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REVIEWED BLD BLD2021-0895+GeoReport LOCKED+6.26.2021_10.12.55_AM+2271114COBALT 0 E 0 S C I E N C E S Geotechnical Investigation Proposed Residence 129 Skyline Drive Edmonds, Washington December 16, 2020 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON Table of Contents 1.o INTRODUCTION............................................................................................................. 1 2.0 PROJECT DESCRIPTION.............................................................................................. 1 3.0 SITE DESCRIPTION....................................................................................................... 1 4.o FIELD INVESTIGATION............................................................................................... 2 4.1.1 Site Investigation Program................................................................................... 2 5.0 SOIL AND GROUNDWATER CONDITIONS.............................................................. 2 5.1.1 Area Geology........................................................................................................ 2 5.1.2 Groundwater........................................................................................................ 2 6.o GEOLOGIC HAZARDS................................................................................................... 3 6.1 Steep Slope Hazard.............................................................................................. 3 6.2 Erosion Hazard.................................................................................................... 6 6.3 Seismic Hazard.................................................................................................... 6 7.o DISCUSSION................................................................................................................... 6 7.1.1 General.................................................................................................................6 8.o RECOMMENDATIONS.................................................................................................. 7 8.1.1 Site Preparation................................................................................................... 7 8.1.2 Temporary Excavations........................................................................................ 7 8.1.3 Erosion and Sediment Control.............................................................................. 8 8.1.4 Foundation Design............................................................................................... 9 8.1.5 Reinforced Concrete Retaining Walls..................................................................10 8.1.6 Stormwater Management....................................................................................11 8.1.7 Slab-on-Grade.....................................................................................................11 8.1.8 Utilities...............................................................................................................12 8.1.9 Groundwater Influence on Construction.............................................................12 9.o CONSTRUCTION FIELD REVIEWS...........................................................................12 moCLOSURE...................................................................................................................13 LIST OF APPENDICES Appendix A — Statement of General Conditions Appendix B — Figures Appendix C — Exploration Logs GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON December 16, 2020 i.o Introduction COBALT GEOSCIENCES In accordance with your authorization, Cobalt Geosciences, LLC (Cobalt) has completed a geotechnical investigation for the proposed residence located at 129 Skyline Drive in Edmonds, Washington (Figure 1). The purpose of the geotechnical investigation was to identify subsurface conditions and to provide preliminary geotechnical recommendations for foundation design, retaining walls, earthwork, soil compaction, and suitability of the on -site soils for use as fill. The scope of work for the geotechnical investigation consisted of a site investigation followed by engineering analyses to prepare this report. Recommendations presented herein pertain to various geotechnical aspects of the proposed development, including foundation design, retaining walls, drainage, and earthwork. 2.0 Project Description The project includes construction of a new residence within the property. Based on topography, a daylight basement will likely be incorporated into the design. A new driveway will extend into the property from the north. We anticipate that structural loads will generally be light and site grading will include cuts and fills on the order of io feet or less for daylight basement construction. We should be provided with the plans once they become available so that we may update our recommendations, if necessary. Stormwater runoff will be infiltrated, if determined to be feasible. 3.0 Site Description The site is located at 129 Skyline Drive in Edmonds, Washington (Figure 1). The property consists of one irregularly shaped parcel (No. 00423000005300) with a total area of 0.32 acres. The property is currently undeveloped and vegetated with grasses. The site slopes downward from east to west at magnitudes of io to about 6o percent and total relief of about 27 feet. The steeper slopes are located in the western third to half of the property. There is a rockery in the western half of the property south of the subject property. This wall is 4 to io feet tall near the southwest corner of the subject property. The site is bordered to the north and east by Skyline Drive, to the south by a residence, and to the west by right-of-way and residences. PO Box 82243 Kenmore, WA 98028 cobaltgeoRgmail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON December 16, 2020 4.o Field Investigation 4.1.1 Site Investigation Program COBALT GEOSCIENCES The geotechnical field investigation program was completed on December 1, 202o and included excavating two test pits within the property for subsurface analysis. The soils encountered were logged in the field and are described in accordance with the Unified Soil Classification System (USCS). A Cobalt Geosciences field representative conducted the explorations, collected disturbed soil samples, classified the encountered soils, kept a detailed log of the explorations, and observed and recorded pertinent site features. The results of the test pit sampling are presented on the exploration logs enclosed in Appendix C. 5.0 Soil and Groundwater Conditions 5aa Area Geology The site lies within the Puget Lowland. The lowland is part of a regional north -south trending trough that extends from southwestern British Columbia to near Eugene, Oregon. North of Olympia, Washington, this lowland is glacially carved, with a depositional and erosional history including at least four separate glacial advances/retreats. The Puget Lowland is bounded to the west by the Olympic Mountains and to the east by the Cascade Range. The lowland is filled with glacial and non -glacial sediments consisting of interbedded gravel, sand, silt, till, and peat lenses. The Geologic Map of the Edmonds East and West Quadrangle, indicates that the site is near the contacts between Vashon Glacial Till and Vashon Advance Outwash. Vashon Advance Outwash consists of sand with variable amounts of gravel which are generally dense to very dense and relatively permeable. Vashon Glacial Till includes a mixture of silt, sand, gravel, and clay. These deposits are typically dense to very dense below a weathered zone. Explorations The test pits encountered approximately 6 inches of topsoil and grass underlain by approximately 3 to 5 feet of loose to medium dense, silty -fine to medium grained sand with gravel (Fill). This layer was underlain by medium dense to dense, silty -fine to medium grained sand with gravel (Glacial Till), which continued to the termination depths of the test pits. 5.1.2 Groundwater Groundwater was not encountered during our investigation. be encountered within the dense to very dense glacial till encountered, it would be expected to be light in volume. Light amounts of perched groundwater could during the wet season. If groundwater is 2 PO Box 82243 Kenmore, WA 98028 cobaltgeoRgmail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON December 16, 2020 COBALT GEOSCIENCES 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. 6.o Geologic Hazards 6.1 Steep Slope Hazards The site contains local steep slope areas based on magnitude and height. These areas are underlain by fill and glacial till. During our site visit, we observed the areas for signs of instability. We did not observe areas of erosion, landslide activity, or other signs of instability or historic landslide activity. The site appears stable and is underlain by dense glacial till which is not highly susceptible to global instability. The proposed construction includes a new residence with daylight basement level. We recommend a minimum effective setback of io feet from slopes with magnitudes of 30 percent or greater. The effective setback is the horizontal distance measured from the outside of the foundation areas laterally to the adjacent slope face. Deeper foundation embedment may be required for the footings in the western margin of the new residence (downslope side). Relevant code items with discussion for steep slope and erosion hazards are as follows: 23.80.o6o Development standards — General requirements. A. Alterations of geologically hazardous areas or associated buffers may only occur for activities that: 1. Will not increase the threat of the geological hazard to adjacent properties beyond predevelopment conditions; 2. Will not adversely impact other critical areas; 3. Are designed so that the hazard to the project is eliminated or mitigated to a level equal to or less than predevelopment conditions; and 4. Are certified as safe as designed and under anticipated conditions by a qualified engineer or geologist, licensed in the state of Washington. The site is situated within and contains moderately steep slope areas that were created over time through natural erosion along with historic grading activities. The site is not situated within an active or historic landslide and the risk of landslide activity is ver low. ow. The proposed residence will not increase the threat of geologic hazards on adjacent properties, will not impact other critical areas, and are safe as designed under anticipated conditions. The level of risk due to the proposed development will be the same or lower than what is currently present provided temporary and permanent runoff is fully controlled. 3 PO Box 82243 Kenmore, WA 98028 cobaltgeoRgmail.com 206-331-1097 COBALT GEOTECHNICAL INVESTIGATION GEOSCIENCES EDMONDS, WASHINGTON December 16, 2020 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 2,1.80.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; A minimum effective building setback of io feet is recommended. 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; The site has been previously graded and does not contain areas of native vegetation. No specific buffer is required from a geotechnical standpoint. We recommend that all site soils be vegetated or protected from erosion through placement of surface landscaping materials/hardscapes. 3. Alterations. Alterations of an erosion or landslide hazard area, minimum building setback and/or buffer may only occur for activities for which a hazards analysis is submitted and certifies that: a. The alteration will not increase surface water discharge or sedimentation to adjacent properties beyond predevelopment conditions; b. The alteration will not decrease slope stability on adjacent properties; and c. Such alterations will not adversely impact other critical areas; Provided earthwork activities are Derformed in accordance with the aDDroved Dlans, all runoff is full control, and periodic geotechnical oversight is performed, the development will not decrease slope stability on adjacent properties, will not increase surface water discharge or sedimentation bg and 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: 4 PO Box 82243 Kenmore, WA 98028 cobaltgeoRgmail.com 206-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON December 16, 2020 COBALT GEOSCIENCES 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; Based on site topography, underling soil conditions, and our experience with similar projects, the estimated static and seismic factors of safety are well above minimum requirements. The factor of safety for landslide activity will not be affected by the proposed development. The building loads and where they will be located will not affect global stability. Performed with proper erosion control and engineering controls, the proposed development does not pose a risk to critical areas or the need to increase buffers on adjacent properties. 5. Vegetation Retention. Unless otherwise provided or as part of an approved alteration, removal of vegetation from an erosion or landslide hazard area or related buffer shall be prohibited; 6. Seasonal Restriction. Clearing shall be allowed only from May 1st to October 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; 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 5 PO Box 82243 Kenmore, WA 98028 cobaltgeoRgmail.com 2o6-331-1097 COBALT GEOTECHNICAL INVESTIGATION GEOSCIENCES EDMONDS, WASHINGTON December 16, 2020 We concur with the above code items and have no additional comments at this time. 6.2 Erosion Hazard The Natural Resources Conservation Services (NRCS) maps for Snohomish County indicate that the property is underlain by Alderwood-Everett gravelly sandy loams (25 to 70 percent slopes). These soils can have a severe erosion potential when exposed. These soils are included in the Edmonds Municipal Code section 23.80.020 under Al. It is our opinion that typical erosion control measures will be suitable to limit/reduce erosion potential during construction. It is our opinion that soil erosion potential at this project site can be reduced through landscaping and surface water runoff control. Typically, erosion of exposed soils will be most noticeable during periods of rainfall and may be controlled by the use of normal temporary erosion control measures, such as silt fences, hay bales, mulching, control ditches and diversion trenches. The typical wet weather season, with regard to site grading, is from October 31st to April 1st. Erosion control measures should be in place before the onset of wet weather. 6.3 Seismic Hazard The overall subsurface profile corresponds to a Site Class D as defined by Table 1613.5.2 of the 2015 International Building Code (2015 IBC). A Site Class D applies to an overall profile consisting of medium dense or stiff soils within the upper too feet. We referenced the U.S. Geological Survey (USGS) Earthquake Hazards Program Website to obtain values for Ss, S,, FQ, and F,,. The USGS website includes the most updated published data on seismic conditions. The site specific seismic design parameters and adjusted maximum spectral response acceleration parameters are as follows: PGA (Peak Ground Acceleration, in percent of g) Ss 127.30% of g S, 49.8o% of g FA 1.00 Fv 1.502 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 relatively dense glacially consolidated materials that underlie the site have a low potential for liquefaction. 6 PO Box 82243 Kenmore, WA 98028 cobaltgeoRgmail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON December 16, 2020 7.o DISCUSSION 7.1.1 General COBALT GEOSCIENCES It is our opinion that the proposed residence may be supported on a shallow foundation system bearing on medium dense or firmer native soils. These soils will likely be encountered 3.5 to 5.5 feet below existing site elevations in areas underlain by loose native soils and fill materials. The basement excavation work will likely remove most of the undocumented fill that appears to be present over much of the site. Local overexcavation of fill will likely be necessary for shallow foundation elements near the low side of the building (west end). We recommend a minimum effective building setback of io feet from any slopes of 30 percent magnitude or greater. This may require greater foundation embedment along the western margin of the residence. It is our opinion that the proposed development will not increase the potential for soil movement or erosion at the property, and the risk of damage to the proposed development and adjacent properties will be minimal, provided the work is performed in accordance with approved plans and our recommendations and is monitored by the geotechnical engineer. 8.o Recommendations 8.m Site Preparation Trees, shrubs and other vegetation should be removed prior to stripping of surficial organic -rich soil. Based on observations from the site investigation program, it is anticipated that the stripping depth will range from 6 to 18 inches. Deeper excavations should be expected below larger vegetation and where undocumented fill is present. The near -surface soils consist of silty -sand with gravel (Fill and Native Soils). These soils are suitable for use as structural fill if they are compacted when the moisture levels are within a few percent of optimum. This is typically only possible during mid -late summer and early fall when there are extended periods of warm, dry weather. All fill soils should be compacted when they are within 3 percent of the optimum moisture as determined by the ASTM D 1557 proctor analysis test. Note that existing fill may need to be exported as fill typically has higher moisture contents 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. 8.1.2 Temporary Excavations Based on our understanding of the project, we anticipate that the grading could include local cuts on the order of approximately 10 feet or less for basement foundation placement, if proposed. These excavations should be sloped no steeper than 1.5H:1V (Horizontal:Vertical) in loose native or fill soils (upper 3 to 5 feet), 1H:1V in medium dense or firmer native soils and 3/4H:1V in dense to very dense native soils. If an 7 PO Box 82243 Kenmore, WA 98028 cobaltgeoRgmail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON December 16, 2020 COBALT GEOSCIENCES excavation is subject to heavy vibration or surcharge loads, we recommend that the excavations be sloped no steeper than 1.5H:1V, where room permits. The geotechnical engineer should be on site during excavation work to determine safe temporary slope angles. If the proposed residence will have a basement level and be located within about 10 feet of the south or east property lines, temporary shoring will likely be required. We can provide shoring recommendations upon request. 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. 8.1.3 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. 8 PO Box 82243 Kenmore, WA 98028 cobaltgeoRgmail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON December 16, 2020 COBALT GEOSCIENCES • 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. 8.1.4 Foundation Design The proposed residence 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. If structural fill is used to support foundations, then the zone of structural fill should extend beyond the faces of the footing a lateral distance at least equal to the thickness of the structural fill. 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 structures. 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. 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 250 pounds per cubic foot (pcf) acting against the appropriate vertical footing faces (neglect the upper 12 inches below grade in exterior areas). The allowable friction factor and allowable equivalent fluid passive pressure values include a factor of safety of 1.5. The frictional and passive resistance of the soil may be combined without reduction in determining the total lateral resistance. A 1/3 increase in the above values may be used for short duration transient loads. 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 9 PO Box 82243 Kenmore, WA 98028 cobaltgeoRgmail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON December 16, 2020 COBALT GEOSCIENCES 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. 8.1.5 Reinforced 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. 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) Level Ground; 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 2,500 year seismic event (2 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. The 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 fl- 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 10 PO Box 82243 Kenmore, WA 98028 cobaltgeo(d)gmail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON December 16, 2020 COBALT GEOSCIENCES 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. 8.1.6 Stormwater Management The site is underlain by fill which is underlain by weathered and unweathered glacial till. The glacial till has a low permeability which decreases with depth as soil density increases. The site contains steep slope areas which extend off site onto adjacent properties to the west and south. We do not recommend utilizing infiltration or dispersion devices due to the soil conditions and steep topography. We recommend collection of all runoff from new impervious surfaces with direct connection to City stormwater infrastructure. We can provide additional recommendations upon request. 8-1-7 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). If loose fill soils or deep topsoil/roots are encountered, they should be removed and replaced with 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 in Section 8.1. A minimum 4 inch thick capillary break is recommended over the prepared subgrade. This 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. 11 PO Box 82243 Kenmore, WA 98028 cobaltgeoRgmail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON December 16, 2020 COBALT GEQSCIENCES 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. 8.1.8 Groundwater Influence on Construction Groundwater was not encountered in the test pits. There is a very slight chance that light volumes of perched groundwater could be encountered in the dense native soils. If groundwater is encountered during construction, we anticipate that sump excavations and small diameter pumps systems will adequately de -water short-term excavations, if required. Any system should be designed by the contractor. We can provide additional recommendations upon request. 8a.9 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 and silty soils were encountered at shallow depths in the explorations at this site. These soils have variable cohesion and density and may 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. 9.o 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: 12 PO Box 82243 Kenmore, WA 98028 cobaltgeoRgmail.com 2o6-331-1097 COBALT GEOTECHNICAL INVESTIGATION GEOSCIENCES EDMONDS, WASHINGTON December 16, 2020 ■ Monitor and test structural fill placement and soil compaction ■ Monitor temporary excavation stability ■ Verify footing drain installation ■ Observe bearing capacity at footing locations 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. mo Closure This report was prepared for the exclusive use of Sue Eastman -King and her 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 Sue Eastman -King 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. Respectfully submitted, Cobalt Geosciences, LLC Original signed by: VkONY wAsyi�� -d 2513-`' 00 54896 IST 12/16/2020 Phil Haberman, PE, LG, LEG Principal 13 PO Box 82243 Kenmore, WA 98028 cobaltgeoRgmail.com 2o6-331-1097 APPENDIX A Statement of General Conditions 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 party. BASIS OF THE REPORT: The information, opinions, and/or recommendations made in this report are in accordance with Cobalt Geosciences present understanding of the site specific project as described by the Client. The applicability of these is restricted to the site conditions encountered at the time of the investigation or study. If the proposed site specific project differs or is modified from what is described in this report or if the site conditions are altered, this report is no longer valid unless Cobalt Geosciences is requested by the Client to review and revise the report to reflect the differing or modified project specifics and/or the altered site conditions. STANDARD OF CARE: Preparation of this report, and all associated work, was carried out in accordance with the normally accepted standard of care in the state of execution for the specific professional service provided to the Client. No other warranty is made. INTERPRETATION OF SITE CONDITIONS: Soil, rock, or other material descriptions, and statements regarding their condition, made in this report are based on site conditions encountered by Cobalt Geosciences at the time of the work and at the specific testing and/or sampling locations. Classifications and statements of condition have been made in accordance with normally accepted practices which are judgmental in nature; no specific description should be considered exact, but rather reflective of the anticipated material behavior. Extrapolation of in situ conditions can only be made to some limited extent beyond the sampling or test points. The extent depends on variability of the soil, rock and groundwater conditions as influenced by geological processes, construction activity, and site use. VARYING OR UNEXPECTED CONDITIONS: Should any site or subsurface conditions be encountered that are different from those described in this report or encountered at the test locations, Cobalt Geosciences must be notified immediately to assess if the varying or unexpected conditions are substantial and if reassessments of the report conclusions or recommendations are required. Cobalt Geosciences will not be responsible to any party for damages incurred as a result of failing to notify Cobalt Geosciences that differing site or sub -surface conditions are present upon becoming aware of such conditions. PLANNING, DESIGN, OR CONSTRUCTION: Development or design plans and specifications should be reviewed by Cobalt Geosciences, sufficiently ahead of initiating the next project stage (property acquisition, tender, construction, etc), to confirm that this report completely addresses the elaborated project specifics and that the contents of this report have been properly interpreted. Specialty quality assurance services (field observations and testing) during construction are a necessary part of the evaluation of sub -subsurface conditions and site preparation works. Site work relating to the recommendations included in this report should only be carried out in the presence of a qualified geotechnical engineer; Cobalt Geosciences cannot be responsible for site work carried out without being present. 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Box 82243 Kenmore, WA 98028 COBALT 129 Skyline Drive MAP (2o6) 331-1097 G E • S C I E N C k S Edmonds, Washington FIGURE i t^'ww.cobaltgeo.com cobaltgeoogmail.com Approximate TP_ Test Pit Location Proposed Residence 129 Skyline Drive Edmonds, Washington SITE PLAN FIGURE 2 N A Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, WA 98028 (206) 331-1097 www.cobaltgeo.com cobaltgeoogmail.com APPENDIX C Exploration Logs Unified Soil Classification System (USCS) MAJOR DIVISIONS SYMBOL TYPICAL DESCRIPTION GW Clean Gravels Well -graded gravels, gravels, gravel -sand mixtures, little or no fines Gravels (more than 50% (less than 5% _ GP fines) Poorly graded gravels, gravel -sand mixtures, little or no fines COARSE GRAINED SOILS of coarse fraction retained on No. 4 sieve) GM Gravels with Fines (more than 12% GC fines) Silty gravels, gravel -sand -silt mixtures Clayey gravels, gravel -sand -clay mixtures (more than 50% retained on ;°e sw Clean Sands Well -graded sands, gravelly sands, little or no fines No. 200 sieve) Sands (50% or more of coarse fraction (less than 5% SP fines) Poorly graded sand, gravelly sands, little or no fines passes the No. 4 sieve) sM Sands with Fines Silty sands, sand -silt mixtures (more than 12% sc fines) Clayey sands, sand -clay mixtures ML Inorganic silts of low to medium plasticity, sandy silts, gravelly silts, FINE GRAINED SOILS (50% or more Silts and Clays (liquid limit less than 50) Inorganic cL or clayey silts with slight plasticity Inorganic clays of low to medium plasticity, gravelly clays, sandy clays silty clays, lean clays oL Organic Organic silts and organic silty clays of low plasticity passes the MH Inorganic silts, micaceous or diatomaceous fine sands or silty soils, No. 200 sieve) Silts and Clays (liquid limit 50 or more) Inorganic CH elastic silt Inorganic clays of medium to high plasticity, sandy fat clay, or gravelly fat clay OH Organic Organic clays of medium to high plasticity, organic silts HIGHLY ORGANIC SOILS primarily organic matter, dark in color, PT and organic odor Peat, humus, swamp soils with high organic content (ASTM D4427) 1 Classification of Soil Constituents 1 MAJOR constituents compose more than 50 percent, by weight, of the soil. Major constituents are capitalized (i.e., SAND). Minor constituents compose 12 to 50 percent of the soil and precede the major constituents (i.e., silty SAND). Minor constituents preceded by "slightly" compose 5 to 12 percent of the soil (i.e., slightly silty SAND). Trace constituents compose o to 5 percent of the soil (i.e., slightly silty SAND, trace gravel). Relative Density (Coarse Grained Soils) Consistency (Fine Grained Soils) N, SPT, Relative N, SPT, Relative Blows/FT Density Blows/FT Consistency 0-4 Very loose Under 2 Very soft 4 -10 Loose 2-4 Soft 10 - 30 Medium dense 4-8 Medium stiff 30 - 50 Dense 8 -15 Stiff Over 50 Very dense 15 - 30 Very stiff Over 3o Hard Grain Size Definitions Description Sieve Number and/or Size Fines <#200 (0.08 mm) Sand -Fine #200 to #40 (0.08 to 0.4 mm) -Medium #40 to #10 (0.4 to 2 MM) -Coarse #10 to #4 (2 to 5 mm) Gravel -Fine #4 to 3/4 inch (5 to 19 mm) -Coarse 3/4 to 3 inches (19 to 76 mm) Cobbles 3 to 12 inches (75 to 305 mm) Boulders >12 inches (305 mm) Moisture Content Definitions Dry Absence of moisture, dusty, dry to the touch Moist Damp but no visible water Wet Visible free water, from below water table Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, WA98028 Soil Classification Chart Figure C1 (206) 331-1097 _ www.cobalt eg o.com cobaltgeo(d-) gmail.com Test Pit TP-1 Date: December 1, 2020 Depth. 9' Groundwater: None Contractor: Jim Elevation: Logged By: PH Checked By: SC a)J0 0) o Q Moisture Content (%) Plastic 1 Liquid � U E 3 Limit Limit } CI a a, L N Material Description c ? o DCP Equivalent N-Value o C 0 10 20 30 40 50 ------ — -- To�soilVetation -------------------------------- 1 SM Loose to medium dense, silty -fine to medium grained sand with gravel dark yellowish brown to grayish brown, moist. 2 (Fill) 3 ------ q ---- -- SM --------------------------------------------- Medium dense to dense, silty -fine to medium grained sand with gravel grayish brown, moist. 5 (Glacial Till) 6 8 End of Test Pit 9' 10 Test Pit TP-2 Date: December 1, 2020 Depth: 9' Groundwater: None Contractor: Jim Elevation: Logged By: PH Checked By: SC N 0 o N Moisture Content (%) u E 3 Plastic 1 Liquid Limit Limit L L N Material Description DCP Equivalent N-Value Q o �, ? o C 0 10 20 30 40 5 -- TopsoilVegetation ________________________________ 1 SM Loose to medium dense, silty -fine to medium grained sand with gravel dark yellowish brown to grayish brown, moist. 2 (Fill) 3 4 5 ------- 6 ---- -- SM --------------------------------------------- Medium dense to dense, silty -fine to medium grained sand with gravel grayish brown, moist. (Glacial Till) 8 End of Test Pit 9' 10 Cobalt Geosciences, LLC Proposed Residence P.O. Box 82243 Test Pit Kenmore, WA 98028 COBALT129 Skyline Drive (2o6) 331-1097 G E • S C I E N C k S Edmonds, Washington Logs www.cobaltgeo.com cobaltgeoogmail.com