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HomeMy WebLinkAboutExh.15_RS_Geotechnical_Report_190221_v1.pdfCobalt Geosciences Geotechnical Investigation Proposed 7-Lot Plat 19805 – 108th Avenue SE Renton, Washington September 18, 2018 GEOTECHNICAL INVESTIGATION RENTON, WASHINGTON i Table of Contents 1.0 INTRODUCTION ............................................................................................................. 1 2.0 PROJECT DESCRIPTION .............................................................................................. 1 3.0 SITE DESCRIPTION ....................................................................................................... 1 4.0 FIELD INVESTIGATION ............................................................................................... 1 4.1.1 Site Investigation Program ................................................................................... 1 5.0 SOIL AND GROUNDWATER CONDITIONS .............................................................. 2 5.1.1 Area Geology ........................................................................................................ 2 5.1.2 Groundwater ........................................................................................................ 3 6.0 GEOLOGIC HAZARDS ................................................................................................... 3 6.1 Erosion Hazard .................................................................................................... 3 6.2 Seismic Hazard .................................................................................................... 3 7.0 DISCUSSION ................................................................................................................... 4 7.1.1 General................................................................................................................. 4 8.0 RECOMMENDATIONS .................................................................................................. 4 8.1.1 Site Preparation ................................................................................................... 4 8.1.2 Temporary Excavations ........................................................................................ 5 8.1.3 Erosion and Sediment Control.............................................................................. 5 8.1.4 Foundation Design ............................................................................................... 6 8.1.5 Stormwater Management ..................................................................................... 7 8.1.6 Slab-on-Grade ...................................................................................................... 7 8.1.7 Utilities ................................................................................................................ 8 8.1.8 Groundwater Influence on Construction .............................................................. 8 8.1.9 Pavement Recommendations ............................................................................... 8 9.0 CONSTRUCTION FIELD REVIEWS ...........................................................................10 10.0 CLOSURE ...................................................................................................................10 LIST OF APPENDICES Appendix A — Statement of General Conditions Appendix B — Figures Appendix C — Test Pit Logs GEOTECHNICAL INVESTIGATION RENTON, WASHINGTON September 18, 2018 1 PO Box 82243 Kenmore, WA 98028 206-331-1097 cobaltgeo@gmail.com 1.0 Introduction In accordance with your authorization, Cobalt Geosciences, LLC (Cobalt) has completed a geotechnical investigation for the proposed 7-lot residential plat located at 19805 – 108thAvenue SE in Renton, Washington (Figure 1). The purpose of the geotechnical investigation was to identify subsurface conditions and to provide geotechnical recommendations for foundation design, earthwork, soil compaction, utilities, general pavement guidelines, stormwater management, 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 , drainage, and earthwork. 2.0 Project Description The proposed development includes a new access roadway, 7 residential building lots, and a detention pond located in the southeast corner of the property. We anticipate that foundation loads will be generally light and that site grading will include cuts and fills on the order of 4 feet or less for foundation placement and roadway construction. Deeper excavations may be proposed for detention facilities. We should be provided with the final plans in order to update our recommendations, if necessary. 3.0 Site Description The site is located at 19805 – 108th Avenue SE in Renton, Washington (Figure 1). The property consists of one rectangular parcel (No. 0522059078) with a total area of about 2.2 acres. The northeast corner of the property is developed with a single -family residence and driveway. The remainder of the property is undeveloped and vegetated with blackberry vines, ferns, grasses, Cottonwood trees, and other small diameter deciduous trees. The site is nearly level to gently undulating with slopes less than 20 percent in magnitude and an overall topographic relief of less than 8 feet. The site is bordered to the north and south by single -family residences, to the east by 108th Avenue SE, and to the west by 106th Avenue SE. 4.0 Field Investigation 4.1.1 Site Investigation Program The geotechnical field investigation program was completed on February 17, 2018 and included excavating and sampling four test pits within the property, where accessible. GEOTECHNICAL INVESTIGATION RENTON, WASHINGTON September 18, 2018 2 PO Box 82243 Kenmore, WA 98028 206-331-1097 cobaltgeo@gmail.com 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 , classified the encountered soils, kept a detailed log of each test pit, and observed and recorded pertinent site features. The results of the test pit explorations are presented 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 histor y 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 Composite Geologic Map of King County, 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. Test Pit TP-1 Test Pit TP-1 encountered approximately 2.5 feet of loose, silty -fine to medium grained sand with gravel and organic debris (Fill) underlain by about 1.5 feet of loose to medium dense, silty-fine to medium grained sand with gravel (Weathered Glacial Till). This layer was underlain by dense, silty -fine to medium grained sand with gravel (Glacial Till), which continued to the termination depth of the test pit. Test Pits TP-2 through TP-4 These test pits encountered approximately 1.5 to 2 feet of topsoil and vegetation underlain by about 1.5 to 2 feet of loose to medium dense, silty-fine to medium grained sand with gravel (Weathered Glacial Till). These materials were underlain by dense to very dense, silty -fine to medium grained sand with gravel (Glacial Till), which continued to the termination depths of the test pits. GEOTECHNICAL INVESTIGATION RENTON, WASHINGTON September 18, 2018 3 PO Box 82243 Kenmore, WA 98028 206-331-1097 cobaltgeo@gmail.com 5.1.2 Groundwater Groundwater was encountered in Test Pits TP-2 and TP-3 and 1 and 1.5 feet below existing grades, respectively. Groundwater was not encountered in the other test pits. We anticipate that perched groundwater will be encountered during the winter and spring months at this site. 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. 6.0 Geologic Hazards 6.1 Erosion Hazard The Natural Resources Conservation Services (NRCS) maps for King County indicate that the site is underlain by Alderwood gravelly sandy loam (0 – 8 percent slopes). 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 1st. Erosion control measures should be in place before the onset of wet weather. 6.2 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 dense to very dense soils within the upper 100 feet. We referenced the U.S. Geological Survey (USGS) Earthquake Hazards Program Website to obtain values for SS, S1, Fa, and Fv. 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 136.00% of g S1 51.11% of g FA 1.00 FV 1.506 GEOTECHNICAL INVESTIGATION RENTON, WASHINGTON September 18, 2018 4 PO Box 82243 Kenmore, WA 98028 206-331-1097 cobaltgeo@gmail.com 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 groundwate r table. The relatively dense soil deposits that underlie the site have a low potential for liquefaction. 7.0 DISCUSSION 7.1.1 General It is our opinion that the proposed residences may be supported on shallow foundation systems bearing on medium dense or firmer native soils, re-compacted native soils, or on structural fill placed on native soils per the recommendations in Section 8.1.1 and 8.1.4 of this report. We encountered local areas of loose fill as well as thick topsoil and locally loose native soils within the property. Local overexcavation below footing, slab, and paved areas will likely be necessary depending on planned finish grade elevations. In general, bearing soils were observed between 2 and 3.5 feet below existing site elevations at the test pit locations. All excavations within the building footprint should be backfilled and compacted according to the recommendations in this report. Any filled areas beneath foundation elements should extend a lateral distance equal to the depth of the overexcavation in all directions from the fa ces of the footings. Soils below foundation elements should be compacted to at least 95 percent of the modified proctor or consist of medium dense to very dense native soils. 8.0 Recommendations 8.1.1 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 1 to 2 feet. Deeper excavations should be expected where undocumented fill is present or under large trees; on the order of 4 feet or more. The excavated material is not suitable as fill material within the proposed building envelope but could be used as fill material in non-settlement sensitive areas such as landscaping regions. In these non- settlement sensitive areas, the fill should be placed in maximum 12 inch thick lifts that should be compacted to at least 90 percent of the modified proctor (ASTM D 1557 Test Method) maximum dry density. The native soils below the vegetation and topsoil (and fill) consist of glacial till. These materials are generally considered suitable for use as structural fill provided they are within 3 percent of the optimum moisture content. It should be noted that glacial till soil materials are typically suitable for structural fill during the summer months only if they can be dried to optimum moisture levels. GEOTECHNICAL INVESTIGATION RENTON, WASHINGTON September 18, 2018 5 PO Box 82243 Kenmore, WA 98028 206-331-1097 cobaltgeo@gmail.com 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 4 feet or less for foundation placement. Any excavations deeper than 4 feet should be sloped no steeper than 1H:1V (Horizontal:Vertical) in 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:1V, where room permits. Temporary cuts should be in accordance with the Washington Administrative Code (WAC) Part N, Excavation, Trenching, and Shoring. Temporary slopes should be visually inspected daily by a qualified person during construction activities and the inspections should be documented in daily reports. The contractor is responsible for maintaining the stability of the temporary cut slopes and reducing slope erosion during construction. Temporary cut slopes should be covered with visqueen to help reduce erosion during wet weather, and the slopes should be closely monitored until the permanent retaining systems or slope configurations are complete. Materials should not be stored or equipment operated within 1 0 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: GEOTECHNICAL INVESTIGATION RENTON, WASHINGTON September 18, 2018 6 PO Box 82243 Kenmore, WA 98028 206-331-1097 cobaltgeo@gmail.com  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 residences may be supported on shallow spread footing foundation systems bearing on undisturbed medium dense or firmer native soils, re-compacted native soils, or on properly compacted structural fill placed on the suitable native soils. If structural fill is used to supp ort 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. Bearing soils were generally encountered between 2 and 3.5 feet below existing grades, generally below fill and loose weathered native soils. If undocumented fill is encountered below foundation elements, it should be removed and replaced with suitable native soils or imported structural fill. As noted above, overexcavations should extend laterally from the footing edges a distance equal to the depth of the overexcavation. 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 col umn footings, should be less than ½ 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. GEOTECHNICAL INVESTIGATION RENTON, WASHINGTON September 18, 2018 7 PO Box 82243 Kenmore, WA 98028 206-331-1097 cobaltgeo@gmail.com 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 275 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 f ooting excavation and evaluating the bearing surface by the geotechnical engineer or his representative. 8.1.5 Stormwater Management The site is underlain by weathered and unweathered glacial till which is nearly impermeable and not conducive to infiltration of stormwater runoff. We planned on conducting at least one in situ infiltration test to determine infiltration feasibility. However, groundwater was encountered at shallow depths which preclude the suitability of the site for infiltration BMPs. There is inadequate clearance below any system and the observed groundwater. We anticipate that runoff will be managed through the use of a detention pond with overflow to City infrastructure. We can provide additional recommendations for detention vaults, ponds, or other stormwater infrastructure upon request. We should be provided with the final plans to verify suitability of any system from a geotechnical standpoint. 8.1.6 Slab-on-Grade We recommend that the upper 12 inches of the existing soils within any proposed slab areas be re- compacted to at least 95 percent of the modified proctor (ASTM D1557 Test Method). Often, a vapor barrier is considered below concrete slab areas. However, the usage of a vapor barrier could result in curling of the concrete slab at joints. Floor covers sensitive to moisture typically requires the usage of a vapor barrier. A materials or structural engineer should be consulted regarding the detailing of the vapor barrier below concrete slabs. Exterior slabs typically do not utilize vapor barriers. The American Concrete Institutes ACI 360R-06 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. 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. GEOTECHNICAL INVESTIGATION RENTON, WASHINGTON September 18, 2018 8 PO Box 82243 Kenmore, WA 98028 206-331-1097 cobaltgeo@gmail.com Exterior grades surrounding buildings should be sloped at a minimum of one percent to facilitate surface water flow away from these buildings and preferably with a relatively impermeable surface cover immediately adjacent to the buildings. 8.1.7 Utilities Utility trenches should be excavated acc ording to accepted engineering practices following OSHA (Occupational Safety and Health Administration) standards, by a contractor experienced in such work. The contractor is responsible for the safety of open trenches. Traffic and vibration adjacent to trench walls should be reduced; cyclic wetting and drying of excavation side slopes should be avoided. Depending upon the location and depth of some utility trenches, groundwater flow into open excavations could be experienced, especially during or shortly following periods of precipitation. In general, silty and sandy soils were encountered at shallow depths in the explorations at this site. These soils have low cohesion and have a tendency to cave or slough in excavations. Shoring or sloping back trench sidewalls is required within these soils. 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 9 5 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. 8.1.8 Groundwater Influence on Construction At the time of our investigation, groundwater was encountered in two of the test pits between 1 and 2 feet below existing site grades. The groundwater appears to be perched between fill or weathered glacial till and underlying glacial till materials. We anticipate that typical sump excavations and pumping will adequately de -water trenches and other shallow excavations. 8.1.9 Pavement Recommendations The near surface subgrade soils generally consist of silty sand with gravel. These soils are rated as fa ir to good for pavement subgrade material (depending on silt content and moisture conditions). We estimate that the subgrade will have a California Bearing Ratio (CBR) value of 8 and a modulus of subgrade reaction value of k = 180 pci, provided the subgrade is prepared in general accordance with our recommendations. GEOTECHNICAL INVESTIGATION RENTON, WASHINGTON September 18, 2018 9 PO Box 82243 Kenmore, WA 98028 206-331-1097 cobaltgeo@gmail.com We recommend that at a minimum, 12 inches of the existing subgrade material be moisture conditioned (as necessary) and re-compacted to prepare for the construction of pavement sections. Deeper levels of recompaction or overexcavation and replacement may be necessary in areas where fill and/or loose soils are present. The depth to suitable soils in these areas could be up to 4 feet below existing grades. The subgrade should be compacted to at least 95 percent of the maximum dry density as determined by ASTM Test Method D1557. In place density tests should be performed to verify proper moisture content and adequate compaction. However, if the subgrade soil consists of firm and unyielding native glacial soils a proof roll of the pavement subgrade soil may be performed in lieu of compaction tests. The recommended flexible and rigid pavement sections are based on design CBR and modulus of subgrade reaction (k) values that are achieved, only following proper subgrade preparation. It should be noted that subgrade soils that have relatively high silt contents will likely be highly sensitive to moisture conditions. The subgrade strength and performance characteristics of a silty subgrade material may be dramatically reduced if this material becomes wet. Based on our knowledge of the proposed project, we expect the traffic to range from light duty (passenger automobiles) to heavy duty (delivery trucks). The following tables show the recommended pavement sections for light duty and heavy duty use. ASPHALTIC CONCRETE (FLEXIBLE) PAVEMENT LIGHT DUTY Asphaltic Concrete Aggregate Base* Compacted Subgrade* ** 2.0 in. 6.0 in. 12.0 in. HEAVY DUTY Asphaltic Concrete Aggregate Base* Compacted Subgrade* ** 3.0 in. 6.0 in. 12.0 in. PORTLAND CEMENT CONCRETE (RIGID) PAVEMENT Min. PCC Depth Aggregate Base* Compacted Subgrade* ** 6.0 in. 6.0 in. 12.0 in. * 95% compaction based on ASTM Test Method D1557 ** A proof roll may be performed in lieu of in place density tests GEOTECHNICAL INVESTIGATION RENTON, WASHINGTON September 18, 2018 10 PO Box 82243 Kenmore, WA 98028 206-331-1097 cobaltgeo@gmail.com The asphaltic concrete depth in the flexible pavement tables should be a surface course type asphalt, such as Washington Department of Transportation (WSDOT) ½ inch HMA. The rigid pavement design is based on a Portland Cement Concrete (PCC) mix that has a 28 day compressive strength of 4,000 pounds per square inch (psi). The design is also based on a concrete flexural strength or modulus of rupture of 550 psi. 9.0 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  Verify the soil bearing at foundation locations for the buildings  Verify slab subgrade and capillary break material below slab-on-grade  Observe footing drainage placement  Observe proof rolls of roadway subgrade prior to asphalt placement 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 fo r the project. 10.0 Closure This report was prepared for the exclusive use of Elite Homes and their appointed consultants. Any use of this report or the material contained herein by third parties, or for other than the intended purpose, should first be approved in writing by Cobalt Geosciences, LLC. The recommendations contained in this report are based on assumed continuity of soils with those of our test holes, and assumed structural loads. Cobalt Geosciences should be provided with final architectural and civil drawings when they become available in order that we may review our design recommendations and advise of any revisions, if necessary. Use of this report is subject to the Statement of General Conditions provided in Appendix A. It is the responsibility of Elite Homes who is identified as “the Client” within the Statement of General Conditions, and its agents to review the conditions and to notify Cobalt Geosciences should any of these not be satisfied. GEOTECHNICAL INVESTIGATION RENTON, WASHINGTON September 18, 2018 11 PO Box 82243 Kenmore, WA 98028 206-331-1097 cobaltgeo@gmail.com Respectfully submitted, Cobalt Geosciences, LLC Original signed by: Exp 6/26/2020 Phil Haberman, PE, LG, LEG Principal PH/sc 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 pr operly 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. 10.2 PO Box 82243 Kenmore, WA 98028 206-331-1097 cobaltgeo@gmail.com APPENDIX B Figures: Vicinity Map, Site Plan SITE N VICINITY MAP FIGURE 1 P.O. Box 82243 Kenmore, WA 98028 (206) 331-1097 cobaltgeo@gmail.com Cobalt Geosciences Project Location Renton WASHINGTON 7-Lot Residential Plat 19805 - 108th Avenue SE Renton, Washington SITE PLAN FIGURE 2 P.O. Box 82243 Kenmore, WA 98028 (206) 331-1097 cobaltgeo@gmail.com Cobalt Geosciences N 7-Lot Residential Plat 19805 - 108th Avenue SE Renton, Washington TP-4 TP-3 TP-1 TP-2 Subject Property APPENDIX C Test Pit Logs TEST PIT LOGS P.O. Box 82243 Kenmore, WA 98028 (206) 331-1097 cobaltgeo@gmail.com Cobalt Geosciences Test Pit TP-1 0-2.5’ Silty-Sand with Gravel and Wood Debris (SM) Loose, silty-fine to medium grained sand with gravel and debris, dark yellowish brown to yellowish brown, moist. (Fill) 2.5-4’ 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) 4-8’ Silty Sand with Gravel (SM) Dense to very dense, silty-fine to medium grained sand with gravel, yellowish brown to grayish brown, moist. (Glacial Till) End of Test Pit 8’ No Groundwater No Caving SM SM Weathered Glacial Till Glacial Till 2.5’ 4’ USCS Graphic 7-Lot Residential Plat 19805 - 108th Avenue SE Renton, Washington Fill LLC Test Pit TP-2 0-1.5’ Vegetation/Topsoil 1.5-3’ Silty Sand with Gravel (SM) Loose to medium dense, silty-fine to medium grained sand with gravel, mottled yellowish brown to grayish brown, moist to wet. (Weathered Glacial Till) 3-8’ 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’ Groundwater at 1’ No Caving SM SM Weathered Glacial Till Glacial Till 1.5’ 3’ USCS Graphic Topsoil/Vegetation Test Pit TP-3 0-2’ Vegetation/Topsoil 2-4’ Silty Sand with Gravel (SM) Loose to medium dense, silty-fine to medium grained sand with gravel, mottled yellowish brown to grayish brown, moist to wet. (Weathered Glacial Till) 4-8’ 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’ Groundwater at 1.5’ No Caving SM SM Weathered Glacial Till Glacial Till 2’ 4’ USCS Graphic Topsoil/Vegetation Test Pit TP-4 0-1.5’ Vegetation/Topsoil 1.5-3.5’ 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) 3.5-6’ 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 6’ No Groundwater No Caving SM SM Weathered Glacial Till Glacial Till 1.5’ 3.5’ USCS Graphic Topsoil/Vegetation