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REVIEWED BLD2022-0561+Geotechnical_Report+5.3.2022_4.41.00_PM+2845119NELSON GEOTECHNICAL ASSOCIATES. INC. August 27, 2021 Chander Sharma VIA Email: chander.sharma(@fsbwa.com Geotechnical Engineering Evaluation Sharma Residence Development 8422 — 238`h Street SW Edmonds, Washington NGA File No. 1269821 Dear Chander: 17311-135th Ave. N.E. Suite A-500 Woodinville, WA 98072 (425) 486-1669 www.nelsongeotech.com We are pleased to submit the attached report titled "Geotechnical Engineering Evaluation — Sharma Residence Development — 8422 - 238th Street SW — Edmonds, Washington." This report summarizes our observations of the existing surface and subsurface conditions within the property and provides general recommendations for the proposed site development. Our services were completed in general accordance with the proposal signed by you on July 12, 2021. The property is rectangular in shape and covers 0.24 acres in area. It is currently occupied by a single- family residence, detached shed, and concrete driveway within the northeastern corner of the site. The property is bordered by 238th Street SW to the north, neighboring residential properties to the east and south, and by a parking lot to the west. Topographically, the site is relatively flat to gently sloping from northeast to southwest. We understand the plans for development include the construction of a new residence and the relocation of the existing residence to serve as an accessory structure. We explored the site subsurface soil conditions with three trackhoe-excavated test pits, one of which was utilized for infiltration testing. Our explorations indicated that the site was underlain by competent native glacial till soils at relatively shallow depths, below a layer of undocumented fill and/or topsoil. It is our opinion that the proposed site development is feasible from a geotechnical engineering standpoint, provided that our recommendations for site development are incorporated into project plans. We recommend that the new structure foundations be founded on medium dense or better native bearing glacial soils for bearing capacity and settlement considerations. These soils should generally be encountered approximately 1.5- to 2.0-feet below the existing ground surface based on our explorations. However, deeper areas of loose soil and/or undocumented fill could also exist within unexplored areas of the site. If undocumented fill is encountered in unexplored areas of the site, it should be removed and replaced with structural fill for foundation and pavement support. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1269821 Sharma Residence Development August 27, 2021 Edmonds, Washington Summary — Page 2 We also performed on -site infiltration testing in accordance with the 2019 Department of Ecology (DOE) Stormwater Management Manual for Western Washington, utilized by the City of Edmonds. Our on -site testing consisted of one small pilot infiltration tests (PITs) performed in accordance with City of Edmonds requirements. Subsurface soils generally consisted of silty fine to medium sand with varying amounts of gravel and cobbles in a medium dense or better condition which we interpreted as native glacial till soils. Based on our observations, testing in the field, and the silty nature of the native glacial till deposits that underlie the site, it is our opinion that the on -site native soils encountered at depth are not conducive for traditional stormwater infiltration systems; however, shallow, low -impact systems may be feasible, provided they incorporate an overflow component directed to an approved point of discharge. In the attached report, we have also provided general recommendations for site grading, slabs -on - grade, structural fill placement, erosion control, and drainage. We should be retained to review and comment on final development plans and observe the earthwork phase of construction. We recommend that Nelson Geotechnical Associates (NGA) be retained to review the geotechnical aspects of the project plans prior to 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 construction differ from those anticipated, and to evaluate whether or not earthwork and foundation installation activities comply with contract plans and specifications. It has been a pleasure 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. U&� Khaled M. Shawish, PE Principal NELSON GEOTECHNICAL ASSOCIATES, INC. TABLE OF CONTENTS INTRODUCTION..........................................................................................................1 SCOPE........................................................................................................................1 SITECONDITIONS....................................................................................................... 2 SurfaceConditions....................................................................................................... 2 Subsurface Conditions.................................................................................................. 2 Hydrogeologic Conditions............................................................................................ 3 SENSITIVE AREA EVALUATION.................................................................................... 3 SeismicHazard............................................................................................................. 3 ErosionHazard............................................................................................................. 4 CONCLUSIONS AND RECOMMENDATIONS..................................................................4 General......................................................................................................................... 4 ErosionControl............................................................................................................. 5 Site Preparation and Grading....................................................................................... 5 Temporary and Permanent Slopes............................................................................... 6 FoundationSupport..................................................................................................... 7 StructuralFill................................................................................................................ 8 Slab-on-Grade.............................................................................................................. 9 Pavements.................................................................................................................... 9 Utilities....................................................................................................................... 10 SiteDrainage.............................................................................................................. 11 CONSTRUCTION MONITORING................................................................................. 13 USE OF THIS REPORT................................................................................................ 13 LIST OF FIGURES Figure 1—Vicinity Map Figure 2 — Site Plan Figure 3 — Soil Classification Chart Figure 4 — Exploration Logs NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation Sharma Residence Development 8422 — 2381" Street SW Edmonds, Washington INTRODUCTION This report presents the results of our geotechnical engineering investigation and evaluation of the planned residence development project located at 8422 — 238t" Street SW Street in Edmonds, Washington as shown on the Vicinity Map in Figure 1. The Snohomish County parcel number for the subject property is 004633-009-002-00. The site consists of a rectangular property occupied by a single-family residence. Topography within the site is generally level. Vegetation within the site includes grass yard areas bordered by shrubs and young to mature trees. We understand the plans for development include the construction of a new residence and the relocation of the existing residence to serve as an accessory structure. The existing site layout is shown on the Site Plan in Figure 2. SCOPE The purpose of this study was to explore and characterize the site's surface and subsurface conditions and to provide geotechnical recommendations for the planned residence development. Specifically, our scope of services included the following: 1. Reviewing available soil and geologic maps of the area as well as other relevant geotechnical information, as provided. 2. Exploring the subsurface soil and conditions within the site using trackhoe-excavated test pits. Excavation services provided by NGA. 3. Providing long-term design infiltration rates based on on -site Small Pilot Infiltration Testing (PIT) per the 2014 SWMMWW, if feasible. 4. Performing laboratory grain -size sieve analysis on soil samples, as necessary. 5. Providing general recommendations for earthwork and foundation support. 6. Providing recommendations for subsurface utilities and pavement subgrade preparation. 7. Providing our opinion on stormwater infiltration feasibility. 8. Providing recommendations for infiltration/bioretention system installation, if feasible. 9. Providing general recommendations for site drainage and erosion control. 10. Documenting the results of our findings, conclusions, and recommendations in a written geotechnical report. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1269821 Sharma Residence Development August 27, 2021 Edmonds, Washington Page 2 SITE CONDITIONS Surface Conditions The subject site consists of a rectangular -shaped parcel covering an area of approximately 0.24 acres. The site is currently occupied by an existing residence, detached garage structure, and concrete driveway within the northeast corner of the property. Vegetation within the site consists of grass yard areas bordered by shrubs and young to mature trees. Topographically, the site is relatively level to gently sloping from northeast to southwest. The site is bound to the north by 238th Street SW, to the west by a parking lot, and to the east and south by neighboring residential properties. Surface water was not observed throughout the site during our site visits on August 4, 2021. Subsurface Conditions Geology: The geologic units for this site are shown on the Geologic Map of the Edmonds East and Part of the Edmonds West Quadrangles, Washington, by Minard, J.P. (USGS, 1983). The site is mapped as glacial outwash (Qvt). Glacial till is described a dense diamicton, or mixture of silt, clay, sand, and gravel. Our explorations within the site generally encountered surficial topsoil and/or undocumented fill underlain by medium dense or better, fine to coarse sand with varying amounts of silt and gravel that we interpreted as native glacial till deposits throughout the property. Explorations: The subsurface conditions within the site were explored on August 4, 2021 by excavating three test pits extending to depths in the range of 4.5 to 6.0 feet below the existing ground surface, one of which was used for infiltration testing. 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 paragraph. For a detailed description of the subsurface conditions, the exploration logs should be reviewed. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation Sharma Residence Development Edmonds, Washington NGA File No. 1269821 August 27, 2021 Page 3 At the surface of all of our explorations, we encountered approximately 1.5 to 2.0 feet of dark brown, silty fine to medium sand with varying amounts of gravel, roots, organics, and concrete rubble, which we interpreted as undocumented fill soils. These soils were dry to moist and typically encountered in a variable loose to medium dense condition. Underlying the undocumented fill soils in each exploration, we encountered orange -brown to gray, silty, fine to medium sand with varying amounts of gravel, cobbles, roots, and iron oxide staining in an increasingly dense condition. We interpreted these lower soils to be native glacial till deposits. All test pits were terminated within native glacial till soils at depths in the range of 4.5 to 6.0 feet below existing grade. Hydrogeologic Conditions We did not encounter groundwater within our explorations. If groundwater is encountered during construction, we would interpret this as perched groundwater. Perched water occurs when surface water infiltrates through less dense, more permeable soils, such as topsoil and the weathered horizon, and accumulates on top of a less permeable soil. 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 groundwater to decrease during drier times of the year and increase during wetter periods. SENSITIVE AREA EVALUATION Seismic Hazard We reviewed the 2018 International Building Code (IBC) for seismic site classification for this project. Since competent glacial soils are inferred to underlie 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 two percent 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 Spectral Acceleration Site Coefficients Design Spectral at 0.2 sec. (g) at 1.0 sec. (g) Response SS S1 Parameters Fa Fv Sos SM D 1.262 0.493 1.000 1.507 0.841 0.495 NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation Sharma Residence Development Edmonds, Washington NGA File No. 1269821 August 27, 2021 Page 4 The spectral response accelerations were obtained from the ATC Hazards by Location (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 medium dense or better glacial deposits interpreted to underlie the site have a low potential for liquefaction or amplification of ground motion. 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 Natural Resources Conservation Service (NRCS) lists this area of Edmonds in the Soil Survey of Snohomish County as Alderwood-Urban land complex, 2 to 8 percent slopes. The erosion hazard listed for these soils is slight. Based on our observations we would interpret this site as having a low to moderate erosion hazard where surficial soils are exposed. It is our opinion that the erosion hazard for site soils should be low in areas where the site is not disturbed. CONCLUSIONS AND RECOMMENDATIONS General It is our opinion that the planned development within the site is generally feasible from a geotechnical standpoint. Our explorations indicated that the site was generally underlain by competent native glacial till soils at relatively shallow depths. The native glacial soils encountered at depth should provide adequate support for foundation, slab, and pavement loads. We recommend that all structure foundations planned within the site be founded on medium dense or better native bearing glacial soils for bearing capacity and settlement considerations. These soils should generally be encountered approximately 1.5 to 2.0 feet below the existing ground surface based on our explorations. However, deeper areas of loose soil and/or undocumented fill could also exist within unexplored areas of the site. If undocumented fill is encountered in unexplored areas of the site, it should be removed and replaced with structural fill for foundation and pavement support. We recommend that NGA be retained to review proposed grading plans once they are developed and allowed to provide alternative foundation support recommendations as needed. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1269821 Sharma Residence Development August 27, 2021 Edmonds, Washington Page 5 We also performed on -site infiltration testing based on the 2019 Department of Ecology Stormwater Manual for Western Washington. The onsite testing consisted of performing one small-scale PIT within Infiltration Test Pit One. Based on our observations, results of the onsite testing, and the relatively dense, silty nature of the native till deposits that underlie the site, it is our opinion that the on -site native soils encountered at depth are not conducive for traditional stormwater infiltration systems. Low impact systems may be feasible on this site as discussed in the Site Drainage section of this report. The soils encountered on this site are considered moisture -sensitive and will disturb easily when wet. 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, 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 is considered to be low but the actual hazard will be dependent on how the site is graded and how water is allowed to concentrate. Best Management Practices (BMPs) 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. Disturbed areas should be planted as soon as practical, and the vegetation should be maintained until it is established. The erosion potential of areas not stripped of vegetation should be low. Site Preparation and Grading After erosion control measures are implemented, site preparation should consist of stripping the topsoil, undocumented fill and/or loose soils from foundation, slab, pavement areas, and other structural areas, to expose medium dense or better native bearing soils. The stripped soil should be removed from the site or stockpiled for later use as a landscaping fill. Based on our observations, we anticipate stripping depths of approximately 1.5 to 2.0 feet, depending on the specific locations. However, additional stripping may be required if areas of deeper undocumented fill and/or loose soil are encountered in unexplored areas of the site. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1269821 Sharma Residence Development August 27, 2021 Edmonds, Washington Page 6 After site stripping, 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 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 pavement areas, the loose soils should be removed and replaced with rock spalls or granular structural fill. 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 stripping and 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. The site soils are considered to be moisture -sensitive and will disturb easily when wet. We recommend that construction take place during the drier summer months if possible. However, if construction takes place during the wet season, additional expenses and delays should be expected due to the wet conditions. Additional expenses could include the need for placing a blanket of rock spalls on exposed subgrades, construction traffic areas, and paved areas prior to placing structural fill. Wet weather grading will also require additional erosion control and site drainage measures. Some of the on -site soils may be suitable for use as structural fill, depending on the moisture content of the soil at the time of construction. NGA should be retained to evaluate the suitability of all on -site and imported structural fill material during construction. 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. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1269821 Sharma Residence Development August 27, 2021 Edmonds, Washington Page 7 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 the competent unweathered glacial outwash 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. Permanent cut and fill slopes should be no steeper than 2HAV. 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. Foundation Support Conventional shallow spread foundations should be placed on medium dense or better native bearing glacial soils or be supported on structural fill or rock spalls extending to those soils. Medium dense or better bearing soils should be encountered approximately 1.5 to 2.0 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 overexcavation may be filled with structural fill, or the footing may be extended down to the competent native 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 overexcavation 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. 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. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation Sharma Residence Development Edmonds, Washington NGA File No. 1269821 August 27, 2021 Page 8 For foundations constructed as outlined above, we recommend an allowable 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 %-inch differential between adjacent footings or across a distance of about 20 feet, based on our experience with similar projects. 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. 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. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1269821 Sharma Residence Development August 27, 2021 Edmonds, Washington Page 9 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, but this will be highly dependent on the moisture content of these soils at the time of 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 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. 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 may be used to protect the vapor barrier membrane and to aid in curing the concrete. Pavements The pavement subgrade should be prepared as recommended in the Site Preparation and Grading and Structural Fill subsections of this report, including proof -rolling the subgrade with a loaded dump truck and repairing areas observed to pump or weave during the proof -roll test. Also, all fill placed within the pavement areas, including utility trench backfill, should be compacted to 95 percent of the Maximum Dry Density (Modified Proctor). We should be retained to observe the proof -roll test. Any areas observed to pump or weave under the wheels of the loaded dump truck should be over -excavated and replaced with crushed rock. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation Sharma Residence Development Edmonds, Washington NGA File No. 1269821 August 27, 2021 Page 10 We have reviewed the Asphalt Institute Information Series No. 91 Publication "Full -Depth Asphalt for Parking Lots, Service Stations, and Driveways." Based on this publication, and an assumed medium dense to dense native pavement subgrade, we recommend that in light traffic and parking lot areas, the pavement section consist of five inches of crushed rock base -course, overlain by 2.5 inches of Class B Hot Mix Asphalt (HMA). Alternatively, the pavement section could consist of 3.5 inches of ATB, overlain by 2.5 inches of Class B HMA. For heavy truck access areas, the pavement section should consist of eight inches of crushed rock overlain by four inches of HMA. Alternatively, the crushed rock layer could be substituted by five inches of ATB. Pavements should be sloped to provide rapid drainage of surface water. Water allowed to pond on or adjacent to the pavements could saturate the subgrade and contribute to premature pavement deterioration. In addition, the pavement subgrade should be graded to provide positive drainage within the crushed aggregate base section. The pavement sections provided in this report represent minimum recommended thicknesses. Therefore, preventive maintenance should be planned and provided for through an on -going pavement management program. Preventive maintenance activities are intended to slow the rate of pavement deterioration, and to preserve the pavement investment. Preventive maintenance consists of both localized maintenance (e.g., crack and joint sealing and patching) and global maintenance (e.g., surface sealing). Preventive maintenance is usually the first priority when implementing a planned pavement maintenance program. Prior to implementing any maintenance, additional engineering observation is recommended to determine the type and extent of a cost-effective program. Even with periodic maintenance, some movements and related cracking may still occur, and repairs may be required. Utilities We recommend that underground utilities be bedded with a minimum six 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. The trench backfill compaction should be tested. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1269821 Sharma Residence Development August 27, 2021 Edmonds, Washington Page 11 Site Drainage Infiltration: We performed one small PIT within Infiltration Test Pit One. The infiltration pit measured approximately 4.0-feet long by 3.0-feet wide by 4.5-feet deep. The soils within the pit consisted on gray, silty, fine to medium sand with gravel and cobbles that we interpreted to be native glacial till soils. Infiltration Test Pit One was filled with approximately 12 inches of water and this level was maintained for 6 hours for the presoak portion of the test. At this time, the water flow rate into the pit was monitored with a Great Plains Industries (GPI) TM 050 water flow meter 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. The flow rate for Infiltration Pit 1 stabilized at 0.021 gallons per minute (1.26 gallons per hour). This equated to an approximate infiltration rate of 0.17 inches per hour. The water was shut off after the steady-state period and monitored at least every 15 minutes for one hour. After 60 minutes, the water level within the pit dropped approximately 0.125 inches, resulting in a measured infiltration rate of 0.125 inches per hour. In accordance with the Table V-5.1 of the Department of Ecology 2019 Stormwater Management Manual for Western Washington, correction factors of 0.8, 0.5, and 0.9 for site variability and number of locations tested (CFv), testing method (CFt), and degree of influent control to prevent siltation and bio-buildup (CFm), respectively were applied to the field measured minimum infiltration rate of 0.17 inches per hour, calculated from data collected from all days. A total correction factor of 0.36 was applied to the measured field infiltration rate obtained from the falling head portion of the test to determine the long- term design infiltration rate. Using this correction factor, we calculated a long-term design infiltration rate of 0.062 inches per hour. This does not meet the minimum long-term design infiltration rate provided by the 2019 SWMMWW of 0.30 incher per hour. Based on the results of our infiltration testing and the dense silty nature of the onsite native glacial till soils at depth it is our opinion that the native glacial till soils at depth within the site are not conductive to traditional stormwater infiltration systems. However, a low -impact stormwater infiltration system may be feasible, depending on final site layout. The project civil engineer should determine possible methods of low -impact stormwater infiltration in conjunction with our calculated long-term design infiltration rate and the Department of Ecology 2019 Stormwater Management Manual for Western Washington. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1269821 Sharma Residence Development August 27, 2021 Edmonds, Washington Page 12 We did not observe any groundwater during our time on site. It is our opinion that proposed infiltration systems should be able to maintain a minimum separation from the base of the infiltration systems to any impermeable surfaces and/or groundwater table. We recommend that the base of the on -site infiltration systems be terminated in the native deposits. We also recommend that all on -site limited infiltration systems include an overflow component directed into an approved point of discharge, likely within 238tn Street SW. We recommend that any infiltration systems be placed as to not negatively impact any proposed or existing nearby structures and also meet all required setbacks from existing property lines, structures, and sensitive areas as discussed in the drainage manual. In general, infiltration systems should not be located within proposed fill areas within the site associated with site grading or retaining wall backfill as such condition could lead to failures of the placed fills and/or retaining structures. We should be retained to evaluate the infiltration system design and installation during construction, if necessary. Surface Drainage: The finished ground surface should be graded such that stormwater is directed to an approved stormwater collection system. Water should not be allowed to stand in any areas where footings, slabs, or pavements are to be constructed. Final site grades should allow for drainage away from the residences. We suggest that the finished ground be sloped downward at a minimum gradient of three percent, for a distance of at least 10 feet away from the residences. Surface water should be collected by permanent catch basins and drain lines and be discharged into an approved discharge system. 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 and routed into a permanent storm drain. We generally recommend the use of footing drains around the structures. Footing drains may be installed at least one foot below planned finished floor elevation. The drains should consist of a minimum 4-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. Pea gravel is an acceptable drain material. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation Sharma Residence Development Edmonds, Washington NGA File No. 1269821 August 27, 2021 Page 13 The free -draining material should extend up the wall to one foot below the finished surface. The top foot of backfill should consist of impermeable soil placed over plastic sheeting or building paper to minimize surface water or fines migration 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. CONSTRUCTION MONITORING We recommend that we be retained to provide construction monitoring services to evaluate conditions encountered in the field with respect to anticipated conditions, to provide recommendations for design changes should the conditions differ from anticipated, and to evaluate whether construction activities comply with contract plans and specifications. USE OF THIS REPORT NGA has prepared this report for Chandler Sharma and associated agents, for use in the planning and design of the development on these sites 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 we be retained to review the project plans after they have been developed to determine that recommendations in the report were incorporated into project plans. We recommend that NGA be retained to review final plans prior to 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 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 NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation Sharma Residence Development Edmonds, Washington NGA File No. 1269821 August 27, 2021 Page 14 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. Sarah L. Dunn Staff Geologist M. w S , q 35215 w 41 F ti� �s GisT@ C5 s�aNAs, E� 08.27.20221 Khaled M. Shawish, PE Principal SLD:KMS:dy Attachments: Four Figures NELSON GEOTECHNICAL ASSOCIATES, INC. PCC Community Markets - Edmonds 104 North American Martyrs Catholic Church 0 v Westgate Chapel PRC EdmondsQ 234th St SW w � 3 n D D f f Madrona K-8 School 19 VICINITY MAP Not to Scale 9 = Fashion Dim Sum, � 2 230th St SW f s e 231st St SW s Holly Ln n 99 ; �� z3atn st sw Project D r 's Ballinger Park f m n N x 'I 236thSTSW Site !^16thStSW Lake Ballinger` Q Beach T.J. M t 2381h St SW ` $t Sw Edmount m Island a' m_ o P =... 24ttth St'" 24— 5<SV/ o -h � Burlington Q a 1v FIRDALE VILLAGE 9 CDa 242nd St SIN 3i 242nd St SW P Caravan Kebab © Brackett Apartments v f Scotts Bar &Grill f D Panera Bread 244th St sw The Habit Burger Grill m $ a ST N 2041h St O — © I NW 203rd St D 3 > N 203rd St The Home Depot Q ( _ ' i`s m NW 201st St 1.Z Sanmaru Grill © _ N 201 St St b Costco Gasoline Nzot4lst `¢ m NW 200th St III N 200th St Echo < < H I L L W 0 0 D N1"hSt Lake Park Ballinger Commons to z _ f *z bW 198th s, Apartment Homes King's Elementary School n N 19Rth S1 0 Echo Laket. t,l' < 3 Z Elementary School m NV! 1951h SI 1: I ;Sm � D m N 195tFi ' 0 N 1951h St Z— NE , 95[h SI 4 n Middle School0 Echo Lake Apartments ECHO LAKE Le Dale Turner O O $_ C.... J— VIA! A N 193rd St u Edmonds, WA Project Number 1269821 Sharma Residence Development � mson GEOTEnnim ASSOCIATES, inc No. Date Revision By cK Z 1 8/17/21 Original DPN Duo 0 (Y Fiure 1 J Vicinity Map Wo°dl°vllle0r6°e Wenatchee ice 17311-135th Ave. NE, A-500 105 Palouse St. Woodinville, WA 98072 Wenatchee, WA 98801 E www.nelsongeotech.com (425) 486-1669 / Fax. 481-2510 (509) 665-7696 / Fax: 665-7692 L Site Plan HOUSE �FROM vSPLICE v IN 238th St SW OHP WIRES BETWEEN _ N89"48'58"E POLES 525.09' 499— PAINT IN."ARK INV 12" RCP STORM DRAIN —•500` IE 498.7' PAINT MARK SITE BENCH. --*—OPEN DITCH GINNIE ASSUMED e�oa�'Sf- 4(ATE t rb%82 _ ELEV 500' I—�� G V ETE ND 1/2" R/C 50 .13 7 I 501.30 SET 1/2" R/C GTC LS 280711 8" FIR • 500.89 • 500.7E LS 30444 8/13/20 EN%20 PAINT I TP-2 • 500.22 MARK GAS �n 30" CEDA • 500.55 501.20 SIDEWALK 500.73 �� • 5ao.2a GAS z z z TP-1 ME Existing o I.L. House Z �-..... Z • 4 .84 499.63 Z Q Existing Garage O 500.22 � , ` • 5)0.35 02� cn o LOCATION FROM �� w W o SCANNED AS BUILTS �- ,QBTAINED FROM SEWER DIST.- 1-498.76 499.74 z 36" C-D<R • 498.40 Q 'Ag9 z_ ` 'a-"9Z72, S J I Q 497.97 14% INF-1 L� Zp Z o cn o • 497.29 �= i 4�7 4Y� Q W 497.31 497.60� 3 •-4,97..284--GNE A" FIR 498.20, FIND 1/2" R/C 496.90� 75.00' • 497.83 MEYRING LS 2621 N89°49'04"E 8/13/20 FND 1/2" R/C t WSI 1 S 1 fi91 fi LEGEND — — Property line INF-1 0 30 60 Number and approximate U) location of infiltration test pit TP-1 Scale: 1 inch = 30 feet N Number and approximate LL I location of test pit a Reference: Site Plan based on a plan dated February 5, 2021 titled "Lot Survey for Chandler Sharma," prepared by Owyhee Consulting. a Project Number Sharma Residence mson GEOTECNICAL No. Date Revision By CK o N 1269821 Development , ASSOCIATES, inn 1 8/17/21 Original DPN Duo 0 Figure 2 Site Plan - Woodinviileorroe Wenatchee Office 17311-1351h Ave. NE, A-500 105 Palause St. Wood-ille, WA 98072 Wenatchee, WA 98801 Sc www.nelsangeolech.cam (425) 486-1669 / Fax: 481-2510 (509) 665-7696 / Fax: 665-7692 L UNIFIED SOIL CLASSIFICATION SYSTEM GROUP MAJOR DIVISIONS GROUP NAME SYMBOL CLEAN GW WELL -GRADED, FINE TO COARSE GRAVEL COARSE- GRAVEL GRAVEL GP POORLY -GRADED GRAVEL GRAINED MORE THAN 50 % GRAVEL GM SILTY GRAVEL OF COARSE FRACTION RETAINED ON SOILS NO.4 SIEVE WITH FINES GC CLAYEY GRAVEL SAND CLEAN SW WELL -GRADED SAND, FINE TO COARSE SAND SAND SP POORLY GRADED SAND MORE THAN 50 % RETAINED ON MORE THAN 50 % NO. 200 SIEVE OF COARSE FRACTION SAND SM SILTY SAND PASSES NO. 4 SIEVE WITH FINES SC CLAYEY SAND FINE - SILT AND CLAY ML SILT INORGANIC GRAINED LIQUID LIMIT CL CLAY LESS THAN 50 % SOILS ORGANIC OL ORGANIC SILT, ORGANIC CLAY SILT AND CLAY MH SILT OF HIGH PLASTICITY, ELASTIC SILT INORGANIC MORE THAN 50 % PASSES LIQUID LIMIT CH CLAY OF HIGH PLASTICITY, FAT CLAY NO. 200 SIEVE 50 % OR MORE ORGANIC OH ORGANIC CLAY, ORGANIC SILT HIGHLY ORGANIC SOILS PT PEAT NOTES: 1) Field classification is based on visual SOIL MOISTURE MODIFIERS: examination of soil in general accordance with ASTM D 2488-93. Dry - Absence of moisture, dusty, dry to the touch 2) Soil classification using laboratory tests is based on ASTM D 2488-93. Moist - Damp, but no visible water. 3) Descriptions of soil density or Wet - Visible free water or saturated, consistency are based on usually soil is obtained from interpretation of blowcount data, below water table visual appearance of soils, and/or test data. Project Number nELSOn GEOTEnninl_ No. Date Revision By cK Sharma Residence �. 1 8/17/21 Original DPN DJO 1269821 Development ASSOCIATES, inc , Figure 3 Soil Classification Chart "� W. 135th"°e CA Wa"a`°hee°"`Ce 17311-1351h Ave. NE, A-500 105 Palouse St. Woodinville,WA98072 Wenatchee, WA 98801 www.nelsongeoiech.com (425) 486-1669 / Fax: 481-2510 (509) 665-7696 / Fax: 665-7692 LOG OF EXPLORATION DEPTH (FEET) USCS SOIL DESCRIPTION INFILTRATION TEST PIT ONE 0.0 - 2.0 DARK BROWN, SILTY FINE TO MEDIUM SAND WITH ROOTS, ORGANICS, GRAVEL, COBBLES, CONCRETE RUBBLE, AND METAL DEBRIS (LOOSE TO MEDIUM DENSE, MOIST) (FILL) 2.0 - 3.2 SM ORANGE -BROWN TO LIGHT BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, COBBLES, ROOTS, AND IRON -OXIDE STAINING (LOOSE TO MEDIUM DENSE, MOIST) 3.2-4.5 SM GRAY, SILTY FINE TO MEDIUM SAND WITH GRAVEL, COBBLES, AND IRON -OXIDE STAINING (MEDIUM DENSE TO DENSE, MOIST) SAMPLES WERE NOT COLLECTED GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED TEST PIT CAVING WAS NOT ENCOUNTERED TEST PIT WAS COMPLETED AT 4.5 FEET ON 08/04/2021 TEST PIT ONE 0.0 - 1.5 DARK BROWN, SILTY FINE TO MEDIUM SAND WITH ROOTS, ORGANICS, GRAVEL, AND IRON - OXIDE STAINING (LOOSE TO MEDIUM DENSE, DRY TO MOIST) (FILL) 1.5 - 3.0 SM ORANGE -BROWN TO LIGHT BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, COBBLES, ROOTS, AND IRON -OXIDE STAINING (LOOSE TO MEDIUM DENSE, DRY TO MOIST) 3.0-6.0 SM GRAY, SILTY FINE TO MEDIUM SAND WITH GRAVEL, COBBLES, AND TRACE IRON -OXIDE STAINING (MEDIUM DENSE TO DENSE, MOIST) SAMPLE WAS COLLECTED AT 3.5 FEET GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED TEST PIT CAVING WAS NOT ENCOUNTERED TEST PIT MET REFUSAL AT 6.0 FEET ON 08/04/2021 TEST PIT TWO 0.0 - 1.7 DARK BROWN, SILTY FINE TO MEDIUM SAND WITH ROOTS, ORGANICS, GRAVEL, AND IRON - OXIDE STAINING (LOOSE TO MEDIUM DENSE, DRY TO MOIST) (FILL) 1.7 - 3.0 SM ORANGE -BROWN TO LIGHT BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, COBBLES, ROOTS, ORGANICS, AND IRON -OXIDE STAINING (LOOSE TO MEDIUM DENSE, DRY TO MOIST) 3.0-5.0 SM GRAY, SILTY FINE TO MEDIUM SAND WITH GRAVEL, COBBLES, AND TRACE IRON -OXIDE STAINING (MEDIUM DENSE TO DENSE, MOIST) SAMPLE WAS COLLECTED AT 4.5 FEET GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED TEST PIT CAVING WAS NOT ENCOUNTERED TEST PIT WAS COMPLETED AT 5.0 FEET ON 08/04/2021 DPN:DJO NELSON GEOTECHNICAL ASSOCIATES, INC. FILE NO 1269821 FIGURE 4