phase in order to provide a Final Letter for the project. www.cobaltgeo.com (206) 331-1097 August 7, 2022 Page 9 of io Geotechnical Evaluation CLOSURE This report was prepared for the exclusive use of Ron and Maureen Haider and their 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 Ron and Maureen Haider 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 HONry9 WAsy/y CL 54896 sCISTER�� ZONAL�- 8/%/2022 Phil Haberman, PE, LG, LEG Principal Vlashi \��9mmmn9.?odoPN .� nz 0 ��od Geo t PHILUP HASEMAN I www.cobaltgeo.com (2o6) 331-1097 August 7, 2022 Page io of io 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 parry 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 parry 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 Provided Site Plan p' o`V Op WfV 1a7E5 uMraTxrzrolMffl ��' FV" i _ TSO. fJ! I/ HB-1 OLgY: DYES IAfOEw�PPtn.ID cNlE-smut cAA'AE Jf2-2' nfxJa �Ifu' •I I I (S7 \ I II f 44 _ I \ TP-i � RA. g \^O± n Assweer \ 1 nru .nn-zo ` jl N E !xJenl.abYW?.3 y Te a II !M.W' LAB \ LAIN EP Aw Irate -7 Mais ue.o (e'Pls) � / BOT /)f101 _ pl S�3J•1T£ i _—`--�_.---- — aim MAC 6 HAA,/1MA5!Ew SD M LAW MACY J -_ E ! El ),Soo' 1 '- N(MM lE1E ACT / — -� -- C----C E C-__ �__ __ p .-�- e --�w--�-pr--LW w-- / PO. Rio QMfT IAMtm w-`--c --- -w-- �'--w---�w--__ SSW s Iale o i I RMI IT7J I-'O+w rr� e-w' i ww IN.15 I 1 A9wMLr IIOAO m-f I I I EFfrl.!(Izr1cJ 1 I wwl I19Q6 R f , 1 2 w It' .S�2 ipl`oCl v1 1 I K N t/6t (d .rt_sp__ r__�__s� ___ 55---- 55____ 55 ew/fS�((fT'PYC)) I f1oT I/�.J —_ I -�-_ �gp ---- 5p____�____ --- Sp-rt _,gp____Sp___-� ® / _ - ASNTALT� Y'f70 HB-1 TP-1 Approximate Test Pit and lr Provided topographic Hand Boring survey Location Not to Scale Cobalt Geosciences, LLC Proposed Development SITE MAP P.O. Box 82243 915 Brookmere Street Kenmore, WA 98028 COBALT .` Edmonds, Washington FIGURE i (206) 331-1097 www.cobaltgeo.com cobaltgeo(&gmail.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 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 Attachment North Bend, WA 98045 • _ (2o6) 331-1097 GEOSCIENCES www.cobaltgeo.com Philpcobaltgeo.com Unified Soil Classification System (USCS) MAJOR DIVISIONS SYMBOL TYPICAL DESCRIPTION Clean Gravels Gw Well -graded gravels, gravels, gravel -sand mixtures, little or no fines Gravels (more than 50% (less than 5% fines) GP Poorly graded gravels, gravel -sand mixtures, little or no fines COARSE GRAINED SOILS of coarse fraction retained on No. 4 sieve) Gravels with Fines (more than 12% fines) GM Silty gravels, gravel -sand -silt mixtures GC Clayey gravels, gravel -sand -clay mixtures (more than 50% retained on Clean Sands :•: sw Well -graded sands, gravelly sands, little or no fines No. 200 sieve) Sands (50% or more of coarse fraction (less than 5% fines) sP Poorly graded sand, gravelly sands, little or no fines passes the No. 4 sieve) Sands with Fines sM Silty sands, sand -silt mixtures (more than 12% fines) sc Clayey sands, sand -clay mixtures ML Inorganic silts of low to medium plasticity, sandy silts, gravelly silts, FINE GRAINED (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 Organic rganic oL 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 elastic silt CH Inorganic clays of medium to high plasticity, sandy fat clay, or gravelly fat clay Organic OHOrganic clays of medium to high plasticity, organic silts HIGHLY ORGANIC SOILS Primarily organic matter, dark in color, and organic odor PT Peat, humus, swamp soils with high organic content (ASTM D4427) Classification of Soil Constituents 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 (o.o8 mm) Sand -Fine #200 to #40 (o.o8 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) 1 Moisture Content Definitions 1 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, WA 98028 Soil Classification Chart Figure Ci (2o6) 331-1097 _ www.cobaltgeo.com cobaltgeo(&gmail.com Test Pit TP-1 Date: July 2022 Depth: 8' Groundwater: 7' Contractor: Jim Elevation: N/A Logged By: PH Checked By: SC N o Moisture Content (%) Plastic I I Liquid U L E 3 Limit Limit � � N Material Description ? o DCP Equivalent N-Value o C 0 10 20 30 40 50 Cobalt Geosciences, LLC Proposed Additions P.O. Box 82243 COBALT g15 Brookmere Street Test Pit Kenmore, WA 98028 (2o6) 331-109�7 — Edmonds, Washington Logs www.cobaltgeo.com cobaltgeo@gmail.com _______Topsoil and Grass 1 • SM Loose to medium dense, silty -fine to medium grained sand w Hand Boring HB-1 Date: July 2022 Depth: 6' Groundwater: None Contractor: Elevation: Logged By: PH Checked By: SC 0) o Moisture Content (%) a) 0 U -0Plastic E 3 Limit Liquid Limit � � L Q N Material Description DCP Equivalent N-Value E ? o o G 0 10 20 30 40 50 --------------------------------------------- Loose to medium dense, silty -fine to medium grained sand with ------ 1 ---- ,--- SM gravel, dark yellowish brown, moist. (Weathered Glacial Till?) 2 Local gradational with SP-SM 3 ------ ---- _ _ Mottled below 33" --------------------------------------------- 4 SM Dense, silty -fine to medium grained sand with gravel, mottled yellowish brown to grayish brown, moist. (Glacial Till?) 5 Fz End of Hand Boring 6' 7 8 9 10 Cobalt Geosciences, LLC - Proposed Additions Hand Boring P.O. Box 82243 COBALT 915 Brookmere Street Logs Kenmore, WA 98028 (2o6) 331-1097 GEOSCIENCES Edmonds, Washington www.cobaltgeo.com cobaltgeopgmail.com COBALT G E 0 S C I E N C E S August 31, 2023 Ron and Maureen Haider maureen (&haiderconstruction.com ron0haiderconstruction.com C/O Jessica Kashian Jessicaoalexandraimmeldesign.com RE: Plan Review Proposed Additions 915 Brookmere Edmonds Washington Cobalt Geosciences, LLC P.O. Box 1792 North Bend, WA 98045 In accordance with your authorization, Cobalt Geosciences, LLC has prepared a plan review letter for the project. We have reviewed the provided civil plans by Interlaken Engineering and Design, PLLC dated July 28, 2023. The plan shows the locations of dispersion devices and other stormwater systems. We note that the driveway will be surfaced with permeable pavers with a slope toward the structure. There is a drain to be located between the driveway and residence, directing runoff to the adjacent dispersion trench. Provided the drains are constructed properly and in accordance with the approved plans, this is a suitable and feasible system. We should be on site to verify aspects of the construction. These include but are not limited to soil bearing for footing areas, dispersion trench placement, foundation drainage placement, fill compaction (if utilized), slab on grade preparation, and any temporary excavations. Sincerely, Cobalt Geosciences, LLC 44- L Phil Haberman, PE, LG, LEG Principal www.cobaltgeo.com (2o6) 331-1097