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REVIEWED-BLD2023-0623+Geotechnical_Report+5.22.2023_2.01.14_PM+3560883RECEIVED Jun 05 2023 REVIEWED .................................................of oEP�,ME BY BLD2023-0623 CITY OF EDMONDS BUILDING DEPARTMENT COBALT..... , ... ,...................................... Cobalt Geosciences, LLC G E 0 S C I E N C E S P.O. Box82243 Kenmore, Washington 98028 May 22, 2023 Landsverk Quality Homes Attn: Duane Landsverk & Joseph Rowett 24113 56th Avenue West Mountlake Terrace, Washington RE: Plan Review Proposed Development 19515 94th Place West Edmonds, Washington In accordance with your authorization, Cobalt Geosciences, LLC has prepared a plan review letter for the proposed residence. We have reviewed the provided civil plan sby Omeaga Engineering, Inc. which shows stormwater system and building locations/setbacks from the steep slopes. The plans appear to incorporate the relevant recommendations from our geotechnical report for the project. We have no comments at this time. We should be on site during construction to verify soil bearing of foundation areas, infiltration system soil conditions, temporary excavations, building locations, erosion control, any subsurface drainage placement, and soil compaction. Please contact us if you have any questions. Sincerely, Cobalt Geosciences, LLC Pa�NONryq fiF WAsy/y�<^ -off 54896 O, 'PECIST ERGO SS�ONAL 5/22/2023 Phil Haberman, PE, LG, LEG Principal V4 a s h�^ � fC9 25i3� Enpmmtnng �?ndc�p�� �� \ ° ? 0 d Cie° PH`IM.1l"' HABEWAAN www.cobaltgeo.com (2o6) 331-1097 COBALT G E 0 S C I E N C E S January 14, 2023 Landsverk Quality Homes Attn: Duane Landsverk & Joseph Rowett 24113 56th Avenue West Mountlake Terrace, Washington RE: Geotechnical Evaluation Proposed Development 19515 94th Place West 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 19515 94th Place West in Edmonds, Washington. The site consists of one irregularly shaped parcel (No. 004346o00058o1) with a total area of about 1.19 acres. We understand that the project will likely include boundary line adjustments with the parcel to the east and south (No. 00434600005805), with the new parcel including steep slope areas to the south. The western portion of the site is developed with a residence. There is a driveway extending north of the residence providing access for the subject property and property to the east (same owner). The remainder of the site is undeveloped and vegetated with grasses, ferns, ivy, blackberry vines, bushes, and variable diameter trees. The site and developed portions of the property to the east are situated on a west -trending ridge with low slope magnitudes (5 to 12 percent). There are locally steep and very steep natural and partially graded slopes extending downward from the ridge to the north, west, and south. We describe these areas as follows: The steep slopes near the west property line are mostly the result of prior grading associated with the access roadway. This slope is 15 to 20 feet tall and has magnitudes of 50 to 8o percent. The slope is locally faced with rockery walls. The steep slopes near the southern portion of the site and adjacent parcel have magnitudes of up to vertical to overturned as a result of sloughing. Overall, the slope magnitudes range from 6o to 150 percent where vegetation is present. There are rockery walls at the toe of this slope near the area roadways. The areas near the top of the slope have exposed outwash sands in sloughs. These areas are 3 to 7 feet in height with the height increasing to the east. There are trees and other understory hanging over the sloughed zones. The total height of these slopes is 40 to 6o feet. www.cobaltgeo.com (2o6) 331-1097 January 14, 2023 Page 2 of 14 Geotechnical Evaluation The steep slope areas to the north of the site have magnitudes of 40 to 90 percent and relief of 40 to 50 feet from west to east. These slopes continue to the north at lower magnitudes. There are apparent wetland areas and standing water further north in the low lying areas. The site is bordered to the north and east by residential properties, to the west by an access road and residences, and to the south by right of way and Puget Drive. As noted, the proposed development includes boundary line adjustments between the subject parcel and property to the east followed by construction of a new residence, driveway, and garage in the east -central portion of the site. The existing residence will removed. 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 if our recommendations remain valid or require updating. Area Geology The Geologic Map of the Edmonds East and West Quadrangles, indicates that the site is underlain by Vashon Glacial Till and near the contacts with Vashon Advance Outwash. Vashon 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. These deposits are often underlain by Vashon Advance Outwash. The outwash includes fine to medium grained sand with minor gravel and silt. These deposits are typically permeable. Soil & Groundwater Conditions As part of our evaluation, we excavated a test pit within the property areas, where accessible. We also traversed steeper slope areas and advanced shallow hand borings/probes to determine the approximate depth to medium dense soils and general soil composition. The test pit encountered approximately 6 inches of grass and topsoil underlain by approximately 3.5 feet of loose to medium dense, silty -fine to medium grained sand with gravel (Weathered Advance Outwash). These materials were underlain by medium dense to dense, fine to medium grained sand trace to with gravel (Vashon Advance Outwash), which continued to the termination depths of the exploration. Groundwater was not encountered during the exploration work. There is a very slight chance that interflow could develop in some areas of the site on finer grained soils. In general, outwash sands are well draining. Groundwater is most often found at the base of the advance outwash, likely 50 or more feet below site elevations. Based on our review of nearby explorations and slope reconnaissance, regional groundwater is likely present at an elevation of about 210 to 215 feet above sea level. This is about 8o feet below site elevations. 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. www.cobaltgeo.com (206) 331-1097 January 14, 2023 Page 3 of 14 Geotechnical Evaluation Steep Slope and Landslide Hazard Areas Most municipal codes designate slope areas with magnitudes greater than 40 percent and relief of at least io feet as potentially geologically hazardous. The site contains steep slope and erosion hazard areas due to the combination of steeper topography and presence of coarser outwash sands. The site and developed portions of the property to the east are situated on a west -trending ridge with low slope magnitudes (5 to 12 percent). There are locally steep and very steep natural and partially graded slopes extending downward from the ridge to the north, west, and south. We describe these areas as follows: The steep slopes near the west property line are mostly the result of prior grading associated with the access roadway. This slope is 15 to 20 feet tall and has magnitudes of 50 to 8o percent. The slope is locally faced with rockery walls. The steep slopes near the southern portion of the site and adjacent parcel have magnitudes of up to vertical to overturned as a result of sloughing. Overall, the slope magnitudes range from 6o to 150 percent where vegetation is present. There are rockery walls at the toe of this slope near the area roadways. The areas near the top of the slope have exposed outwash sands in sloughs. These areas are 3 to 7 feet in height with the height increasing to the east. There are trees and other understory hanging over the sloughed zones. The total height of these slopes is 40 to 6o feet. The steep slope areas to the north of the site have magnitudes of 40 to 90 percent and relief of 40 to 50 feet from west to east. These slopes continue to the north at lower magnitudes. There are apparent wetland areas and standing water further north in the low lying areas. During our site visit, we observed the overall surface condition of the steep slope areas within and near the site. There is evidence of surface sloughing of outwash sands in the upper portions of the south slope. These areas are consistent with peel off associated with oversteepened slopes through prior grading and erosion. The toe of this slope has been cut as part of area grading for roadways. There is a rockery at the toe of this slope. We discussed historic slide activity on a west -facing steep slope located several hundred feet northwest of the site on an adjacent parcel. We understand that there were drain pipes allowing surface water to saturate the near surface weathered outwash sands. This sloughing resulted in deposition below the slope. We note no similar sloughing on the west or north slopes near the site. We did observe evidence of minor soil creep as indicated by curved trunks of some of the trees on the north and south slope areas. Soil creep is a natural process in which soil very slowly migrates downward and result in local curvature of trees and other vegetation. Soil creep occurs in most slope areas of magnitudes greater than about 50 percent. Code Information 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; www.cobaltgeo.com (2o6) 331-1097 January 14, 2023 Page 4 of 14 Geotechnical Evaluation 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 currently proposed project includes minimal excavation work associated with new foundation areas only. The work will not occur in or near steep slope or significant erosion hazard areas and as such, can be completed without adversely affecting these areas and any geologic hazards. This report provides recommendations to facilitate safe construction, including temporary excavation recommendations, fill compaction and placement, drainage and foundation design. It is our opinion that this work will not affect global or local stabilily provided work is monitored by the geotechnical engineer and erosion control measures are in place during construction. The proposed construction will not increase the threat of geologic hazards on adjacent pro even rties, will not impact other critical areas, and are safe as designed under anticipated conditions. 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 22.80.o6o, Development standards — General requirements, and the specific following requirements: 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; We recommend a minimum building setback of 25 feet from the steep slopes near the south margin of the site: a minimum building setback of 15 feet from the steep slope near the west property line: and a minimum building setback of 25 feet from the steep slope near the north property line. Infiltration systems should be located at least 40 feet from the top of the south steep slope, �o feet from the west steep slope, and 40 feet from the north property line. 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; We recommend that the areas within io feet of the top of the steep slope areas remain fu11X vegetated. This io foot buffer is included within the building setback and is not additive. 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; www.cobaltgeo.com (2o6) 331-1097 January 14, 2023 Page 5 of 14 Geotechnical Evaluation 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 controlled, 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; 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 project scope does not pose a risk to critical areas or the need to increase buffers on adjacent properties. The residence will be located well away from steep slope areas. Factors of safety against landslide movements are well above minimum values based on the soil Wes, topography, and locations of the development relative to the slope systems. 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 1st 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; www.cobaltgeo.com (2o6) 331-1097 January 14, 2023 Page 6 of 14 Geotechnical Evaluation 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. Erosion Hazard The Natural Resources Conservation Services (NRCS) maps for Snohomish County indicate that the site is underlain by Alderwood Everett gravelly sandy loams (25 to 70 percent slopes). These soils would have a moderate to very severe 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 below the fill 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, 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 7-16. 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 F„ SDS SDl D 1.294 0.456 1.0 Null 0.862 Null 0.552 www.cobaltgeo.com (206) 331-1097 January 14, 2023 Page 7 of 14 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 primarily Vashon Advance Outwash. There appear to be areas of Vashon Glacial Till near the western margin and on adjacent parcels to the west. The outwash becomes denser with depth. 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. Infiltration appears to be feasible in the outwash sands that underlie the site at variable depths. The depth to outwash will vary with location and drywells will likely be the most suitable type of infiltration system. We can provide additional recommendations upon request. We should review the plans to verify suitability of the system locations and elevations. We recommend a minimum building setback of 25 feet from the steep slopes near the south margin of the site; a minimum building setback of 15 feet from the steep slope near the west property line; and a minimum building setback of 25 feet from the steep slope near the north property line. Infiltration systems should be located at least 40 feet from the top of the south steep slope, 30 feet from the west steep slope, and 40 feet from the north property line. 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 larger trees, foundation systems, and in any areas underlain by undocumented fill. The native soils consist of silty -sand with gravel and poorly graded sand. The poorly graded sands and native soils are likely suitable for use as fill provided all debris and organic materials are removed. These 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. www.cobaltgeo.com (2o6) 331-1097 January 14, 2023 Page 8 of 14 Geotechnical Evaluation 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 1.511:1V (Horizontal:Vertical) in loose native soils and fill, 111:1V in medium dense native soils and 3/41-1:1V in dense to very dense native soils. 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. 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 structure may be supported on a shallow spread footing foundation system 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. It may be feasible to recompact outwash to a firm and unyielding condition depending on the moisture content. 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. www.cobaltgeo.com (2o6) 331-1097 January 14, 2023 Page 9 of 14 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 0.4o 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 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) 14H* (Uniform Distribution) 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 www.cobaltgeo.com (2o6) 331-1097 January 14, 2023 Page io of 14 Geotechnical Evaluation 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 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. Stormwater Management Feasibility The site is underlain by Vashon Advance Outwash. Infiltration is generally feasible in the outwash sands. Infiltration is not typically feasible or recommended in glacial till soils, if encountered in some locations. The design infiltration rate was determined by applying correction factors to the observed infiltration rate as prescribed in Volume III, Section 3.3.6 of the DOE. The observed rate must be reduced through appropriate correction factors for site variability (CFv), uncertainty of test method (CFT), and degree of influent control (CFM) to prevent siltation and bio-buildup. It should be noted that construction traffic or other disturbance to the target infiltration area could compact the soil, which may decrease the effective infiltration rates. The correction factors and resulting design infiltration rate are also shown in the table below. Test Pit Sample Observed Correction Factors Design Number Depth (ft) Infiltration Infiltration Rate (in/hr) Rate CFv CFT CFM (in/hr) TP-1 5 3.6 0.8 0.5 0.9 1.3 Widespread infiltration is feasible in the outwash sands below about 4 feet (in the test pit location areas). This soil consists of coarser outwash sands. Drywells will likely be most suitable option for stormwater management. www.cobaltgeo.com (206) 331-1097 January 14, 2023 Page 11 of 14 Geotechnical Evaluation We must be on site to verify soil conditions in the drywells during construction. All systems must penetrate into the sands at least 6 inches. The soils are consistent with Medium Sand per the USDA textural triangle if a prescriptive sizing is used. The depth to suitable outwash will vary across the site and overexcavation of till or silty -sands will be required to achieve the outwash soils at depth. We must be on site to confirm soil conditions. 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. Slab -on -Grade We recommend that the upper 12 inches of the native soils within slab areas below the fill 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 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. www.cobaltgeo.com (2o6) 331-1097 January 14, 2023 Page 12 of 14 Geotechnical Evaluation 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, 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 ■ Verify soil conditions in infiltration systems if utilized ■ Monitor foundation drainage placement ■ Observe excavation stability www.cobaltgeo.com (2o6) 331-1097 January 14, 2023 Page 13 of 14 Geotechnical Evaluation 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. CLOSURE This report was prepared for the exclusive use of Landsverk Quality Homes 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 Landsverk Quality Homes 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 r H0Nr� P �F WAs�.q ✓ •1 �t 54896 <� 0�FssLIST ZONAL�- 1/14/2023 Phil Haberman, PE, LG, LEG Principal www.cobaltgeo.com (2o6) 331-1097 January 14, 2023 Page 14 of 14 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 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 19516 ' m All 111111 AA 19511 19518 t9 51 1 52WIN 19524 �� 52 PUGE'T DR Sno. Co. Gis Map TP-i N Approximate Test Pit Location Not to Scale Cobalt Geosciences, LLC Proposed Residence SITE MAP P.O. Box 82243 _ . 19515 94th Place West Ken 3 e, 0978028 = — Edmonds, Washington FIGURE 1 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: December 2022 Depth: 12' Groundwater: None Contractor: Client provided Elevation: N/A Logged By: PH Checked By: SC N 0) o Moisture Content (%) u Q E Plastic I Limit I Liquid Limit � � L Material Description DCP Equivalent N-Value o C ? o O 0 10 20 30 40 50SP Cobalt Geosciences, LLC Proposed Residence P.O. Box 82243 COBALT 19515 94th Place West Test Pit Kenmore, WA 98028 (2o6) 331-1097 GEOSCIENCES Edmonds, Washington Logs www.cobaltgeo.com cobaltgeopgmail.com _______Topsoil and Grass 1 • ' ' : • SP/ Loose to medium dense, silty -fine to medium grained sand trace - • • SM gravel yellowish brown to grayish brown, moist. 2 ;l;-:� � • (Weathered Advance Outwash) 3 • tif . • • ; •• Medium dense to dense, fine to medium grained sand trace gravel 5 grayish brown, moist. ;1,-••'.;:' (Advance Outwash) 10 End of Test Pit 12'