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Geotechnical Report.pdfCOBALT GEOSCIENCES Geotechnical Investigation Proposed Four Lot Plat Parcel No. 004520004Oo800 Edmonds, Washington September 24, 2018 ReCelve® APR 0 4 2919 DEVELOPMENT COUNTER RViCES GEOTECHNICAL INVESTIGATION TUKWILA, 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--------------------- - - - - - - - -.... _ ... ....... ................. .. ................. 3 6.o GEOLOGIC HAZARDS................................................................................................... 3 6.1 Steep Slope Hazard.............................................................................................. 3 6.2 Erosion Hazard.................................................................................................... 3 6.3 Seismic Hazard.................................................................................................... 4 7.o DISCUSSION................................................................................................................... 4 7.1.1 General.................................................................................................................4 8.o RECOMMENDATIONS.................................................................................................. 5 8.1.1 Site Preparation................................................................................................... 5 8.1.2 Temporary Excavations........................................................................................ 5 8.1.3 Erosion and Sediment Control.............................................................................. 6 8.1.4 Foundation Design............................................................................................... 6 8.1.5 Reinforced Concrete Retaining Walls................................................................... 7 8.1.6 Stormwater Management..................................................................................... 9 8.1.7 Slab-on-Grade...................................................................................................... 9 8.1.8 Groundwater Influence on Construction .............................................................10 8.1.9 Utilities...............................................................................................................10 9.o CONSTRUCTION FIELD REVIEWS...........................................................................10 io.o CLOSURE...................................................................................................................11 LIST OF APPENDICES Appendix A — Statement of General Conditions Appendix B — Figures Appendix C — Test Pit Logs GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON September 24, 2018 i.o Introduction ;M COBALT GEOSCIENCES In accordance with your authorization, Cobalt Geosciences, LLC (Cobalt) has completed a geotechnical investigation for the proposed four lot plat located at 7901 - 240th Street SW in Edmonds, Washington (Figure 1). The purpose of the geotechnical investigation was to identify subsurface conditions and to provide geotechnical recommendations for foundation design, stormwater management, earthwork, soil compaction, and suitability of the on -site soils for use as fill. The scope of work for the geotechnical evaluation 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 support of the new buildings. 2.0 Project Description The project includes subdivision of the parcel into four new lots followed by construction of four new residences and an access roadway. The provided site plans indicate that the roadway will extend north along the western portion of the property and the residences will be situated generally to fit a relatively steep slope in the central portion of the property. Anticipated building loads are expected to be light and site grading will include cuts and fills on the order of 10 feet or less. Stormwater management may include infiltration devices (if feasible), detention, rain gardens, and/or detention systems. We should be provided with the plans to verify that our recommendations are valid. 3.0 Site Description The site is located at 7901 — 240th Street SW in Edmonds, Washington (Figure 1). The property consists of one rectangular shaped parcel (No. 00452000400800) with a total area of about o.9 acres. Based on a review of historic aerial photographs, the property has previously been developed with residential structures from at least 1958 up to the early to mid-196os. Currently, the property is undeveloped and vegetated with grasses, ivy, blackberry vines, along with variable diameter deciduous and evergreen trees. The site slopes from west to east at variable magnitudes. There is a steep slope within the central portion of the property that extends north to south and slopes to the east. The slope is generally 8 to 16 feet in height and has magnitudes of 30 to too percent. The western and eastern portions of the property are gently sloping to nearly level and the western portion appears to have been graded as an access roadway at one time. The site is bordered to the north, west, and east by residential developments and to the south by 240th Street SW. PO Box 82243 Kenmore, WA 98028 cobaltgc @Rmail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON September 24, 2018 4.o Field Investigation COBALT GEOSCIENCES 4.1.1 Site Investigation Program The geotechnical field investigation program was completed on August 30, 2018 and included excavating and sampling five test pits within the property for subsurface analysis. Site access was somewhat limited due to heavy vegetation. 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 explorations are presented on the logs enclosed in Appendix C. 5.0 Soil and Groundwater Conditions 5.1.1 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 underlain by Vashon Glacial Till. Vashon Glacial Till is typically characterized by an unsorted, non -stratified mixture of clay, silt, sand, gravel, cobbles and boulders in variable quantities. These materials are typically dense and relatively impermeable. The poor sorting reflects the mixing of the materials as these sediments were overridden and incorporated by the glacial ice. Subsurface Explorations In general, all of the test pits encountered variable thicknesses of topsoil and vegetation underlain by up to 8 feet of loose to dense fill. The fill appears to be thickest along the upper bench in the western portion of the property extending over and into the steep slope in the central portion of the property. The fill was underlain by weathered and/or unweathered glacial till which consists of medium dense to very dense, silty -fine to medium grained sand with gravel. It appears that the site was graded on one or more occasions during previous site development activities. The depth and extent of the fill could not be fully determined through test pit excavations due to access limitations. 2 PO Box 82243 Kenmore, WA 98o28 cobal c Ogmail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON September 24, 2018 5.1.2 Groundwater COBALT GEOSCIENCES Groundwater was not encountered in any of the test pits. We anticipate that perched groundwater will be present at the site within io feet below existing grades during the winter and early spring months. 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 Hazard The site and adjacent areas contain local steep slope/landslide hazard areas and associated buffer zones. Within the property, there are slope areas with magnitudes between 3o and ioo percent and relief of 8 to 16 feet. The steep slope areas appear to have been created through previous grading activities. They appear mostly unnatural in topography and could be comprised of both fill and native glacial till at depth. The site and adjacent areas appear stable at this time. The underlying soils are generally medium dense to very dense and are relatively resistant to global instability. The steep slope locally meets the criteria described in the Edmonds Municipal Code section 23.80.020 under heading B (item number four). This criteria describes slopes with magnitudes of 4o percent and relief of io feet or more, even if created through prior grading. The site does not include the other aspects of this code section. It is our opinion that the relatively steep slope area is stable at this time and slope stability analyses are not warranted. The proposed construction will result in modification of the slope area, resulting in greater stability than what currently exists. The project will not decrease slope stability or pose an unreasonable threat to persons or property on the site or adjacent areas provided the earthwork and grading activities are periodically monitored by the geotechnical engineer and performed according to the plans and this report. 6.2 Erosion Hazard The Natural Resources Conservation Senrices (NRCS) maps for Snohomish County indicate that the site is underlain by Alderwood-Urban land complex (2 to 15 percent slopes). In general, these soils have a slight to moderate erosion potential in a disturbed state. 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 Lst. Erosion control measures should be in place before the onset of wet weather. 3 PO Box 82243 Kenmore, WA 98028 cobaltaw0gmail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON September 24, 2018 6.3 Seismic Hazard COBALT GEOSCIENCES 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 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, S,, Fa, 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) R� 19A 7no/ of a S, 49.20% of g FA 1.00 Fv 1.5o8 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 and very fined grained soil deposits that underlie the site have a low liquefaction potential. 7.o DISCUSSION 7.1.1 General The site is underlain by undocumented fill and at depth by glacial till. The proposed residential structures may be supported on shallow foundation systems bearing on medium dense or firmer native soils and structural fill placed on suitable native soils. Footings should not bear on undocumented fill. Any fill should be removed fully from below new foundation elements. Infiltration of stormwater runoff is not feasible at this site. The shallow soil conditions and high likelihood of shallow seasonal groundwater limit suitability of infiltration devices. We anticipate that detention systems and possibly dispersion systems, if space allows, will be suitable for stormwater management at the site. 4 PO Box 82243 Kenmore, WA 98028 cobai geoP mail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON September 24, 2018 8.o Recommendations 8.1.1 Site Preparation COBALT GEOSCIENCES 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 24 inches. Deeper excavations will be necessary below large trees and in areas underlain by undocumented fill materials. The fill and native soils consist of silty -sand with gravel. These soils may be used as structural fill provided they achieve compaction requirements and are within 3 percent of the optimum moisture. Please note that these soils are highly moisture sensitive. These soils are typically only 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. If existing fill is re -used as fill, debris and any organic material should be removed prior to use. 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 io feet or less for basement foundation and utility placement. Excavations should be sloped no steeper than 1H:1V (Horizontal:Vertical) in medium dense or firmer fill and native soils. If an excavation is subject to heavy vibration or surcharge loads, we recommend that the excavations be sloped no steeper than 1.5H:iV, 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 io 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 5 PO Box 82243 Kenmore, WA 98028 cobal gc (@ mail.cvm 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON September 24, 2018 COBALT GEOSCIENCES 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 permittempor-ary--slopes to be -cut to-the--maximam-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. • 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 single-family residences may be supported on shallow spread footing foundation systems bearing on undisturbed medium dense or firmer native soils or on properly compacted structural fill placed on the suitable native soils. 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. It should be noted that the upper (west) portions of the site are likely underlain by variable thicknesses of fill and overexcavations of 6 feet or more may be required in foundation areas. All footing excavations should be periodically monitored by the geotechnical engineer to verify suitable bearing on native soils is achieved. Backfill and compaction of replaced structural fill should also be monitored and verified on a periodic to full-time basis. 6 PO Box 82243 Kenmore, WA 98028 cobaltgeo(agmail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON September 24, 2018 COBALT GEOSCIENCES For shallow foundation support, we recommend widths of at least 18 and 24 inches, respectively, for continuous wall and isolated column footings supporting the proposed structure. Provided that the footings are supported as recommended above, a net allowable bearing pressure of 2,000 pounds per square foot (psf) may be used for design. A 1/3 increase in the above value may be used for short duration loads, such as those imposed by wind and seismic events. Structural fill placed on bearing, native subgrade should be compacted to at least 95 percent of the maximum dry density based on ASTM Test Method D1557. Footing excavations should be inspected to verify that the foundations will bear on suitable material. 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 0.35 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 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. 7 PO Box 82243 Kenmore, WA 98028 cobal ua)( gmail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON September 24, 2018 COBALT GEOSCIENCES Wall Design Criteria "At -rest" Conditions (Lateral Earth Pressure — EFD+) 55 pcf (Equivalent Fluid Density) "Alive—Conzditions- (L-ateral-Earth Pressure — LFD-) 35 pcf (Equivalent7Fluid-Density) Seismic Increase for "At -rest" Conditions (Lateral Earth Pressure) 21H* (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) Level Ground; Neglect upper 2 feet, then 250 pcf EFD+ Soil -Footing Coefficient of Sliding Friction (Allowable; includes F.S. = 1.5) 0.35 "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 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. 8 PO Box 82243 Kenmore, WA 98028 cohalteco a smail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON September 24, 2018 8.1.6 Stormwater Management COBALT GEOSCIENCES The site is underlain by relatively fine-grained glacial deposits along with variable thicknesses of fill. These soils have a very low permeability which decreases with depth. It is likely that there will be shallow perched groundwater throughout the property that further limits infiltration suitability. We performed an in -situ infiltration test in TP-2 at a depth of 5 feet below grade. Following testing and application of correction factors, the infiltration rate was o.ii inches per hour, which is lower than what the Department of Ecology considers to be feasible. We do not recommend utilizing infiltration systems at the site. We recommend utilizing shallow dispersion trenches or a combination of detention, dispersion and overflow connection to City stormwater infrastructure. We can provide additional information upon request. We should be provided with the final plans for review. 8.1.7 Slab -on -Grade We recommend that the upper 24 inches of the existing fill and/or native soils within slab areas be re - compacted to at least 95 percent of the modified proctor (ASTM D1557 Test Method). Any loose soils should be removed to a depth at least 2 feet below slab areas prior to slab -on -grade preparation. The fill should be evaluated prior to imported fill placement to verify stability. 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 302AR-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 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. 9 PO Box 82243 Kenmore, WA 98028 cobal ge oRmail.com 2o6-331-1097 COBALT GEOTECHNICAL INVESTIGATION G E o S c I E N C E S EDMONDS, WASHINGTON September 24, 2018 8.1.8 Groundwater Influence on Construction Groundwater was not encountered in the test pits. We anticipate that perched groundwater develops during the wetter months between layers weathered glacial till and/or fill and underlying dense to very dense soils. Seasonal groundwater will likely be encountered within io feet of the ground surface. During the winter -spring months, 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. 8.1.9 Utilities Utility trenches should be excavated according to accepted engineering practices following OSHA (Occupational Safety and Hcalth Ad11—i—atim,-andard., by a �.U.—UMr experienced in such work. The contractor is responsible for the safety of open trenches. Traffic and vibration adjacent to trench walls should be reduced; cyclic wetting and drying of excavation side slopes should be avoided. Depending upon the location and depth of some utility trenches, groundwater flow into open excavations could be experienced, especially during or shortly following periods of precipitation. In general, silty and sandy soils were encountered at shallow depths in the explorations at this site. These soils have variable 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. 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: • Monitor and test structural fill placement and soil compaction ■ Observe bearing capacity at foundation locations ■ Observe slab -on -grade preparation 10 PO Box 82243 Kenmore, WA 98028 cobal =(@gnlail.com 2o6-331-1097 GEOTECHNICAL INVESTIGATION EDMONDS, WASHINGTON September 24, 2018 ■ Observe excavation stability COBALT GEOSCIENCES 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. io.o Closure This report was prepared for the exclusive use of Luke Shamp and his appointed consultants. Any use of this report or the material contained herein by third parties, or for other than the intended purpose, should first be approved in writing by Cobalt Geosciences, LLC. The recommendations contained in this report are based on assumed continuity of soils with those of our test holes, and assumed structural loads. Cobalt Geosciences should be provided with final architectural and civil drawings when they become available in order that we may review our design recommendations and advise of any revisions, if necessary. Use of this report is subject to the Statement of General Conditions provided in Appendix A. It is the responsibility of Luke Shamp 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: -,INO N Y ti �pR� � 0� WAS, qIN V �I/►` ,7 2513 /rb` �54896 O ��� �,� «, 'nF FCIST E��- G\� SS/p N A t �-MR0 A. Haberman Exp. 6/26/2020 Phil Haberman, PE, LG, LEG Principal PH/sc 11 PO Box 82243 Kenmore, WA 98028 cobaltp.eo@maii.com 2o6-881-1097 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. APPENDIX B Figures: Vicinity Map, Site Plan 10.2 PO Box 82243 Kenmore, WA 98028 cobal tReo(a)gmail.com 2o6-331-1097 122022.000' W 3584 122019,000' W WASHINGTON _ }'s3 J NL,a •. 1 Q s Q _r., ! rt tl ' , ' •. 20tSlh S1N I, ' fit$, c'i r �,j •' . Edmonds i 1> '� •Q o L > • .;-` `'� ProjectLocation • R tip.• � _ ^iry� • m � _//- o '. ».7c111h [-•W ~ f '•_>r;��i i it h+ I• h p d- 7' �� 1 = - . 1 Y, _ .. 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' I s E ` .`c< W NF is h St v ' 122022.000' W 122121,000' W 122120.000' W WGS84 122019.000' W N TN MN NATIONAL h`" 10t10 0 IIXIO 20011 3fg0 4000 51170 ]S.fl� GEOGRAPHIC rfE' I 0 NiIOME[E9" I M1111r, 118 Cobalt Geosciences, LLC Proposed Four Lot Plat VICINITY P.O. Box 82243 _ 24oth Street SW MAY 206 or 1- oA 98028 ) 33 97 7G E 0 S C I E N C E S Edmonds, Washington FIGURE i wttiv.cobaltg oe com cobalt=oP=aiI,com rN G k !O b J J TP-2; k t� I t t! s �� yam, •1f ii 'ffi ••' %� �', Q o r ,tA f/g41 TP D a0 -1 4 i om 2 � t , � t•Z J � �' •`�` 1 � D f tii �J Gt 19 zoo TP-5 • TP-i s e Approximat Test Pit Location :. 2.07' 127.00' -__;t.._.._4 . I ASPN 240"' sT SVI W e I N 6e76'01' W 51J.26' CALL. A 4 � S 06'3��t7'rC �t-f� e I' 4 ' W Cobalt Geosciences, LLC Proposed Four Lot Plat SITE PLAN P.O. Box 82243 2 oth Street SW Kenmore, WA 98028 • 4 (206) 331-1097 GEOSCIENCES Edmonds, Washington FIGURE 2 %%%%1A''cQt)81tge0.c0m cobal coogniiLcom APPENDIX C Test Pit Logs 1 1 1 Test Pit TP-i USCS op'soil/Veg.et.a.t.io..n 177,777, ...................................... . SMFill 4.. ..................... SM X Weathered Glacial Till? Graphic 6' ................ \ ....................... ..........: SM Glacial Till Test Pit TP-2 USCS Graphic Topsoil/Vegetation 1' ........................................... .. SM Fill 2' ............................................. ....... SM I Weathered Glacial Till 3' ............... .................. ........ SM Glacial Till USCS Test Pit TP-3 Graphic Topsoil/Vegetation z'........... ............................... .... SM Weathered Glacial Till 4" ................ .........� SM Glacial Till* ill ! 0-1' Vegetation/Topsoil 1-4' Silty Sand with Gravel (SM) Loose to medium dense, silty -fine to medium grained sand with gravel and roots, dark yellowish brown to grayish brown, dry. (Fill) 4-6' Silty Sand with Gravel (SM) Loose to medium dense, silty -fine to medium grained sand with gravel, mottled yellowish brown to grayish brown, moist. (Weathered Glacial Till?) 6-8' Silty Sand with Gravel (SM) Dense to very dense, silty -fine to medium grained sand with gravel, grayish brown, moist. (Glacial Till) End of Test Pit 8' No Groundwater No Caving -1' Vegetatio^./Topsoil 1-2' Silty Sand with Gravel (SM) Loose to medium dense, silty -fine to medium grained sand with gravel and roots, dark yellowish brown to grayishb brown, dry. (Fill) 2-3' Silty Sand writh Gravel (SM) Loose to medium dense, silty -fine to medium grained sand writh gravel, mottled yellowish brown to grayish brown, diy to moist. (Weathered Glacial Till) 3-10' Silty Sand with Gravel (SM) Dense to very dense, silty -fine to medium grained sand with gravel, grayish brown, moist. (Glacial Till) End of Test Pit io' No Groundwater No Caving o-z' Vegetation/Topsoil 2-4' Silty Sand with Gravel (SM) Loose to medium dense, silty -fine to medium grained sand writh gravel, mottled yellowish brown to grayish brown, moist. (Glacial Till) 4-7' Silty Sand with Gravel (SM) Dense to very dense, silty -fine to medium grained sand with gravel, grayish brown, moist. (Glacial Till) End of Test Pit 7' No Groundwater No Caving Cobalt Geosciences, LLC Proposed Four Lot Plat P.O. Box TEST PIT Kenmore,, WA WA 98o28 -COBALT 24oth Street SW LOGS (2o6) 331-1097 \7GEOSCIENCES Edmonds, Washington um-w.cobaltgeo com cobal e0(ftmail,com Test Pit TP-4 USCS 1 � Topsoil/Vegetation / SM Fill ........ \ ..... SM Glacial Till Test Pit TP-5 USCS Graphic Graphic ..................................rsiVegt tion 1' ............. ....: SM Fill 8' ................ ..........t SM Glacial Till 0-1' Vegetation/Topsoil 1-6' Silty Sand with Gravel (SM) Loose to medium dense, silty -fine to medium grained sand with gravel, asphalt, debris, and roots, dark yellowish brown to grayish brown, dry. (Fill) 6-8' Silty Sand with Gravel (SM) Dense to very dense, silty -fine to medium grained sand with gravel, grayish brown, moist. (Glacial Till) End of Test Pit 8' No Groundwater No Caving 0-1' Vegetation/Topsoil 1-8' Silty Sand with Gravel (SM) Loose to medium dense, silty -fine to medium grained sand with gravel, debris, and roots, dark yellowish brown to grayishb brown, dry. (Fill) 8-io' Silty Sand with Gravel (SM) Dense to very dense, silty -fine to medium grained sand with gravel, grayish brown, moist. (Glacial Till) End of Test Pit 10' No Groundwater No Caving Cobalt Geosciences, LLC Proposed Four Lot Plat P.O. Box 82243 TEST PIT Kenmore, WA 98028 COBALT 24oth Street SW LOGS (2o6) 331-1097 G E 0 S C I E N C E S Edmonds, Washington 1+'ww'coba►tgeo.com c baltuo(oh mail.com w