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HomeMy WebLinkAboutRS_Geotech Report_050918_v1Cornerstone Geotechnical, Inc. August 18, 2005 Mr. Mike Davis Davis Consulting, Inc. 27013 Pacific Highway South, #353 Des Moines, Washington 98198 Geotechnical Engineering Report Sandhu Property King County, Washington CG File No. 1928 Dear Mr. Davis: INTRODUCTION 17625-13Qth Ave. NE, C102, Woodinville, WA 98072 Phone: 425-844-1977 Fax: 425-844-1987 This report presents the results of our geotechnical engineering investigation at the proposed 16-lot residential project in the Renton area of King County, Washington. The site is located at 12044 SE 184"' Street, as shown on the Vicinity Map in Figure I. You have requested that we complete this report to evaluate subsurface conditions and provide recommendations for site development. For our use in preparing this report, we have been provided with a copy of the planned site layout that shows the locations of the proposed lots, roadway, and stormwater detention system. PROJECT DESCRIPTION The irregularly-shaped property is 6.27 acres in size. Much of this area consists of wetlands, delineated by others, and the eastern half of the site is a Bonneville Power Administration (BPA) utility tract. The area slated for development is predominantly located at the southwestern portion of the property and is planned for 16 single-family residences and a small access road that ends in a cul-de-sac, as shown in Figure 2. We have not been provided with details of the proposed detention system, but understand that the currently planned storm water detention system includes a concrete vault. We understand that site grading will include minor cuts and fills. We understand that you plan to demolish all existing buildings prior to construction of the new residential development. Geotechnical Engineering Report Sandhu Property August 18, 2005 CG File No. 1928 Page 2 SCOPE The purpose of this study is to explore and characterize the subsurface conditions and present recommendations for site development. Specifically, our scope of services as outlined in our Services Agreement, dated July 26, 2005, includes the following; I. Review available geologic maps of the area. 2. Explore the subsurface conditions at the site with backhoe-excavated test pits. 3. Provide recommendations for building foundations. 4. Provide recommendations for site preparation and grading. 5. Provide general recommendations for site drainage. 6. Prepare a written report to document our conclusions and recommendations. SITE CONDITIONS Surface Conditions The overall property is 6.27 acres in size and has maximum dimensions of approximately 600 feet in the east-west direction and 580 feet in the north-south direction, with the proposed developed area consisting of a roughly 300-foot-square area extending from the southwest comer of the property. Access to the site is provided by SE 184th Street, which runs along the southern edge of the site. Single-family residences border the western property line, and BPA powerlines trend roughly north-south through the eastern portion of the property. The project site is bordered to the north by wooded land. A proposed layout of the main development area is shown on the Site Plan in Figure 2. The site is mostly flat lying to gently sloping downward to the east. Two single-family residences are currently located near to the southern property line, along with a few outbuildings and several vehicles. We understand that all structures are to be removed. We observed a partially open pit near to the westernmost residence that contained buried household debris. The area around the easternmost residence is mostly grass covered with small-to medium-sized trees. Much of the property outside of this area consists of an open grassy field with a few trees along the north and west borders of the planned developed area. We understand that this open field area had previously been graded for the purposes of creating a motocross track. Wetlands, delineated by others, are located to the north and east of the planned developed area. Cornerstone Geotechnical, Inc. Geotechnical Engineering Report Sandhu Property August 18, 2005 CG File No. 1928 Page 3 Geology Most of the Puget Sound Region was affected by past intrusion of continental glaciation. The last period of glaciation, the Vashon Stade of the Fraser Glaciation, ended approximately 11,000 years ago. Many of the geomorphic features seen today are a result of scouring and overriding by glacial ice. During the Vashon Stade, much of the Puget Sound region was overridden by over 3,000 feet of ice. Soil layers overridden by the ice sheet were compacted to a much greater extent than those that were not. Part of a typical glacial sequence includes recessional outwash sand over glacial till. Glacial till is an unsorted mixture of sand, silt, and gravel that is deposited at the bottom of the glacier, which is commonly referred to as "hardpan". Glacial drift, similar to glacial till, will display better sorting and is typically used as a more general term. Some glacial drift is deposited directly from the melting ice during glacial retreat. The geologic unit mapped for this area is shown on the Surficial Geologic Map of the Renton Quadrangle, King County, Washington, by D.R. Mullineaux (U.S.G.S., 1965). The site is shown to be underlain by glacial till. Our site explorations encountered glacial till and glacial drift, along with recessional outwash. Recessional outwash generally consists of sand and gravel deposited by meltwater streams during glacial retreat. At glacial margins, it is not unusual to find interbedded sands and gravels with glacial drift, suggesting a fluctuating glacial margin. The glacial drift/till has been consolidated under the weight of the continental glaciers. The drift/till exhibits both high strength and low permeability. Explorations Subsurface conditions were explored at the site on August 11, 2005, by excavating a total of four test pits. The test pits were excavated to depths of 6.0 to 13.0 feet below the ground surface. The explorations were located in the field by a geologist from this firm who also examined the soils and geologic conditions encountered, and maintained logs of the test pits. The approximate locations of the explorations are shown on the Site Plan in Figure 2. The soils were visually classified in general accordance with the Unified Soil Classification System, a copy of which is presented as Figure 3. Subsurface Conditions A brief description of the conditions encountered in our explorations is included below. For a more detailed description of the soils encountered, review the test pit logs in Figure 4. Cornerstone Geotechnical, Inc. Geotechnical Engineering Report Sandhu Property August 18, 2005 CG File No. 1928 Page4 All of our explorations encountered roughly 2.0 to 2.5 feet of fill material, presumably placed during the construction, and subsequent removal, of the relict motocross track. This fill displayed a weakly- cemented "till-like" appearance. In Test Pit 4, this fill included assorted household debris. At Test Pits 1 and 4, the fill overlies a buried layer of topsoil, consisting of dark brown silty sand with roots and organics. Below the topsoil, we encountered a weathered zone of loose to medium dense, red- brown silty fine to medium sand with occasional gravel, approximately 1.0 to 3.0 feet in thickness. Below this weathered zone, we encountered medium dense to dense, slightly rust-stained gray fine to medium sand with gravel, interpreted as recessional outwash, that extended to roughly 5 .0 to 6.0 feet below the ground surface. Underlying this outwash, Test Pit 1 encountered dense to very dense, gray silty sand with gravel, interpreted as glacial till, to the depth explored. At Test Pit 4, the material underlying the outwash consisted of gray silty fine to medium sand with gravel, with a somewhat sandier zone between 7.5 and 11.0 feet below the ground surface. We interpret this material as glacial drift deposited during glacial ablation that can vary in thickness and density. Test Pit 4 was completed in this drift. At Test Pits 2 and 3, the fill zone directly overlies dense to very dense silty sand with gravel, with the soil at Test Pit 3 being somewhat sandier than that found at Test Pit 2. We have interpreted the soil encountered at Test Pit 2 to be glacial till and the soil encountered at Test Pit 3 to be glacial drift. Due to access constraints, we were unable to perform a test pit at the planned vault location. However, we advanced Test Pit 4 to a depth of 13.0 feet near to the planned vault area, We expect that the soil conditions at the planned vault area will be similar to those encountered in Test Pit 4. This should be verified during construction. Hydrologic Conditions We encountered ground water seepage in Test Pit 4 at 11.0 feet. We consider this water to be perched within a sandier portion of the drift. We also encountered rust staining in the recessional outwash overlying the till or drift. This mottled zone is also a sign of perched water during the wet season. The thickness of the mottled layer does not indicate full saturation of the unit. The dense to very dense drift/till below this mottled zone is considered poorly draining. During the wetter times of the year, we expect perched water conditions will occur as pockets of water on top of the till layer. Perched water does not represent a regional ground water "table" within the upper soil horizons. Volumes of perched ground water vary depending upon the time of year and the upslope recharge conditions. Cornerstone Geotechnical, Inc. Geotechnical Engineering Report Sandhu Property August 18, 2005 CG File No. 1928 Page 5 GEOLOGIC HAZARDS Landslide Hazards The subject site is underlain by dense to very dense glacial soils at shallow depths. These materials typically exhibit very high shear strength and have a high resistance to slope failure. The terrain within the site is flat-lying to gently sloping, posing little risk of slope failure. Erosion Hazard The erosion hazard criteria used for determination of affected areas includes soil type, slope gradient, vegetation cover, and ground water conditions. The erosion potential is related to vegetative cover and the specific surface soil types (group classification), which are related to the underlying geologic units. The Soil Survey of King County Area Washington by the Soil Conservation Service (SCS) was reviewed to determine the erosion hazard of the on-site soils. The site surface soils were classified using the SCS classification system as Alderwood gravelly sandy loam (AgB). The corresponding geologic unit for these soils is till, which is in general agreement with the soils encountered in our site explorations. The erosion hazard for the soil is listed as being slight for the generally flat-lying to gently sloping conditions at the site. Seismic Hazard The site is classified based on its overall soil profile using Table 1615.1.1 of the 2003 International Building Code (IBC). It is our opinion, based on our subsurface explorations, that the soil profile in accordance with Table 1615.1.1 of the 2003 IBC is Site Class C. We referenced the 2002 map from the US Geological Survey website to obtain the following seismic parameters. The USGS website includes the most updated published data on seismic conditions. The seismic design parameters are: S, 135.16% g F, Fv 46.06% g 1.0 1.3 Based on Table 1615.1.2(1) of the 2003 IBC Based on Table 1615.1.2(2) of the 2003 IBC Site specific coefficients and adjusted maximum considered earthquake spectral response acceleration parameters apply as shown in Section 1615.1 of the IBC. Cornerstone Geotechnical, Inc. Geotechnical Engineering Report Sandhu Property August 18, 2005 CG File No. 1928 Page 6 Additional seismic considerations include liquefaction potential and amplification of ground motions by soft soil deposits. The liquefaction potential is highest for loose sand with a high ground water table. The underlying dense soil is considered to have a very low potential for liquefaction and amplification of ground motion. CONCLUSIONS AND RECOMMENDATIONS General It is our opinion that the site is compatible with the planned development. The underlying dense to very dense, silty sands are capable of supporting the planned structures and pavements. We recommend that the foundations for the structures extend through any topsoil, fill, loose, or disturbed soils, and bear on the underlying medium dense to very dense, native soils, or on structural fill extending to these soils. Given the known history of previous grading at the site, we expect that there may be areas with deeper fill wnes than those encountered by our explorations. Also, based on our observations of household debris at the site, it should be anticipated that one or more "garbage pits" may exist at the site. The soils likely to be exposed during construction are highly moisture sensitive and will disturb easily when wet or during wet conditions. We recommend that construction take place during the drier summer months, if possible. If construction takes place during the wet season, additional expenses and delays should be expected due to the wet conditions. Additional expenses could include additional depth of site stripping, export of on-site soil, the import of clean granular soil for fill, and the need to place a blanket of rock spalls in the access roads and paved areas prior to placing structural fill. Site Preparations and Grading The first step of site preparation should be to strip the vegetation, topsoil, fill or loose soils to expose medium dense to very dense native soils in pavement and building areas. A geotechnical engineer should evaluate the subgrade to confirm bearing soils. Additional explorations may be needed during construction to confirm native bearing soils. This material should be removed from the site, or stockpiled for later use as landscaping fill. The resulting subgrade should be compacted to a firm, non-yielding condition. Areas observed to pump or weave should be repaired prior to placing hard surfaces. The on-site soil likely to be exposed during construction is considered moisture sensitive, and the surface will disturb easily when wet. We expect these soils would be difficult, if not impossible, to compact to Cornerstone Geotechnical, Inc. Geotechnical Engineering Report Sandhu Property August 18, 2005 CG File No. 1928 Page 7 structural fill specifications in wet weather. We recommend that earthwork be conducted during the drier months. The sand deposit will be slightly moisture sensitive. Additional expenses of wet weather or winter construction would include extra excavation and use of imported fill or rock spalls. During wet weather, alternative site preparation methods may be necessary. These methods may include utilizing a smooth-bucket trackhoe to complete site stripping and diverting construction traffic around prepared subgrades. Disturbance to the prepared subgrade may be minimized by placing a blanket of rock spalls or imported sand and gravel in traffic and roadway areas. Cutoff drains or ditches can also be helpful in reducing grading costs during the wet season. These methods can be evaluated at the time of construction. Structural Fill General: All fill placed beneath buildings, pavements or other settlement sensitive features should be placed as structural fill. Structural fill, by definition, is placed in accordance with prescribed methods and standards, and is monitored by an experienced geotechnical professional or soils technician. Field- monitoring procedures would include the performance of a representative number of in-place density tests to document the attainment of the desired degree of relative compaction. Materials: Imported structural fill should consist of a good quality, free-draining granular soil, free of organics and other deleterious material, and be well graded to a maximum size of about 3 inches. Imported, all-weather structural fill should contain no more than 5 percent fines (soil finer than a Standard U.S. No. 200 sieve), based on that fraction passing the U.S. 3/4-inch sieve. The use of on-site soil as structural fill will be dependent on moisture content control. Some drying of the native soils may be necessary in order to achieve compaction. During warm, sunny days this could be accomplished by spreading the material in thin lifts and compacting. Some aeration and/or addition of moisture may also be necessary. We expect that compaction of the native soils to structural fill specifications would be difficult, if not impossible, during wet weather. Fill Placement: Following subgrade preparation, placement of the structural fill may proceed. Fill should be placed in 8-to 10-inch-thick uniform lifts, and each lift should be spread evenly and be thoroughly compacted prior to placement of subsequent lifts. All structural fill underlying building areas, and within a depth of 2 feet below pavement and sidewalk subgrade, should be compacted to at least 95 percent of its maximum dry density. Maximum dry density, in this report, refers to that density as Cornerstone Geotechnical, Inc. Geotechnical Engineering Report Sandhu Property August 18, 2005 CG File No. 1928 Page 8 determined by the ASTM D 1557 compaction test procedure. Fill more than 2 feet beneath sidewalks and pavement subgrades should be compacted to at least 90 percent of the maximum dry density. The moisture content of the soil to be compacted should be within about 2 percent of optimum so that a readily compactable condition exists. It may be necessary to overexcavate and remove wet surficial soils in cases where drying to a compactable condition is not feasible. All compaction should be accomplished by equipment of a type and size sufficient to attain the desired degree of compaction. Temporary and Permanent Slopes Temporary cut slope stability is a function of many factors, such as the type and consistency of soils, depth of the cut, surcharge loads adjacent to the excavation, length of time a cut remains open, and the presence of surface or ground water. It is exceedingly difficult under these variable conditions to estimate a stable temporary cut slope geometry. Therefore, it should be the responsibility of the contractor to maintain safe slope configurations, since the contractor is continuously at the job site, able to observe the nature and condition of the cut slopes, and able to monitor the subsurface materials and ground water conditions encountered. We anticipate temporary cuts for installation of utilities. For planning purposes, we recommend that temporary cuts in the fill or near-surface weathered soils be no greater than 1.5 Horizontal to I Vertical l.5H:IV). Cuts in dense to very dense soil may stand at a IH:lV inclination or possibly steeper. If ground water seepage is encountered, we would expect that flatter inclinations would be necessary. We recommend that cut slopes be protected from erosion. Measures taken may include covering cut slopes with plastic sheeting and diverting surface runoff away from the top of cut slopes. We do not recommend vertical slopes for cuts deeper than 4 feet, if worker access is necessary. We recommend that cut slope heights and inclinations conform to local and WISHNOSHA standards. Final slope inclinations for structural fill and the cuts in the native soils should be no steeper than 2H: 1V. Lightly compacted fills or common fills should be no steeper than 3H: IV. Common fills are defined as fill material with some organics that are "trackrolled" into place. They would not meet the compaction specification of structural fill. Final slopes should be vegetated and covered with straw or jute netting. The vegetation should be maintained until it is established. Cornerstone Geotechnical, Inc. Geotechnical Engineering Report Sandhu Property August 18, 2005 CG File No. 1928 Page 9 Foundations Conventional, shallow spread foundations should be founded on undisturbed, medium dense to very dense, native soils, or be supported on structural fill extending to those soils. If the soil at the planned bottom of footing elevation is not medium dense to very dense, it should be overexcavated to expose suitable bearing soil, and the excavation should be filled with structural fill, or the footing may be overpoured with extra concrete. Footings should extend at least 18 inches below the lowest adjacent finished ground surface for frost protection and bearing capacity considerations. International Building Code (IBC) guidelines for minimum foundation widths should be followed for both continuous and isolated spread footings. Standing water should not be allowed to accumulate in footing trenches. All loose or disturbed soil should be removed from the foundation excavation prior to placing concrete. For foundations constructed as outlined above, we recommend an allowable design bearing pressure of 2,000 pounds per square foot (psf) be used for the footing design. International Building Code (IBC) guidelines should be followed when considering short-term transitory wind or seismic loads. Potential foundation settlement using the recommended allowable bearing pressure is estimated to be less than !- inch total and \!,-inch differential between footings or across a distance of about 30 feet. Higher soil bearing values may be appropriate for footings founded on the unweathered drift/till, and with wider footings. These higher values can be determined after a review of a specific design. Lateral loads can be resisted by fiiction between the foundation and subgrade soil, and by passive soil resistance acting on the below-grade portion of the foundation. For the latter, the foundation must be poured "neat" against undisturbed soil or backfilled with clean, free-draining, compacted structural fill. Passive resistance may be calculated as a triangular equivalent fluid pressure distribution. We recommend that an equivalent fluid density of 225 pounds per cubic foot (pcf) be used to calculate the allowable lateral passive resistance for the case of a level ground surface adjacent to the footing. An allowable coefficient of fiiction between footings and soil of 0.45 may be used, and should be applied to the vertical dead load only. A factor of safety of 2.0 has been applied to the passive pressure to account for required movements to generate these pressures. The friction coefficient does not include a factor of safety. Cornerstone Geotechnical. Inc. Geotechnical Engineering Report Sandhu Property August 18, 2005 CG File No. 1928 Page 10 Slabs-On-Grade Slab-on-grade areas should be prepared as recommended in the Site Preparation and Grading subsection. Slabs should be supported on medium dense to very dense native soils, or on structural fill extending to these soils. Where moisture control is a concern, we recommend that slabs be underlain by 6 inches of free-draining coarse sand or pea gravel for use as a capillary break. A suitable vapor barrier, such as heavy plastic sheeting, should be placed over the capillary break. Drainage We recommend that runoff from impervious surfaces, such as roofs, driveway and access roadways, be collected and routed to an appropriate storm water discharge system. Final site grades should allow for drainage away from any buildings. We suggest that the finished ground surface be sloped at a gradient of 3 percent minimum for a distance of at least 10 feet away from the buildings. Surface water should be collected by permanent catch basins and drain lines, and be discharged into a storm drain system. We recommend that footing drains be used around all of the structures where moisture control is important. The underlying silty sand will pond water that accumulates in the crawl space. It is good practice to use footing drains installed at least I foot below the planned finished floor slab or crawl space elevation to provide drainage for the crawl space. At a minimum, the crawl space should be sloped to drain to an outlet tied to the drainage system. If drains are omitted around slab-on-grade floors where moisture control is important, the slab should be a minimum of I foot above surrounding grades. Where used, footing drains should consist of 4-inch-diameter, perforated PVC pipe that is surrounded by free-draining material, such as pea gravel. Footing drains should discharge into tightlines leading to an appropriate collection and discharge point. Crawl spaces should be sloped to drain, and a positive connection should be made into the foundation drainage system. For slabs-on-grade, a drainage path should be provided from the capillary break material to the footing drain system. Roof drains should not be connected to wall or footing drains. Detention Vault If a concrete detention vault is to be constructed, the concrete walls of the vault may be supported on footing foundations bearing on the underlying dense to very dense glacial soils. The allowable soil bearing pressure should not exceed 4,000 pounds per square foot (psf) for the design of the wall footings poured on undisturbed glacial soil. Cornerstone Geotechnical, Inc. Geotechnical Engineering Report Sandhu Property August 18, 2005 CG File No. 1928 Page 11 We recommend that footing drains be installed on the outside of perimeter footings. The footing drains should be at least 4 inches in diameter and should consist of perforated or slotted, rigid, smooth-walled PVC pipe, laid at the bottom of the footings. The drain line should be surrounded with free-draining pea gravel or coarse sand and wrapped with a layer of non-woven filter fabric. A vertical drainage blanket at least 12 inches thick, consisting of compacted pea gravel or other free-draining granular soils, should be placed against the walls. A vertical drain mat, such as Miradrain 6000 by Mirafi Inc., may be placed against the walls in lieu of the vertical drainage blanket. Structural fill is then placed behind the vertical drainage blanket or drain mat to backfill the walls. The vertical drainage blanket or drain mat should be hydraulically connected to the drain line at the base of the walls. Sufficient number of cleanouts at strategic locations should be installed for periodical cleaning of the wall drain line to prevent clogging. The perimeter walls of the concrete vault with a lid would be restrained at their top from horizontal movement and should be designed for at-rest lateral soil pressure, while the perimeter walls of a vault without a lid would be unrestrained at the top and may be designed for active lateral soil pressure. Active earth pressure and at-rest earth pressure can be calculated based on equivalent fluid density. Equivalent fluid densities for active and at-rest earth pressure of 35 pcf and 55 pcf, respectively, may be used for design for a level backslope. These values assume that the on-site soils are used for backfill, and that the wall backfill is drained. The preceding values do not include the effects of surcharges due to foundation loads, traffic or other surface loads. Surcharge effects should be considered where appropriate. For undrained soil conditions, the active and at-rest pressures should be increased to 80 pcf and 90 pcf, respectively. Undrained conditions may occur in the lower portion of the vault if there is not suitable fall to place a wall drain at the footing elevation. All wall backfill should be well compacted. Care should be taken to prevent the buildup of excess lateral soil pressures due to overcompaction of the wall backfill. This can be accomplished by placing wall backfill in 8-inch loose lifts and compacting with small, hand-operated compactors. We recommend that an equivalent fluid density of 225 pcf be used to calculate the allowable lateral passive resistance for the case of a level ground surface adjacent to the footing. An allowable coefficient of friction between footings and soil of 0.45 may be used, and should be applied to the vertical dead load only. A factor of safety of 2.0 has been applied to the passive pressure to account for required movements to generate these pressures. The friction coefficient does not include a factor of safety. Cornerstone Geotechnical, Inc. Geotechnical Engineering Report Sandhu Property August 18, 2005 CG File No. 1928 Page 12 Utilities Our explorations indicate that specific deep dewatering will not be needed to install utilities. Anticipated ground water is expected to be handled with pumps in the trenches. We also expect that some ground water seepage may develop during and following the wetter times of the year. We expect this seepage to mostly occur in pockets. We do not expect significant volumes of water in these excavations. The soils likely to be exposed in utility trenches after site stripping are considered highly moisture sensitive. We recommend that they be considered for trench backfill during the drier portions of the year. Provided these soils are within 2 percent of their optimum moisture content, they should be suitable to meet compaction specifications. During the wet season, it may be difficult to achieve compaction specifications; therefore, soil amendment with kiln dust or cement may be needed to achieve proper compaction with the on-site materials. Pavement The performance of roadway pavement is critically related to the conditions of the underlying subgrade. We recommend that the subgrade soils within the roadways be treated and prepared as described in the Site Preparation and Grading subsection of this report. Prior to placing base material, the subgrade soils should be compacted to a non-yielding state with a vibratory roller compactor and then proof-rolled with a piece of heavy construction equipment, such as a fully-loaded dump truck. Any areas with excessive weaving or flexing should be overexcavated and recompacted or replaced with a structural fill or crushed rock placed and compacted in accordance with recommendations provided in the Structural Fill subsection ofthis report. MONITORING We should be retained to provide monitoring and consultation services during construction to confirm that the conditions encountered are consistent with those indicated by the explorations, and to provide recommendations for design changes, should the conditions revealed during the work differ from those anticipated. As part of our services, we would also evaluate whether or not earthwork and foundation installation activities comply with contract plans and specifications. Cornerstone Geotechnical, Inc. Geotechnical Engineering Report Sandhu Property August 18, 2005 CG File No. 1928 Page 13 USE OF THIS REPORT We have prepared this report for Davis Consulting, Inc., and its agents, for use in planning and design of this project. The data and report should be provided to prospective contractors for their bidding and estimating purposes, but our report, conclusions and interpretations should not be construed as a warranty of subsurface conditions. The scope of our work does not include services related to construction safety precautions, and our recommendations are not intended to direct the contractors' methods, techniques, sequences or procedures, except as specifically described in our report, for consideration in design. There are possible variations in subsurface conditions. We recommend that project planning include contingencies in budget and schedule, should areas be found with conditions that vary from those described in this report. Within the limitations of scope, schedule and budget for our work, we have strived to take care that our work has been completed in accordance with generally accepted practices followed in this area at the time this report was prepared. No other conditions, expressed or implied, should be understood. oOo Cornerstone Geotechnical, Inc. Geotechnical Engineering Report Sandhu Property August 18, 2005 CG File No. 1928 Page 14 We appreciate the opportunity to be of service to you. If there are any questions concerning this report or ifwe can provide additional services, please call. Sincerely, Cornerstone Geotechnical, Inc. c;l·U/ Jeff Laub, LG Project Geologist Rick B. Powell, PE Principal JPL:RBP:nt Three Copies Submitted Four Figures Information about this Geotechnical Engineering Report Cornerstone Geotechnical, Inc. Vicinity Map N Cornerstone Phone: (425) 844-1977 Geotechnical, Inc. Fax ( 425 844 1987 17625-130\h Ave NE, C-102 • Woodinville, WA• 98072 Davis -Sandhu Property 1928 Figure 1 File Number Site Plan so' <. i,§ LOT l~ Reference: Site Plan based on a scanned version of Topographic Site Survey prepared by Cramer Northwest, Inc. and dated June 2005 Cornerstone Phone (425) 844-1977 Geotechnical, Inc. Fax ' 425 84 4- 1987 17625·130th Ave NE, C-102 • Woodinville, WA• 98072 File Number 0 N LEGEND Number and Approximate Location of Test Pit 60 120 I Scale 1" = 60' Davis -Sandhu Property Figure 1928 2 Unified Soil Classification System COARSE- GRAINED SOILS MORE THAN 50% RETAINED ON number 200 SIEVE FINE- GRAINED SOILS MAJOR DIVISIONS GRAVEL MORE THAN 50% OF COARSE FRACTION RETAINED ON NO. 4 SIEVE SAND MORE THAN 50% OF COARSE FRACTION PASSES NO. 4 SIEVE SILT AND CLAY LIQUID LIMIT LESS THAN 50% MORE THAN 50% SILT AND CLAY PASSES NO. 200 SIEVE LIQUID LIMIT 50%0RMORE CLEAN GRAVEL GRAVEL WITH FINES CLEAN SAND SAND WITH FINES INORGANIC ORGANIC INORGANIC ORGANIC HIGHLY ORGANIC SOILS NOTES: 1) Field classification is based on visual examination of soil in general accordance with ASTM D 2488-83. 2) Soil classification using laboratory tests is based on ASTM D 2487-83. 3) Descriptions of soil density or consistency are based on interpretation of blowcount data. visual appearance of soils. and/or test data. GROUP SYMBOL GW GP GM GC SW SP SM SC ML CL OL MH CH OH PT GROUP NAME WELL-GRADED GRAVEL, FINE TO COARSE GRAVEL POORLY-GRADED GRAVEL SILTY GRAVEL CLAYEY GRAVEL WELL-GRADED SAND, FINE TO COARSE SAND POORLY-GRADED SAND SILTY SAND CLAYEY SAND SILT CLAY ORGANIC SILT, ORGANIC CLAY SILT OF HIGH PLASTICITY, ELASTIC SILT CLAY OF HIGH PLASTICITY, FAT CLAY ORGANIC CLAY, ORGANIC SILT PEAT SOIL MOISTURE MODIFIERS Dry-Absence of moisture, dusty, dry to the touch Moist-Damp, but no visible water Wet-Visible free water or saturated, usually soil is obtained from below water table Phone: (425) 844-1977 Fax: (425) 844-1987 Unified Soil Classification SystemCornerstone Ill Geotechnical, Inc. 17625-130thAve NE, C-102 • Woodinville, WA" 98072 Figure 3 DEPTH TEST PITONE 0.0-1.2 1.2-2.5 2.5-3.0 3.0-6.0 6.0-8.0 8.0-8.5 TEST PIT TWO 0.0-2.0 2.0-6.0 TEST PIT THREE 0.0-2.5 2.5-7.5 TEST PIT FOUR 0.0-2.0 2.0-2.5 2.5-3.5 3.5-5.0 5.0-13.0 USC SM SP SM SM SP SM SM SM SM SM SM SM SM SP SM LOG OF EXPLORATION SOIL DESCRIPTION GRAY SILTY SAND WITH GRAVEL (LOOSE, DRY) (FILL) BROWN-GRAY GRAVELY FINE TO MEDIUM SAND (LOOSE, DRY TO MOIST) (FILL) DARK BROWN SILTY SAND WITH ORGANICS AND ROOTS (LOOSE, MOIST) (TOPSOIL) RED-BROWN SILTY FINE TO MEDIUM SAND (LOOSE TO MEDIUM DENSE, MOIST) WEATHERED OUTWASH) SLIGHTY RUST-STAINED GRAY FINE TO MEDIUM SAND (MEDIUM DENSE TO DENSE, MOIST) (RECESSIONAL OUTWASH) GRAY SILTY SAND WITH GRAVEL (VERY DENSE, MOIST) (TILL) SAMPLES WERE COLLECTED AT 3.5, 7.0 AND 8.5 FEET GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED SLIGHT TEST PIT CAVING WAS ENCOUNTERD FROM 5.0 TO 7.0 FEET TEST PITWAS COMPLETED AT 8.5 FEET ON 8111/05 GRAY SILTY SAND WITH GRAVEL (LOOSE, DRY) (FILL) GRAY SILTY SAND WITH GRAVEL (DENSE TO VERY DENSE, MOIST) (TILL) NO SAMPLES WERE COLLECTED GROUND WATER SEEPAGE WAS NOT ENCOUNTERED TEST PIT CAVING WAS NOT ENCOUNTERED TEST PIT WAS COMPLETED AT 6.0 FEET ON 8111/05 GRAY SILTY SAND WITH GRAVEL (LOOSE, DRY) (FILUMODIFIED GROUND) GRAY SILTY FINE TO MEDIUM SAND WITH GRAVEL (DENSE TO VERY DENSE, MOIST) DRIFT) SAMPLE WAS COLLECTED AT 7.5 FEET GROUND WATER SEEPAGE WAS NOT ENCOUNTERED TEST PIT CAVING WAS NOT ENCOUNTERED TEST PIT WAS COMPLETED AT 7.5 FEET ON 8111/05 GRAY SILTY SAND WITH GRAVEL AND ASSORTED DEBRIS (LOOSE, DRY) (FILL) DARK BROWN SILTY SAND WITH ORGANICS AND ROOTS (LOOSE, MOIST) (TOPSOIL) RED-BROWN SILTY FINE TO MEDIUM SAND (LOOSE TO MEDIUM DENSE, MOIST) WEATHERED OUTWASH) SLIGHTY RUST-STAINED GRAY FINE TO MEDIUM SAND (MEDIUM DENSE TO DENSE, MOIST) (RECESSIONAL OUTWASH) GRAY SILTY FINE TO MEDIUM SAND WITH GRAVEL (DENSE TO VERY DENSE, MOIST wet from 7.5 to 11.0 feet)) (DRIFn somewhat sandier from 7. 5 to 11. Ofeet NO SAMPLES WERE COLLECTED GROUND WATER SEEPAGE WAS ENCOUNTERED AT 11.0 FEET TEST PIT CAVING WAS ENCOUNTERED FROM 7.5 TO 12.0 FEET TEST PIT WAS COMPLETED AT 13.0 FEET ON 8111/05 CORNERSTONE GEOTECHNICAL, INC. FILE NO 1928 FIGURE 4 Important Information About Your Geotechnical Engineering RepOrt Geotechnical Services Are Performed for Specific Purposes, Persons, and Projects Geotechnical engineers structure their services to meet the specific needs of their clients. A geotechnical engineering study conducted for acivil engi- neer may not fulfill the needs of aconstruction contractor or even another civil engineer. Because Bach geotechnical engineering study is unique, each geotechnical engineering report is unique, prepared solelyfor the client. No one except you should rely on your geotechnical engineering report without first conferring with the geotechnical engineer who prepared it. And no one not even you-should apply the report for any purpose or project except the one originally contemplated. Read the Full Report Serious problems have occurred because those relying on a geotechnical engineering report did not read it all. Do not rely on an executive summary. Do not read selected elements only. AGeotechnical Engineering Report Is Based on AUnique Set of Project-Specific Factors Geotechnical engineers consider a number of unique, project-specific fac- tors when establishing the scope of astudy. Typical factors include: the client's goals, objectives, and risk management preferences; the general nature of the structure involved. its size, and configuration; the location of the structure on the site; and other planned or existing site improvements, such as access roads, parking lots, and underground utilities. Unless the geotechnical engineer who conducted the study specifically indicates oth- erwise, do not rely on ageotechnical engineering report that was: not prepared for you, not prepared for your project, not prepared for the specific site explored, or completed before important project changes were made. Typical changes that can erode the reliability of an existing geotechnical engineering report include those that affect: the function of the proposed structure, as when it's changed from a parking garage to an office building, or from a light industrial plant to a refrigerated warehouse, elevation, configuration, location, orientation, or weight of the proposed structure, composition of the design team, or project ownership. As ageneral rule, always inform your geotechnical engineer of project changes--ilven minor ones--and request an assessment of their impact. Geotechmcal engineers cannot accept responsibility or liability for problems that occur because their reports do not consider developments of which they were not informed. Subsurface Conditions Can Change Ageotechnical engineering report is based on conditions that existed at the time the study was pertormed. Do not rely on ageotechnica/ engineer- ing report whose adequacy may have been affected by: the passage of time; by man-made.events, such as construction on or adjacent to the site; or by natural events, such as floods, earthquakes, or groundwater fluctua- tions. Always contact the geotechnical engineer before applying the report to determine if it is still reliable. Aminor amount of additional testing or analysis could prevent major problems. Most Geotechnical Findings Are Professional Opinions Site exploration identifies subsurtace conditions only at those points where subsurtace tests are conducted or samples are taken. Geotechnical engi- neers review field and laboratory data and then apply their professional judgment to render an opinion about subsurtace conditions throughout the site. Actual subsurtace conditions may differ-sometimes significantly- from those indicated in your report. Retaining the geotechnical engineer who developed your report to provide construction observation is the most effective method of managing the risks associated with unanticipated conditions. AReport's Recommendations Are Not Final Do not overrely on the construction recommendations included in your report. Those recommendations are not final, because geotechnical engi- neers develop them principally from judgment and opinion. Geotechnical engineers can finalize their recommendations only by observing actual subsurtace conditions revealed during construction. The geotechnical engineer who developed your report cannot assume responsibility or liability for the report's recommendations if that engineer does not perform construction observation. AGeotechnical Engineering Report Is Subject to Misinterpretation Other design team members' misinterpretation of geotechnical engineering reports has resulted in costly problems. Lower that risk by having your geo- technical engineer confer with appropriate members of the design team after submitting the report. Also retain your geotechnical engineer to review perti- nent elements of the design team's plans and specifications. Contractors can also misinterpret ageotechnical engineering report. Reduce that risk by having your geotechnical engineer participate in prebid and preconstruction conferences, and by providing construction observation. Do Not Redraw the Engineer's Logs Geotechnical engineers prepare final boring and testing logs based upon their interpretation of field logs and laboratory data. To prevent errors or omissions, the logs included in a geotechnical engineering report should never be redrawn for inclusion in architectural or other design drawings. Only photographic or electronic reproduction is acceptable, but recognize that separating logs from the report can elevate risk. Give Contractors a Complete Report and Guidance Some owners and design professionals mistakenly believe they can make contractors liable for unanticipated subsurtace conditions by limiting what they provide for bid preparation. To help prevent costly problems, give con- tractors the complete geotechnical engineering report, bu/preface it with a clearly written letter of transmittal. In that letter, advise contractors that the report was not prepared for purposes of bid development and that the report's accuracy is limited; encourage them to confer with the geotechnical engineer who prepared the report (a modest fee may be required) and/or to conduct additional study to obtain the specific types of information they need or prefer. Aprebid conference can also be valuable. Be sure contrac- tors have su/fic1ent time to pertorm additional study. Only then might you be in a position to give contractors the best information available to you, while requiring tnem to at least share some of the financial responsibilities stemming from unanticipated conditions. Read Responsibility Provisions Closely Some clients, design professionals, and contractors do not recognize that geotechnical engineering is far less exact than other engineering disci- plines. This lack of understanding has created unrealistic expectations that have led to disappointments, claims, and disputes: To help reduce the risk of such outcomes, geotechnical engineers commonly include avariety of explanatory provisions in their reports. Sometimes labeled "limitations" many of these provisions indicate where geotechnical engineers' responsi- bilities begin and end, to help others recognize their own responsibilities and risks. Read these provisions closely Ask questions. Your geotechnical engineer should respond fully and frankly. Geoenvironmental Concerns Are Not Covered The equipment, techniques, and personnel used to pertorm a geoenviron- mental study differ significantly from those used to pertorm ageotechnical study. For that reason, a geotechnical engineering report does not usually relate any geoenvironmental findings, conclusions, or recommendations; e.g., about the likelihood of encountering underground storage tanks or regulated contaminants. Unanticipated environmental problems have led to numerous project failures. If you have not yet obtained your own geoen- vironmental information, ask your geotechnical consultant for risk man- agement guidance. Do not rely on an environmental report prepared tor someone else. Obtain Professional Assistance To Deal with Mold Diverse strategies can be applied during building design, construction, operation, and maintenance to prevent significant amounts of mold from growing on indoor surtaces. To be effective, all such strategies should be devised for the express purpose of maid prevention, integrated into acom- prehensive plan, and executed with diligent oversight by aprofessional mold prevention consultant. Because just asmall amount of water or moisture can lead to the development of severe mold infestations, a num- ber of mold prevention strategies focus on keeping building surtaces dry. While groundwater, water infiltration, and similar issues may have been addressed as part of the geotechnical engineering study whose findings are conveyed in this report, the geotechnical engineer in charge of this project is not amold prevention consultant; none of the services per- formed in connection with the geotechnical engineer's study were designed or conducted for the purpose of mold preven- tion. Proper implementation of the recommendations conveyed in this report will not of itself be sufficient to prevent mold from growing in or on the structure involved. Rely, on Your ASFE-Member Geotechncial Engmeer for Additional Assistance Membership in ASFE/The Best People on Earth exposes geotechnical engineers to awide array of risk management techniques that can be of genuine benefit for everyone involved with a construction project. Confer with you ASFE-member geotechnical engineer for more information. ASFE TIit lesl reopln an Earih 8811 Colesville Road/Suite G106, Silver Spring, MD 20910 Telephone: 301/565-2733 Facsimile: 301/589-2017 / e-mail: info@asfe.org www.asfe.org Copyright 2004 by ASFE, Inc. Duplication, reproduction, or copying of tnis aocumenr. in whole or in part, by any means whatsoever, is strictly prohibited, except with ASFE's specific written permission. Excerpting, quoting, or otherwise extracting wording from this document is permitted only with the express wn"tten permission of ASFE, and only for purposes of scholarly research or book review. Only membsrs of ASFE may use this document as a complement to or as an element of & geotechfiical engineering report. Any other firm, individual, or other entity that so uses this document without being an ASFE member could be committing negligent or intentional (fraudulent) misrepresentation. IIGER06045.0M