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Home My WebLink AboutRS- GeoTech Report January 3, 2019 Mr. Edward Pozniak 14311 SE 16th St Bellevue, WA 98007 Geotechnical Engineering Evaluation Swanson Gardner Meyers Office Addition 4512 Talbot Road South Renton, Washington NGA File No. 1071118 Dear Mr. Pozniak, We are pleased to submit the attached report titled “Geotechnical Engineering Evaluation – Swanson Gardner Meyers Office Addition – 4512 Talbot Road South – Renton, Washington.” This report summarizes our observations of the existing surface and subsurface conditions within the site, and provides general recommendations for the proposed site development. Our services were completed in general accordance with the proposal signed by Mrs. Polly Gardner on November 8, 2018. The property consists of a rectangular parcel of 0.62 acres. The ground surface within the property slopes gently upward from the northwest to southeast. Vegetation generally consists of grass yard areas, scattered young to mature trees, and landscaping plants. The property is currently occupied by an office building in the northeast portion of the site, and a paved parking area to the west. We understand that the proposed development will consist of an addition connecting to the south end of the current building. We explored the site subsurface soil conditions in the vicinity of the proposed development on December 7, 2018 with four trackhoe excavated test pits, with one being used as an infiltration pit. The test pit explorations ranged in depths from 3.5 to 8.0 feet below the existing ground surface. Our explorations exposed a surficial layer of topsoil/undocumented fill, with native glacial soils underlying the fill layer. It is our opinion that the proposed addition is feasible from a geotechnical standpoint, provided that our recommendations are incorporated into the design and construction of this project. We have recommended that the new structures be founded on medium dense or better native soils for bearing capacity and settlement considerations. These native bearing soils should generally be encountered Geotechnical Engineering Evaluation NGA File No. 1071118 Talbot Road Office Addition January 3, 2019 Renton, Washington Summary - Page 2 _____________________________________________________________________________________________ approximately 1.0 to 3.0 feet below the existing ground surface, based on our explorations. However, deeper areas of loose soil or undocumented fill could be encountered in unexplored areas of the property. If loose soils or undocumented fill are encountered in unexplored areas of the site, they should be removed and replaced with structural fill for foundation and pavement support. Specific grading and stormwater plans have not been finalized at the time this report was prepared. However, we understand that stormwater from the proposed development may be directed into on-site infiltration systems, if feasible. We collected samples and performed on-site infiltration testing to determine the infiltration rate based on the 2017 City of Renton Surface Water Design Manual. This is discussed in detail in the attached report. In the attached report, we have also provided general recommendations for site grading, slabs-on-grade, structural fill placement, retaining walls, erosion control, and drainage. We should be retained to review and comment on final development plans and observe the earthwork phase of construction. We also recommend that NGA be retained to provide monitoring and consultation services during construction to confirm that the conditions encountered are consistent with those indicated by the explorations, to provide recommendations for design changes should the conditions revealed during the work differ from those anticipated, and to evaluate whether or not earthwork and foundation installation activities comply with contract plans and specifications. We appreciate the opportunity to provide service to you on this project. Please contact us if you have any questions regarding this report or require further information. Sincerely, NELSON GEOTECHNICAL ASSOCIATES, INC. Khaled M. Shawish, PE Principal TABLE OF CONTENTS NELSON GEOTECHNICAL ASSOCIATES, INC. INTRODUCTION............................................................................................................. 1 SCOPE ............................................................................................................................... 1 SITE CONDITIONS ......................................................................................................... 2 Surface Conditions ....................................................................................................... 2 Subsurface Conditions.................................................................................................. 2 Hydrogeologic Conditions ........................................................................................... 3 SENSITIVE AREA EVALUATION ............................................................................... 3 Seismic Hazard ............................................................................................................. 3 Erosion Hazard ............................................................................................................. 4 INFILTRATION TESTING ............................................................................................ 4 CONCLUSIONS AND RECOMMENDATIONS .......................................................... 5 General ......................................................................................................................... 5 Erosion Control ............................................................................................................ 5 Site Preparation and Grading ....................................................................................... 5 Temporary and Permanent Slopes ............................................................................... 6 Foundations .................................................................................................................. 7 Retaining Walls ............................................................................................................ 8 Structural Fill................................................................................................................ 9 Slab-on-Grade ............................................................................................................ 10 Pavements................................................................................................................... 10 Utilities ....................................................................................................................... 10 Site Drainage .............................................................................................................. 10 CONSTRUCTION MONITORING ............................................................................. 11 USE OF THIS REPORT ................................................................................................ 12 LIST OF FIGURES Figure 1 – Vicinity Map Figure 2 – Site Plan Figure 3 – Soil Classification Chart Figures 4 – Exploration Log NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation Swanson Gardner Meyers Office Addition 4512 Talbot Road South Renton, Washington INTRODUCTION This report presents the results of our geotechnical engineering investigation and evaluation of the Swanson Gardner Meyers Office Addition project in Renton, Washington. The project site is located at the address of 4512 Talbot Road South, as shown on the Vicinity Map in Figure 1. The purpose of this study is to explore and characterize the site’s surface and subsurface conditions and to provide geotechnical recommendations for the planned site development. The property consists of a rectangular parcel of 0.62 acres. The ground surface within the property slopes gently upward from the northwest to southeast. Vegetation generally consists of grass yard areas, scattered young to mature trees, and landscaping plants. The property is currently occupied by an office building in the northeast portion of the site, and a paved parking area to the west. We understand that the proposed development will consist of an addition connecting to the south end of the current building. SCOPE The purpose of this study is to explore and characterize the site surface and subsurface conditions, and provide general recommendations for site development. Specifically, our scope of services included the following: 1. A review of available soil and geologic maps of the area. 2. Exploring the subsurface soil and groundwater conditions within the site with trackhoe pits. The trackhoe was provided by NGA. 3. Mapping the conditions on the site slopes, as necessary. 4. Provide recommendations for earthwork, foundation support, retaining walls, and slabs-on-grade. 5. Provide recommendations for temporary and permanent slopes. 6. Provide recommendations for pavement subgrade. 7. Provide recommendations for site drainage and erosion control. 8. Provide our opinion on the feasibility of infiltration for the onsite soils. 9. Provide long-term design infiltration rates based on on-site Small Pilot Infiltration Testing (PIT) per the 2017 City of Renton Surface Water Design Manual. One test to be performed on the property. 10. Documenting our findings, conclusions, and recommendations in a written geotechnical report. Geotechnical Engineering Evaluation NGA File No. 1071118 Swanson Gardner Meyers Office Addition January 3, 2019 Renton, Washington Page 2 _____________________________________________________________________________________________ NELSON GEOTECHNICAL ASSOCIATES, INC. SITE CONDITIONS Surface Conditions The property consists of a rectangular parcel of 0.62 acres. The ground surface within the property slopes gently upward from the northwest to southeast. Vegetation generally consists of grass yard areas, scattered young to mature trees, and landscaping plants. The property is currently occupied by an office building in the northeast portion of the site, and a paved parking area to the west. We did not observe surface water throughout the site during our site visit on December 7, 2018. Subsurface Conditions Geology: The geologic units for this site are shown on Geologic Map of the Renton Quadrangle, Washington, by Mullineaux, D.R. (USGS, 1965). The site is mapped as Ground Moraine Deposits (Qgt). Ground Moraine Deposits are described as a compact, coherent, and unsorted mixture of sand, silt, clay, and gravel. Our explorations within the property encountered native deposits consistent with the mapped soil unit beneath surficial undocumented fill soils. Explorations: The subsurface conditions within the site were explored on December 7, 2018 by excavating four test pits with a mini trackhoe to depths in the range of 3.5 to 8.0 feet below the existing ground surface. The approximate locations of our explorations are shown on the Site Plan in Figure 2. A geologist from NGA was present during the explorations, examined the soils and geologic conditions encountered, obtained samples of the different soil types, and maintained logs of the explorations. The soils were visually classified in general accordance with the Unified Soil Classification System, presented in Figure 3. The logs of our explorations are attached to this report and are presented as Figure 4. We present a brief summary of the subsurface conditions in the following paragraphs. For a detailed description of the subsurface conditions, the logs of the explorations should be reviewed. At the surface of each test pit we generally encountered approximately 0.8 to 3.0 feet of loose to medium dense, silty sand with varying amounts of roots, gravel, organics, which we interpreted as undocumented fill soils. Underlying the fill soils we generally encountered orange-brown to gray-brown, relatively granular silty fine to medium sand with varying amounts of gravel and iron-oxide staining, which we interpreted as native ground marine deposits type soils. Test Pits 1 through 3 terminated at respective depths of 6.0, 8.0, and 5.5 feet below the existing ground surface. Infiltration Pit 1 terminated at a depth of 3.5 feet below the surface. Geotechnical Engineering Evaluation NGA File No. 1071118 Swanson Gardner Meyers Office Addition January 3, 2019 Renton, Washington Page 3 _____________________________________________________________________________________________ NELSON GEOTECHNICAL ASSOCIATES, INC. Hydrogeologic Conditions Groundwater seepage was not encountered within our soil explorations. If groundwater is encountered during construction we would interpret this water to be perched water. Perched water occurs when surface water infiltrates through less dense, more permeable soils and accumulates on top of a relatively low permeability material. Perched water does not represent a regional groundwater "table" within the upper soil horizons. Perched water tends to vary spatially and is dependent upon the amount of rainfall. We would expect the amount of perched groundwater to decrease during drier times of the year and increase during wetter periods. SENSITIVE AREA EVALUATION Seismic Hazard The 2018 International Building Code (IBC) seismic design section provides a basis for seismic design of structures. Since medium stiff to hard soils were generally encountered underlying the site at depth, the site conditions best fit the IBC description for Site Class D. Table 1 below provides seismic design parameters for the site that are in conformance with the 2018 IBC, which specifies a design earthquake having a 2% probability of occurrence in 50 years (return interval of 2,475 years), and the 2008 USGS seismic hazard maps. Table 1 – 2018 IBC Seismic Design Parameters Site Class Spectral Acceleration at 0.2 sec. (g) Ss Spectral Acceleration at 1.0 sec. (g) S1 Site Coefficients Design Spectral Response Parameters Fa Fv SDS SD1 D 1.398 0.521 1.000 1.5 0.932 .521 The spectral response accelerations were obtained from the USGS Earthquake Hazards Program Interpolated Probabilistic Ground Motion website (2008 data) for the project latitude and longitude. Hazards associated with seismic activity include liquefaction potential and amplification of ground motion. Liquefaction is caused by a rise in pore pressures in a loose, fine sand deposit beneath the groundwater table. It is our opinion that the competent native soils interpreted to underlie the site has a low potential for liquefaction or amplification of ground motion. Geotechnical Engineering Evaluation NGA File No. 1071118 Swanson Gardner Meyers Office Addition January 3, 2019 Renton, Washington Page 4 _____________________________________________________________________________________________ NELSON GEOTECHNICAL ASSOCIATES, INC. Erosion Hazard The criteria used for determination of the erosion hazard for affected areas include soil type, slope gradient, vegetation cover, and groundwater conditions. The erosion sensitivity is related to vegetative cover and the specific surface soil types, which are related to the underlying geologic soil units. The Soil Survey of King County Area, Washington by the Natural Resources Conservation Service (NRCS), was reviewed to determine the erosion hazard for the surficial soils found within the subject site. The site is listed as Alderwood gravelly sandy loam at 8 to 15 percent slopes. The erosion hazards are classified as moderate. Based on our experience in the area and our observations in the field, it is our opinion that the site would have moderate erosion hazard for areas where the soils are exposed. It is our opinion that the erosion hazard for site soils should be low in areas where vegetation is not disturbed. INFILTRATION TESTING We conducted our onsite infiltration testing on December 7, 2018. The 2017 City of Renton Surface Water Design Manual was utilized to determine the long term design infiltration rate of the site soils. In accordance with this manual, on-site infiltration testing consisting of the Small Scale Pilot Infiltration Test (Small PIT) was used to determine the long-term design infiltration rates within the property. The subsurface soils at depth generally consisted of silty fine to medium sand with gravel that we interpreted as ground marine deposits to the depths explored. We conducted a Small PIT within Infiltration Pit 1 as shown on the attached Schematic Site Plan in Figure 2. Infiltration Pit 1 measured 4.0-feet long by 3.0-feet wide by 3.5-feet deep. The test pit was filled with approximately 12-inches of water and this level was maintained for six hours for the pre-soak period. After the 6-hour soaking period was completed, the water level was maintained at approximately 12- inches for one hour for the steady-state period of the test. The flow rate for Infiltration Pit 1 stabilized at 0.020 gallons per minute (0.120 gallons per hour), which equates to an approximate infiltration rate of 0.160 inches per hour. The water was shut off after the steady-state period and the water level within the pit was monitored every 15 minutes for one hour. After one hour, the water level within the pit had dropped 0.25 inches, resulting in a measured infiltration rate of 0.25 inches per hour. Using appropriate correction factors and based on our experience in this area, we recommend using a long-term design rate of 0.1 inches per hour. Geotechnical Engineering Evaluation NGA File No. 1071118 Swanson Gardner Meyers Office Addition January 3, 2019 Renton, Washington Page 5 _____________________________________________________________________________________________ NELSON GEOTECHNICAL ASSOCIATES, INC. CONCLUSIONS AND RECOMMENDATIONS General It is our opinion that the proposed addition is feasible from a geotechnical standpoint. It is also our opinion that the native soils that underlie the site can provide adequate support for the new addition. The native soils underlying undocumented fill within the proposed development area should provide adequate support for foundation, slab, and pavement loads. We recommend that the structure be designed utilizing shallow foundations. Footings should extend through any loose surficial soil and be keyed into the underlying competent native bearing soils. These soils should be encountered roughly 1-3 feet below the existing ground surface within the planned addition area, with some potential localized areas of deeper loose soils in unexplored areas of the addition. The soils encountered on this site are considered moisture-sensitive and will disturb easily when wet. To reduce the potential impacts of construction on the steep slope and to reduce cost overruns and delays, we recommend that construction take place during the drier summer months. If construction takes place during the rainy months, additional expenses and delays should be expected. Additional expenses could include the need for placing erosion control and temporary drainage measures to protect the slopes, the need for placing a blanket of rock spalls on exposed subgrades and construction traffic areas prior to placing structural fill, and the need for importing all-weather material for structural fill. Erosion Control The erosion hazard for the on-site soils are interpreted to be moderate for exposed soils, but the actual hazard will be dependent on how the site is graded and how water is allowed to concentrate. Best Management Practices (BMPs) measures should be used to control erosion. Areas disturbed during construction should be protected from erosion. Erosion control measures may include diverting surface water away from the stripped or disturbed areas. Silt fences and/or straw bales should be erected to prevent muddy water from leaving the site. Stockpiles should be covered with plastic sheeting during wet weather. Disturbed areas should be planted as soon as practical and the vegetation should be maintained until it is established. The erosion potential for areas not stripped of vegetation should be low. Site Preparation and Grading After erosion control measures are implemented, site preparation should consist of removing loose soils, topsoil, and any undocumented fill from foundations, slab, and pavement areas, to expose medium dense or better native soils at depth. The stripped soil should be removed from the site or stockpiled for later use as a landscaping fill. Based on our observations, we anticipate native, medium dense or better soil to be encountered at approximately 1-3 feet across the planned addition area, but this depth could increase in unexplored areas of the site. After site preparation, if the exposed subgrade is deemed loose, it should be compacted to a non-yielding condition and then proof-rolled with a heavy rubber-tired piece of Geotechnical Engineering Evaluation NGA File No. 1071118 Swanson Gardner Meyers Office Addition January 3, 2019 Renton, Washington Page 6 _____________________________________________________________________________________________ NELSON GEOTECHNICAL ASSOCIATES, INC. equipment. Areas observed to pump or weave during the proof-roll test should be reworked to structural fill specifications or over-excavated and replaced with properly compacted structural fill or rock spalls. If loose soils are encountered in the foundation areas, the loose soils should be removed and replaced with rock spalls. If significant surface water flow is encountered during construction, this flow should be diverted around areas to be developed, and the exposed subgrades should be maintained in a semi-dry condition. If wet conditions are encountered, alternative site grading techniques might be necessary. These could include using large excavators equipped with wide tracks and a smooth bucket to complete site grading, and covering exposed subgrade with a layer of crushed rock for protection. If wet conditions are encountered or construction is attempted in wet weather, the subgrade should not be compacted, as this could cause further subgrade disturbance. In wet conditions, it may be necessary to cover the exposed subgrade with a layer of crushed rock as soon as it is exposed to protect the moisture sensitive soils from disturbance by machine or foot traffic during construction. The prepared subgrade should be protected from construction traffic and surface water should be diverted around areas of prepared subgrade. Temporary and Permanent Slopes Temporary cut slope stability is a function of many factors, including 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 groundwater. It is exceedingly difficult under these variable conditions to estimate a stable, temporary cut slope angle. Therefore, it should be the responsibility of the contractor to maintain safe slope configurations at all times as indicated in OSHA guidelines for cut slopes. The following information is provided solely for the benefit of the owner and other design consultants and should not be construed to imply that Nelson Geotechnical Associates, Inc. assumes responsibility for job site safety. Job site safety is the sole responsibility of the project contractor. For planning purposes, we recommend that temporary cuts in the upper undocumented fill soils be no steeper than 2 Horizontal to 1 Vertical (2H:1V). Temporary cuts in competent, native soils at depth should be no steeper than 1.5H:1V. If significant groundwater seepage or surface water flow were encountered, we would expect that flatter inclinations would be necessary. We recommend that cut slopes be protected from erosion. The slope protection measures 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 four feet, if worker access is necessary. We recommend that cut slope heights and inclinations conform to appropriate OSHA/WISHA regulations. Geotechnical Engineering Evaluation NGA File No. 1071118 Swanson Gardner Meyers Office Addition January 3, 2019 Renton, Washington Page 7 _____________________________________________________________________________________________ NELSON GEOTECHNICAL ASSOCIATES, INC. Permanent cut and fill slopes should be no steeper than 2H:1V. However, flatter inclinations may be required in areas where loose soils are encountered. Permanent slopes should be vegetated and the vegetative cover maintained until established. Foundations Conventional shallow spread foundations should be placed on medium dense or better native bearing soils, or be supported on structural fill or rock spalls extending to those soils. Medium dense soils should be encountered approximately one to three feet below ground surface based on our explorations. Where undocumented fill or less dense soils are encountered at footing bearing elevation, the subgrade should be over-excavated to expose suitable bearing soil. The over-excavation may be filled with structural fill, or the footing may be extended down to the competent native bearing soils. The downhill foundation lines should bear at a minimum of an additional two feet embedment into the native bearing soils. If footings are supported on structural fill, the fill zone should extend outside the edges of the footing a distance equal to one half of the depth of the over-excavation below the bottom of the footing. Footings should extend at least 18 inches below the lowest adjacent finished ground surface for frost protection and bearing capacity considerations. The addition foundation should be connected to the existing building foundation so the entire foundation system acts as one unit for settlement consideration. Foundations should be designed in accordance with the 2018 IBC. Footing widths should be based on the anticipated loads and allowable soil bearing pressure. 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 not more than 2,000 pounds per square foot (psf) be used for the design of footings founded on the medium dense or better native bearing soils or structural fill extending to the competent native bearing material. The foundation bearing soil should be evaluated by a representative of NGA. We should be consulted if higher bearing pressures are needed. Current IBC guidelines should be used when considering increased allowable bearing pressure for short-term transitory wind or seismic loads. Potential foundation settlement using the recommended allowable bearing pressure is estimated to be less than 1-inch total and ½-inch differential between adjacent footings or across a distance of about 20 feet, based on our experience with similar projects. Geotechnical Engineering Evaluation NGA File No. 1071118 Swanson Gardner Meyers Office Addition January 3, 2019 Renton, Washington Page 8 _____________________________________________________________________________________________ NELSON GEOTECHNICAL ASSOCIATES, INC. Lateral loads may be resisted by friction on the base of the footing and passive resistance against the subsurface portions of the foundation. A coefficient of friction of 0.35 may be used to calculate the base friction and should be applied to the vertical dead load only. Passive resistance may be calculated as a triangular equivalent fluid pressure distribution. An equivalent fluid density of 200 pounds per cubic foot (pcf) should be used for passive resistance design for a level ground surface adjacent to the footing. This level surface should extend a distance equal to at least three times the footing depth. These recommended values incorporate safety factors of 1.5 and 2.0 applied to the estimated ultimate values for frictional and passive resistance, respectively. To achieve this value of passive resistance, the foundations should be poured “neat” against the native medium dense soils or compacted fill should be used as backfill against the front of the footing. We recommend that the upper one foot of soil be neglected when calculating the passive resistance. Retaining Walls Specific grading plans for this project were not available at the time this report was prepared, but retaining walls may be incorporated into project plans. In general, the lateral pressure acting on subsurface retaining walls is dependent on the nature and density of the soil behind the wall, the amount of lateral wall movement which can occur as backfill is placed, wall drainage conditions, and the inclination of the backfill. For walls that are free to yield at the top at least one thousandth of the height of the wall (active condition), soil pressures will be less than if movement is limited by such factors as wall stiffness or bracing (at-rest condition). We recommend that walls supporting horizontal backfill and not subjected to hydrostatic forces, be designed using a triangular earth pressure distribution equivalent to that exerted by a fluid with a density of 40 pcf for yielding (active condition) walls, and 60 pcf for non- yielding (at-rest condition) walls. These recommended lateral earth pressures are for a drained granular backfill and are based on the assumption of a horizontal ground surface behind the wall for a distance of at least the total height of the wall, and do not account for surcharge loads. Additional lateral earth pressures should be considered for surcharge loads acting adjacent to subsurface walls and within a distance equal to the total height of the wall. This would include the effects of surcharges such as traffic loads, floor slab loads, slopes, or other surface loads. We could consult with the structural engineer regarding additional loads on retaining walls during final design, if needed. A seismic design loading of 8H in psf should also be included in the wall design, where “H” is the total height of the wall. The lateral pressures on walls may be resisted by friction between the foundation and subgrade soil, and by passive resistance acting on the below-grade portion of the foundation. Recommendations for frictional and passive resistance to lateral loads are presented in the Foundations subsection of this report. Geotechnical Engineering Evaluation NGA File No. 1071118 Swanson Gardner Meyers Office Addition January 3, 2019 Renton, Washington Page 9 _____________________________________________________________________________________________ NELSON GEOTECHNICAL ASSOCIATES, INC. All wall backfill should be well compacted as outlined in the Structural Fill subsection of this report. Care should be taken to prevent the buildup of excess lateral soil pressures due to over-compaction of the wall backfill. This can be accomplished by placing wall backfill in 8-inch loose lifts and compacting the backfill with small, hand-operated compactors within a distance behind the wall equal to at least one-half the height of the wall. The thickness of the loose lifts should be reduced to accommodate the lower compactive energy of the hand-operated equipment. The recommended level of compaction should still be maintained. Permanent drainage systems should be installed for retaining walls. Recommendations for these systems are found in the Subsurface Drainage subsection of this report. We recommend that we be retained to evaluate the proposed wall drain backfill material and observe installation of the drainage systems. Structural Fill General: Fill placed beneath foundations, pavement, or other settlement-sensitive structures 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. The area to receive the fill should be suitably prepared as described in the Site Preparation and Grading subsection prior to beginning fill placement. Sloping areas to receive fill should be benched using a minimum 8-foot wide horizontal benches into competent soils. Materials: Structural fill should consist of a good quality, granular soil, free of organics and other deleterious material, and be well graded to a maximum size of about three inches. All-weather fill should contain no more than five-percent fines (soil finer than U.S. No. 200 sieve, based on that fraction passing the U.S. 3/4-inch sieve). Some of the more granular on-site soils may be suitable for use as structural fill depending on the moisture content of the soil during construction. We should be retained to evaluate all proposed structural fill material prior to placement. Fill Placement: Following subgrade preparation, placement of structural fill may proceed. All filling should be accomplished in uniform lifts up to eight inches thick. Each lift should be spread evenly and be thoroughly compacted prior to placement of subsequent lifts. All structural fill underlying building areas and pavement subgrade should be compacted to a minimum of 95 percent of its maximum dry density. Maximum dry density, in this report, refers to that density as determined by the ASTM D-1557 Compaction Test procedure. The moisture content of the soils to be compacted should be within about two percent of optimum so that a readily compactable condition exists. It may be necessary to over- excavate and remove wet soils in cases where drying to a compactable condition is not feasible. All Geotechnical Engineering Evaluation NGA File No. 1071118 Swanson Gardner Meyers Office Addition January 3, 2019 Renton, Washington Page 10 _____________________________________________________________________________________________ NELSON GEOTECHNICAL ASSOCIATES, INC. compaction should be accomplished by equipment of a type and size sufficient to attain the desired degree of compaction and should be tested. Slab-on-Grade Slabs-on-grade should be supported on subgrade soils prepared as described in the Site Preparation and Grading subsection of this report. We recommend that all floor slabs be underlain by at least six inches of free-draining gravel with less than three percent by weight of the material passing Sieve #200 for use as a capillary break. We recommend that the capillary break be hydraulically connected to the footing drain system to allow free drainage from under the slab. A suitable vapor barrier, such as heavy plastic sheeting (6-mil minimum), should be placed over the capillary break material. An additional 2-inch-thick moist sand layer may be used to cover the vapor barrier. This sand layer is optional, and is intended to be used to protect the vapor barrier membrane and to aid in curing the concrete. Pavements Pavement subgrade preparation and structural filling where required, should be completed as recommended in the Site Preparation and Grading and Structural Fill subsections of this report. The pavement subgrade should be proof-rolled with a heavy, rubber-tired piece of equipment, to identify soft or yielding areas that require repair. The pavement section should be underlain by a minimum of six inches of clean granular pit run or crushed rock. We should be retained to observe the proof-rolling and recommend repairs prior to placement of the asphalt or hard surfaces. Utilities We recommend that underground utilities be bedded with a minimum 12 inches of pea gravel prior to backfilling the trench with on-site or imported material. Trenches within settlement sensitive areas should be compacted to 95% of the modified proctor as described in the Structural Fill subsection of this report. Trenches located in non-structural areas should be compacted to a minimum 90% of the maximum dry density. Site Drainage Surface Drainage: The control of surface water and near-surface groundwater is very important for the long-term stability of the site slopes. We recommend that temporary and final site grading be designed to direct surface water away from the structures and away from any site slopes. Geotechnical Engineering Evaluation NGA File No. 1071118 Swanson Gardner Meyers Office Addition January 3, 2019 Renton, Washington Page 11 _____________________________________________________________________________________________ NELSON GEOTECHNICAL ASSOCIATES, INC. Roof drains should be installed around the site structures. The roof drains should consist of gutters and downspouts to collect stormwater runoff from the roof. The downspouts should connect with 4-inch diameter, rigid PVC tightline pipes to the main catch basin. The footing and roof drains should be routed in separate tightlines into catch basins/cleanouts. Stormwater from the driveway and yard drains should also be collected and directed through tightlines into the main catch basin and then through the controlled drainage system. In our opinion, due to the low permeability of the glacial soils interpreted to underlie the planned development area, on-site stormwater infiltration is considered only marginally feasible for this project. Low impact infiltration like rain gardens, bioswales, or pervious pavement may be used for the onsite infiltration system. The low impact infiltration system should be provided with an overflow directed to the offsite storm drains. Subsurface Drainage: If groundwater is encountered during construction, we recommend that the contractor slope the bottom of the excavation and collect the water into ditches and small sump pits where the water can be pumped out of the excavation and routed to an approved outlet. Perched groundwater conditions are anticipated on this site and footing drains are recommended for this project. We recommend the use of footing drains around the structure foundations. Footing drains should be installed at least one foot below planned finished floor elevation. The drains should consist of a minimum four-inch-diameter, rigid, slotted or perforated, PVC pipe surrounded by free-draining material wrapped in a filter fabric. We recommend that the free-draining material consist of an 18-inch-wide zone of clean (less than three-percent fines), granular material placed along the back of walls. Washed rock is an acceptable drain material, or a drainage composite may be used instead. The free-draining material should extend up the wall to one foot below the finished surface. The top foot of soil should consist of low permeability soil placed over plastic sheeting or building paper to minimize the migration of surface water or silt into the footing drain. Footing drains should discharge into tightlines leading to an approved collection and discharge point with convenient cleanouts to prolong the useful life of the drains. Roof drains should not be connected to wall or footing drains. Under no circumstances should runoff be allowed to flow over site slopes. CONSTRUCTION MONITORING We should be retained to provide construction monitoring services during the earthwork phase of the project to evaluate subgrade conditions, temporary cut conditions, fill compaction, and drainage system installation. Geotechnical Engineering Evaluation NGA File No. 1071118 Swanson Gardner Meyers Office Addition January 3, 2019 Renton, Washington Page 12 _____________________________________________________________________________________________ NELSON GEOTECHNICAL ASSOCIATES, INC. USE OF THIS REPORT NGA has prepared this report for Mr. Edward Pozniak and his agents, for use in the planning and design of the development on this site only. 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 between the explorations and also with time. Our report, conclusions, and interpretations should not be construed as a warranty of subsurface conditions. A contingency for unanticipated conditions should be included in the budget and schedule. We recommend that NGA be retained to provide monitoring and consultation services during construction to confirm that the conditions encountered are consistent with those indicated by the explorations, to provide recommendations for design changes should the conditions revealed during the work differ from those anticipated, and to evaluate whether or not earthwork and foundation installation activities comply with contract plans and specifications. We should be contacted a minimum of one week prior to construction activities and could attend pre-construction meetings if requested. Within the limitations of scope, schedule, and budget, our services have been performed in accordance with generally accepted geotechnical engineering practices in effect in this area at the time this report was prepared. No other warranty, expressed or implied, is made. Our observations, findings, and opinions are a means to identify and reduce the inherent risks to the owner. o-o-o Geotechnical Engineering Evaluation NGA File No. 1071118 Swanson Gardner Meyers Office Addition January 3, 2019 Renton, Washington Page 13 _____________________________________________________________________________________________ NELSON GEOTECHNICAL ASSOCIATES, INC. It has been a pleasure to provide service to you on this project. If you have any questions or require further information, please call. Sincerely, NELSON GEOTECHNICAL ASSOCIATES, INC. Logan A. Heine, GIT Staff Geologist I Maher A. Shebl, PhD, PE, M.ASCE Senior Engineer Khaled M. Shawish, PE Principal LAH:MAS:KMS:dy Four Figures Attached Not to Scale VICINITY MAP Swanson Gardner Meyers Office Addition Vicinity Map Project Site 1 No.Project Number Date By CKRevision Geotechnical Engineers & Geologists Nelson GeotechnicalAssociates, Inc.NGA Woodinville Office 17311-135th Ave. NE, A-500 Woodinville, WA 98072 (425) 486-1669 / Fax: 481-2510 East Wenatchee Office 5526 Industry Lane, #2 East Wenatchee, WA 98802 (509) 665-7696 / Fax: 665-7692www.nelsongeotech.com \\HILL\company\2018 NGA Project Folders\10711-18 Talbot Road Addition Geotech Renton\Drafting\VM.dwg12/13/18 DPN LHOriginal Figure 1 1071118 Renton, WA Reference: Site plan based on a plan dated August 3, 2018 titled "Office Remodel - Sanson Gardner Meyers," prepared by Architectural Innovations.1No.Project NumberDateByCKRevisionNelson GeotechnicalAssociates, Inc.Geotechnical Engineers & GeologistsGNAWoodinville Office17311-135th Ave. NE, A-500Woodinville, WA 98072(425) 486-1669 / Fax: 481-2510East Wenatchee Office5526 Industry Lane, #2East Wenatchee, WA 98802(509) 665-7696 / Fax: 665-7692www.nelsongeotech.com\\HILL\company\2018 NGA Project Folders\10711-18 Talbot Road Addition Geotech Renton\Drafting\SP.dwgFigure 2107111812/14/18DPNLHOriginal Swanson Gardner MeyersOffice AdditionSite PlanLEGEND INF-1 Number and approximate location of infiltration test pit Property lineTalbot Rd SExisting Building Proposed Addition TP-1 TP-3 INF-1 TP-2 TP-1 Number and approximate location of test pit Scale: 1 inch = 30 feet 0 30 60 GW GP GM GC SW SP SM SC ML CL OL MH CH OH PT PEAT ORGANIC CLAY, ORGANIC SILT CLAY OF HIGH PLASTICITY, FAT CLAY SILT OF HIGH PLASTICITY, ELASTIC SILT SILTY SAND SILT ORGANIC SILT, ORGANIC CLAY CLAY CLAYEY SAND POORLY GRADED SAND WELL-GRADED SAND, FINE TO COARSE SAND CLAYEY GRAVEL SILTY GRAVEL POORLY-GRADED GRAVEL WELL-GRADED, FINE TO COARSE GRAVELCLEAN GRAVEL GRAVEL WITH FINES CLEAN SAND SAND WITH FINES INORGANIC ORGANIC INORGANIC ORGANIC HIGHLY ORGANIC SOILS GRAVEL SAND SILT AND CLAY SILT AND CLAY MORE THAN 50 % OF COARSE FRACTION RETAINED ON NO. 4 SIEVE PASSES NO. 4 SIEVE LIQUID LIMIT LESS THAN 50 % 50 % OR MORE LIQUID LIMIT MORE THAN 50 % OF COARSE FRACTION COARSE - GRAINED SOILS FINE - GRAINED SOILS MORE THAN 50 % RETAINED ON NO. 200 SIEVE PASSES NO. 200 SIEVE MORE THAN 50 % MAJOR DIVISIONS GROUP SYMBOL GROUP NAME UNIFIED SOIL CLASSIFICATION SYSTEM NOTES: 1) Field classification is based on visual examination of soil in general accordance with ASTM D 2488-93. 2) Soil classification using laboratory tests is based on ASTM D 2488-93. 3) Descriptions of soil density or consistency are based on interpretation of blowcount data, visual appearance of soils, and/or test data. 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 1 No.Project Number Date By CKRevision Geotechnical Engineers & Geologists Nelson GeotechnicalAssociates, Inc.NGA Woodinville Office 17311-135th Ave. NE, A-500 Woodinville, WA 98072 (425) 486-1669 / Fax: 481-2510 East Wenatchee Office 5526 Industry Lane, #2 East Wenatchee, WA 98802 (509) 665-7696 / Fax: 665-7692www.nelsongeotech.com \\HILL\company\2018 NGA Project Folders\10711-18 Talbot Road Addition Geotech Renton\Drafting\SC.dwgFigure 3 1071118 12/13/18 DPN LHOriginal Swanson Gardner Meyers Office Addition Soil Classification Chart LOG OF EXPLORATION DEPTH (FEET) USC SOIL DESCRIPTION LAH:KMS NELSON GEOTECHNICAL ASSOCIATES, INC. FILE NO 1071118 FIGURE 4 TEST PIT ONE 0.0 – 0.8 TOPSOIL (MOIST, LOOSE) (FILL) 0.8 – 3.0 SM ORANGE-BROWN SILTY SAND WITH TRACE ROOTS (MOIST, MEDIUM DENSE) 3.0 – 6.0 SM GRAY TO GRAY-BROWN FINE TO MEDIUM WELL CEMENTED SILTY SAND WITH TRACE GRAVELS (DRY TO MOIST, MEDIUM DENSE) SAMPLES WERE COLLECTED AT 2.3 AND 6.0 FEET GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED TEST PIT CAVING WAS NOT ENCOUNTERED TEST PIT WAS COMPLETED AT 6.0 FEET ON 12/7/18 TEST PIT TWO 0.0 – 3.0 DARK BROWN SILTY SAND WITH TRACE ROOTS, ASPHALT PIECES, AND GRAVELS (MOIST, LOOSE TO MEDIUM DENSE) (FILL) 3.0 – 3.5 SM GRAY-BROWN SILTY FINE TO MEDIUM SAND WITH TRACE GRAVELS (WET, MEDIUM DENSE) 3.5 – 8.0 SM GRAY-BROWN WELL CEMENTED SILTY SAND WITH SOME GRAVELS, TRACE COBBLES (MOIST, DENSE) SAMPLES WERE COLLECTED AT 3.5 AND 8.0 FEET GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED TEST PIT CAVING WAS NOT ENCOUNTERED TEST PIT WAS COMPLETED AT 8.0 FEET ON 12/7/18 TEST PIT THREE 0.0 – 2.5 DARK BROWN-ORANGE SILTY FINE TO MEDIUM SAND WITH TRACE ORGANICS AND PEA GRAVEL (MOIST, LOOSE) (FILL) 2.5 – 5.5 SM LIGHT BROWN WELL CEMENTED FINE TO MEDIUM SILTY SAND WITH TRACE GRAVELS (DRY, DENSE) SAMPLES COLLECTED AT 3.0 AND 5.5 FEET GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED TEST PIT CAVING WAS NOT ENCOUNTERED TEST PIT WAS COMPLETED AT 5.5 FEET ON 12/7/18 INFILTRATION PIT 0.0 – 1.3 BLACK SILTY SAND WITH TRACE GRAVEL AND ASPHALT PIECES (MOIST, LOOSE) (FILL) 1.3 – 3.5 SM GRAY-BROWN SILTY SAND WITH IRON OXIDE STAINING (MOIST, MEDIUM DENSE) NO SAMPLES WERE COLLECTED GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED TEST PIT CAVING WAS NOT ENCOUNTERED TEST PIT WAS COMPLETED AT 3.5 FEET ON 12/7/18