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GEOTECH REPORT 4.20.20NELSON GEOTECHNICAL N }. A A5SQGIATES, INC. GEOTECHNICAL ENGINEERS & GEOLOGISTS Main office 17311 -- 135" Ave NE, A-500 Woodinville, WA 98072 (425) 486-1669 FAX (425) 481-2510 February 19, 2020 Ms. Deborah Binder 8816 - 20th Place SW Edmonds, Washington VIA Email::jaideborah(a yahoo.com Geotechnical Engineering Evaluation Binder Property Residence Development 8912 —192nd Street SW Edmonds, Washington NGA File No. 1154419 Dear Ms. Binder: Engineering-Gcology Branch 5526 Industry Lane, 42 Fast Wenatchee, WA 98802 (509) 665-7696 • FAX (509) 665-7692 We are pleased to submit the attached report titled "Geotechnical Engineering Evaluation — Binder Property Residence Development — 8912 - 192nd Street SW — Edmonds, 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 you on January 23, 2020. The relatively level to gently sloping, rectangular -shaped subject site is currently occupied by a single- family residence within the northern and south central portion of the site. The property is bordered to the west and south by existing residential properties, to the east by an alley way and to the north by 192" d Street Southwest. We understand that the proposed improvements within the site will consist of removing the existing structures and constructing a new single-family residence. Specific grading and stormwater plans were not available when this report was prepared, however we understand that stormwater may be directed to on -site infiltration facilities, if feasible. In addition to providing recommendations for the development of the new residential structure, we have been requested to evaluate the infiltration capacity of the site soils. The City of Edmonds utilizes the 2019 WSDOE Stormwater Management Manual for Western Washin tg on to determine the design of infiltration facilities. According to this manual, on -site infiltration testing consisting of the small Pilot Infiltration Test (PIT) is used to determine the long-term design infiltration rates. We monitored the excavation of four test pit explorations throughout the property, one of which we utilized for our small-scale PIT testing. Our explorations indicated that the site was underlain by surficial undocumented fill with competent, native glacial soils at depth. 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. In general, the competent bearing native glacial soils underlying the site should adequately support the planned structures. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1154419 Binder Property Residence Development February 19, 2020 Edmonds, Washington Summary — Page 2 Foundations should be advanced through any loose and/or undocumented fill soils down to the competent native bearing material for bearing capacity and settlement considerations. These soils should generally be encountered at approximate depths between 2.0 and 4.5 feet below the existing ground surface, based on our explorations. 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. Final stormwater plans have also not been developed, but we understand that on -site infiltration is being considered for this site. Based on our onsite testing, it is our opinion that the onsite soils are not conducive to traditional methods of stormwater infiltration, however low -impact design systems may be feasible. The subsurface soils generally consisted of surficial undocumented fill soils underlain by silty fine to medium sand with varying amounts of gravel and iron -oxide weathering that we interpreted as native glacial soils at relatively shallow depths. We recommend that any low impact stormwater infiltration systems be designed in accordance with the 2019 WSDOE Stormwater Management Manual for Western Washington. 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 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. Khaled M. Shawish, PE Principal Engineer TABLE OF CONTENTS INTRODUCTION............................................................................................................. 1 SCOPE............................................................................................................................... 1 SITECONDITIONS......................................................................................................... 2 SurfaceConditions....................................................................................................... 2 Subsurface Conditions.................................................................................................. 2 HydrogeologicConditions........................................................................................... 3 SENSITIVE AREA EVALUATION............................................................................... 3 SeismicHazard............................................................................................................. 3 ErosionHazard............................................................................................................. 4 CONCLUSIONS AND RECOMMENDATIONS.......................................................... 4 General......................................................................................................................... 4 ErosionControl............................................................................................................ 5 SitePreparation and Grading....................................................................................... 5 Temporary and Permanent Slopes............................................................................... 6 Foundations.................................................................................................................. 7 RetainingWalls............................................................................................................ 8 StructuralFill................................................................................................................ 9 Slab-on-Grade.............................................................................................................. 9 Pavement Subgrade and Other Hard Surfaces........................................................... 10 Utilities....................................................................................................................... 10 SiteDrainage.............................................................................................................. 10 CONSTRUCTION MONITORING............................................................................. 12 USEOF THIS REPORT................................................................................................ 12 LIST OF FIGURES Figure 1 — Vicinity Map Figure 2 — Site Plan Figure 3 — Soil Classification Chart Figure 4 — Test Pit Logs NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation Binder Property Residence Development 8912 — 192nd Street SW Edmonds, Washington INTRODUCTION This report presents the results of our geotechnical engineering investigation and evaluation of the proposed Binder Residence Development project in the Edmonds, Washington. The project site is located at 8912 — 192nd Street SW, 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. For our use in preparing this report, we have been provided a survey of the property titled "Topographic Survey for: Deborah Binder" dated December 18, 2019 and prepared by Pacific Coast Surveys, Inc. The property is currently occupied by a single-family residence and asphalt driveway and parking area within the south-central and eastern portions of the site, respectively. Vegetation within the site consist mostly of grass yard areas, landscaping areas, and young to mature trees scattered throughout the site. The site is generally level to gently sloping down from the northeast to the southwest. We understand that the proposed development within the site will include constructing a new single-family residence with the possibility of a daylight basement. Final development and grading plans have not been prepared at the time this report was issued. Final stormwater plans have also not been developed, however, we understand that stormwater may be directed to on -site infiltration systems, if feasible. The existing site layout is shown on the Site Plan in Figure 2. 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 includes the following: Reviewing available soil and geologic maps of the area. 2. Exploring the subsurface soil and groundwater conditions within the site with trackhoe excavated test pits. Trackhoe was subcontracted by NGA. 3. Providing recommendations for earthwork. 4. Providing recommendations for temporary and permanent slopes. 5. Providing recommendations for slab subgrade preparation. 6. Providing recommendations for retaining walls. 7. Determining feasibility of on -site stormwater infiltration. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation Binder Property Residence Development Edmonds, Washington NGA File No. 1154419 February 19, 2020 Page 2 8. Providing long-term design infiltration rates based on one on -site Small -Scale Pilot Infiltration Test (PIT) per the 2019 DOE SWMMWW Manual. Location and depth of test was determined by the civil engineer. Water for the test was secured by the client. 9. Providing recommendations for infiltration system installation, as needed. 10. Providing recommendations for site drainage and erosion control. IL Documenting the results of our findings, conclusions, and recommendations in a written geotechnical report. SITE CONDITIONS Surface Conditions The subject site consists of a rectangular -shaped parcel covering approximately 0.28 acres. The site is generally level to gently sloping down from the northeast to the southwest and is currently occupied by a single-family residence and asphalt driveway parking area within the south-central and eastern portions of the site, respectively. Vegetation within the site consists of mostly grass yard areas with landscaping areas and young to mature trees scattered throughout the site. The site is bordered to the west and south by other residences, to the east by an alley, and to the north by 192' Street SW. We did not observe surface water throughout the site during our visit on February 5, 2020. Subsurface Conditions Geology: The geologic units for the overall site are shown on 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 Vashon till (Qvt). The glacial till is generally described as a non -sorted mixture of clay, silt, sand, pebbles, cobbles, and boulders, all in variable amounts. Our explorations typically encountered surficial undocumented fill underlain by silty fine to medium sand with varying amounts of gravel, which we interpreted as native glacial till soils at depth. Explorations: The subsurface conditions within the site were explored on February 5, 2020 by excavating four test pits to approximate depths in the range of 5.0 to 9.0 feet below the existing ground surface using a mini-trackhoe. 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 test pits. The soils were visually classified in general accordance with the Unified Soil Classification System, presented in Figure 3. The logs of our test pits 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 logs of the test pits should be reviewed. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1154419 Binder Property Residence Development February 19, 2020 Edmonds, Washington Page 3 At the surface of Infiltration Pits 1 and Test Pits 1, 2, and 3, we generally encountered 2.0 to 4.6 feet of loose, dark brown to brown, silty fine to medium sand with gravel, roots, and organics, which we interpreted as undocumented fill soils. Underlying the fill soils in each of our explorations, we generally encountered gray, silty fine to medium sand with gravel, cobbles, and iron -oxide staining, which we interpreted as native glacial soils. Infiltration Pit 1 and Test Pit 1, 2, and 3 were terminated within the native glacial till soils at depths in the range of 5.0 to 9.0 feet below the existing ground surface. Hydrogeologic Conditions We encountered moderate groundwater seepage within Test Pits 2 and 3 at depths in the range of 2.7 to 3.0 feet below the existing ground surface. We interpret the water observed as a perched groundwater condition. Perched water occurs when surface water infiltrates through less dense, more permeable soils and accumulates on top of relatively low permeability materials. The more permeable soils consist of the topsoil/weathered soils and undocumented fill. The low permeability soil consists of relatively silty native deposits. 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 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 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 Spectral Acceleration Site Coefficients Design Spectral at 0.2 sec. (g) at 1.0 sec. (g) Response S, S 1 Parameters Fa Fv SDs SD1 D 1.282 0.503 1.000 1.500 0.855 0.503 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. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1154419 Binder Property Residence Development February 19, 2020 Edmonds, Washington Page 4 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 native 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 Soil Survey of Snohomish County Area, Washington, by the Natural Resources Conservation Service (NRCS) was reviewed to determine the erosion hazard of the on -site soils. The surface soils for this site were mapped as Alderwood-Urban land complex, 8 to 15 percent slopes. The erosion hazard for this material is listed as slight to moderate. This site is relatively level to gently sloping and there are no steep slopes on the property. 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 is feasible from a geotechnical standpoint. Our explorations indicated that the site is generally underlain by competent native bearing glacial soils at relatively shallow depths. The native soils encountered at depth should provide adequate support for foundation, slab, and pavement loads. We recommend that the planned structure be designed utilizing shallow foundations. Footings should extend through any loose soil or undocumented fill soils and be founded on the underlying medium dense or better native bearing glacial soils, or structural fill extending to these soils. The medium dense or better native glacial soils should typically be encountered approximately 2.0 to 4.5 feet below the existing surface, based on our explorations. We should note that localized areas of deeper unsuitable soils and/or undocumented fill could be encountered at this site. This condition would require additional excavations in foundation, slab, and pavement areas to remove the unsuitable soils. We have provided detailed recommendations regarding slab -on -grade, pavement and hard surfacing in the Slab -on -Grade and Pavement subgrade and Other Hard Surfaces subsections of this report. Based on the results of our infiltration testing and soil explorations throughout the site, it is our opinion that the onsite native soils are not conducive for traditional methods of stormwater infiltration. This is further discussed in the Site Drainage section of this report. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation Binder Property Residence Development Edmonds, Washington NGA File No. 1154419 February 19, 2020 Page 5 Erosion Control The erosion hazard for the on -site soils is interpreted to be moderate for exposed soils, but actual erosion potential 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 loose soils from foundation, slab, pavement areas, and other structural areas, to expose medium dense or better native bearing glacial 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 five 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. 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. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1154419 Binder Property Residence Development February 19, 2020 Edmonds, Washington Page 6 The site soils are considered to be moisture -sensitive and will disturb easily when wet. We recommend that construction takes 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 native on -site soils may be suitable for use as structural fill, depending on the moisture content of the soil at the time of construction. It is our opinion that the undocumented fill soils encountered within the site are not suitable for structural fill. 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. 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:IV). Temporary cuts in the competent native glacial soils at depth should be no steeper than 1.5H:IV. 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 2H:1 V. 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. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation Binder Property Residence Development Edmonds, Washington NGA File No. 1154419 February 19, 2020 Page 7 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 2.0 to 4.5 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 footings may be extended down to the competent 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. 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 bearing pressure of not more than 2,500 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 1/2-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. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1154419 Binder Property Residence Development February 19, 2020 Edmonds, Washington Page 8 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 the project plans. In general, the lateral pressure acting on 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. A seismic design loading of 8H should also be included in the wall design, where H represents the total height of the wall. 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 height of the wall, and do not account for surcharge loads. Additional lateral earth pressures should be considered for surcharge loads acting adjacent to walls and within a distance equal to the 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. 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. 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 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. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation Binder Property Residence Development Edmonds, Washington NGA File No. 1154419 February 19, 2020 Page 9 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. 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 native 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. In our opinion, the surficial undocumented fill soils are not suitable for use as structural fill. 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. However, undocumented fill soils may be encountered at slab subgrade elevations. If some future slab settlement and/or cracking cannot be tolerated, we recommend that at a minimum, the slab subgrade be overexcavated by two feet and the overexcavation backfilled with 1'/4-inch crushed rock compacted to structural fill specifications. The resulting overexcavation subgrade should be compacted to an unyielding condition prior to placement of the crushed rock backfill. Additional reinforcement and doweled cold joints should be incorporated into the slab design. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1154419 Binder Property Residence Development February 19, 2020 Edmonds, Washington Page 10 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 may be used to protect the vapor barrier membrane and to aid in curing the concrete. Pavement Subgrade and Other Exterior Hard Surfaces Pavement and walkway subgrade preparation should be completed as recommended in the Site Preparation and Grading and Structural Fill subsections of this report. Depending on tolerance to cracking, we recommend that at least the upper two feet of the existing material be removed and replaced with granular structural fill or crushed rock. The subgrades should be proof -rolled with a heavy, rubber - tired piece of equipment, to identify soft or yielding areas that may require repair prior to placing any structural fill and prior to placing the pavement base course. We should be retained to observe the proof - rolling and to recommend repairs prior to placement of the asphalt or hard surfaces. The hard surface section should be thickened and reinforced with rebar where applicable to further reduce the effects of settlement due to the loose/soft soils, but potential long-term cracking should still be expected if any undocumented fill is left in place below the upper crushed rock backfill. 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. Trench backfill should be compacted to a minimum of 95% of the modified proctor maximum dry density. Trenches located in non-structural areas and five feet below roadway subgrade should be compacted to a minimum 90% of the maximum dry density. The trench backfill compaction should be tested. Site Drainage Infiltration: The subsurface soils generally consisted of undocumented fill soils underlain by silty sand with varying amounts of gravel that we interpreted as native glacial deposits to the depths explored. The 2019 WSDOE Stormwater Management Manual for Western Washington 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. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1154419 Binder Property Residence Development February 19, 2020 Edmonds, Washington Page 11 We conducted one Small PIT within Infiltration Pit 1 located within the north -central portion of the site, as shown on the attached Site Plan in Figure 2. The pit where the test was conducted measured 4.0-feet long by 3.0-feet wide by 5.0-feet deep. The pit was filled with 12-inches of water at the beginning of the day and we began the soaking period of the PIT for approximately 6 hours. At this time, the water flow rate into the holes was monitored with a Great Plains Industries (GPI) TM 075 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 of the test. The flow rate for Infiltration Pit 1 stabilized at 0.012 gallons per minute (0.74 gallons per hour), which equates to an approximate infiltration rate of 0.10 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 not dropped from 12 inches, resulting in an infiltration rate of 0.0 inches per hour during the falling head portion of the test. Due to the very low rate from the steady-state portion of the test and the zero infiltration during the falling head, it is our opinion that the native glacial till soils encountered at depth throughout the site are not conducive to traditional onsite infiltration. However low -impact design infiltration systems, such as pervious pavements, bioswales, and rain gardens, could be feasible. This should be determined by the civil engineer during the final design. We recommend that any proposed low -impact design 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 the fill areas 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. 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 at a minimum downward 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. NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation NGA File No. 1154419 Binder Property Residence Development February 19, 2020 Edmonds, Washington Page 12 We recommend the use of footing drains around the structures. Footing drains should 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. 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 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. USE OF THIS REPORT NGA has prepared this report for Ms. Deborah Binder and her 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 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. Geotcchnical Engineering Evaluation Binder Property Residence Development Edmonds, Washington NGA File No. 1154419 February 19, 2020 Page 13 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. Daniel J. O'Dell Staff Geologist I Carston T. Curd, GIT Project Geologist Maher A. Shebl, PhD, PE, M.ASCE Senior Engineer DJO:CTC:MAS:dy Four Figures Attached NELSON GEOTECHNICAL ASSOCIATES, INC. VICINITY MAP N Not to Scale United States Postal Service The Hook Sth PI SWO 18 3� Hutt Park learn St SW 9, SuperBooks Company s FQ 187th SI SW Seaview n n 187th SI SW v s c Elementary School P E R R I N V I L L E m a ca n Project Edmonds Bay D 1891t1 PI S:. Site ou < 189th PI Slq 1901nslsw Adult Care Home "'ek`P' n o_ st' Ctlenry St 19 l os� �a 191s1 192nd St SW C 192nd PI 51\' m N n Just Flowers Q dDe Portofino © Edmonds Q g Q Maplewood Presbyterian Church PLn Restaurant & Bar Adventist Church Faye'Lla' 196m St sw 524 524 Grace Lutheran Church a Iody Ln OEdmonds10 9 Elementary School Tr 198111 S1 SW Hlndley Ln D Viewland Way 3 d) Maplewood 3 ocA e °' z was Park a imonds United t E2 thodist Church Q Maplewood Parent 0 o %oo r 0 � Cooperative v s VW.PI q a' W fHE BOWL o a 0° E Z ' EDMONDS y� 202nd SI SW a o D m nlonas Carol Way F w n 4 o Caen St Glen St ?�rSt SW C ut 9 Pine Ridge a� � -Park Edmonds, WA m Project Number NELSON GEOTECHNICAL No. Date Revision By CK o Binder Residence ASSOCIATES, INC. 1154419 NGA 1 2/12/20 Original DPN DJO Development p GEOTECHNICAL ENGINEERS & GEOLOGISTS E Figure 1 Vicinity Map Woodinville Office East Wenatchee Office OU J 17311-135th Ave. NE, A-500 5526Industry Lane, #2 Woodinville, WA 98072 East Wenatchee, WA 98802 J (425) 486-1669 / Fax: 481-2510 w.nelsonyeotech.com (509) 665-7696 / Fax: 665=7692 = Site Plan N 192nd St SW 27 S� S SS S SS a / d N ` � — N 89'30'26" W 115.00' I DEC 8 M(J14" DEC 8" -- 1 INF-1 J LOT AREA_;_ Q- P-1 I 1 ZO75 SF I o° 16 f k I' 27041800303700 B 1 o �( 16" B 0 ; � 1 18" Aa B 0 o I ROOF PEAK 0 o ELEV.=362.22' o 0 ELEV 344.55' TP-3i z I i ; { I HOUSE J8912 I I - 14� TP-2 M 8.8' I 3 8' 8.2' 6.4 j m I � � 16"x3 ASPHALT N 89'30'26" W 115.00 W X WOOD FENCE x x' r i a 0 LEGEND 0 U — — Property line C Ul INF-1 0 20 40 Number and approximate I location of infiltration test pit TP-1 Scale: 1 inch = 20 feet N —�— Number and approximate o location of test pit o Reference: Site Plan based on a plan dated December 18, 2019 titled "Deborah Binder," prepared by Pacific Coast Surveys, Inc. a` 10 Project Number Binder Residence NELSON GEOTECHNICAL No. Date Revision By CK o 1154419 Development ASSOCIATES, INC. NGA GEOTECHNICAL ENGINEERS & GEOLOGISTS 1 2/12/20 Original DPN DJO E Figure 2 Site Plan Woodinville Office 17311-135th Ave. NE, A-500 Woodinville, WA 98072 East Wenatchee Office 55261ndost y Lane, #2 East Wenatchee, WA 98802 UO J J (425)486-16691 Fax. 481-2510 www.nelsongeotech.com (509)665-7696/Fax: 665-7692 = 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 GM OF COARSE FRACTION OF SILTY GRAVEL 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 CH 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 GEOTECHNICAL No. Date Revision By CK 1154419 Binder Residence NGA ASSOCIATES, INC. Development GEOTECHNICAL ENGINEERS & GEOLOGISTS 1 2/12/20 Original DPN DJO Figure 3 Soil Classification Chart Woodinville Office East Wenatchee Office 311-135th Ave. NE, A-500 5526 Lane, 88 Woodinville, WA 98072 East Wenatchee, WA 98602 (425) 486-16691 Fax: 481-2510 w.nelsongeotech.com (509) 665-76961 Fax: 665-7692 LOG OF EXPLORATION DEPTH (FEET) USC SOIL DESCRIPTION INFILTRATION PIT ONE 0.0 - 3.0 GRASS UNDERLAIN BY DARK BROWN TO BROWN, SILTY FINE TO MEDIUM SAND WITH ORGANICS, GRAVEL, ROOTS, AND COBBLES (LOOSE TO MEDIUM DENSE, MOIST) (UNDOCUMENTED FILL) 3.0 - 5.0 SM GRAY TO GRAY -BROWN, SILTY FINE TO MEDIUM SAND WITH IRON OXIDE STAINING, GRAVEL AND COBBLES (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 5.0 FEET ON 2/5/2020 TEST PIT ONE 0.0-2.0 GRASS UNDERLAIN BY DARK BROWN TO BROWN, SILTY FINE TO MEDIUM SAND, TRACE COBBLES, GRAVEL, ORGANICS, IRON OXIDE STAINING (LOOSE TO MEDIUM DENSE, MOIST) (UNDOCUMENTED FILL) 2.0-9.0 SM GRAY, SILTY FINE TO MEDIUM SAND WITH GRAVEL, TRACE COBBLES, AND IRON OXIDE STAINING (MEDIUM DENSE TO DENSE, MOIST) SAMPLE WAS COLLECTED AT 8.0 FEET GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED TEST PIT CAVING WAS NOT ENCOUNTERED TEST PIT WAS COMPLETED AT 9.0 FEET ON 2/5/2020 TEST PIT TWO 0.0-4.0 GRASS UNDERLAIN BY DARK BROWN, SILTY FINE TO MEDIUM SAND WITH ROOTS, GRAVEL, COBBLES, AND IRON OXIDE STAINING (LOOSE TO MEDIUM DENSE, MOIST) (UNDOCUMENTED FILL) 4.0 - 6.0 SM GRAY, SILTY FINE TO MEDIUM SAND WITH COBBLES, GRAVEL, IRON OXIDE STAINING (MEDIUM DENSE TO DENSE, MOIST) SAMPLES WERE NOT COLLECTED GROUNDWATER SEEPAGE WAS ENCOUNTERED AT 3.0 FEET TEST PIT CAVING WAS NOT ENCOUNTERED TEST PIT WAS COMPLETED AT 6.5 FEET ON 2/5/2020 TEST PIT THREE 0.0-4.6 GRASS UNDERLAIN BY DARK BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, COBBLES, ROOTS, ORGANICS, AND IRON OXIDE WEATHERING (LOOSE TO MEDIUM DENSE, MOIST) (UNDOCUMENTED FILL) 4.6 - 5.5 SM GRAY, SILTY FINE TO MEDIUM SAND WITH COBBLES, GRAVEL, AND IRON OXIDE STAINING (MEDIUM DENSE TO DENSE, MOIST) SAMPLES WERE NOT COLLECTED GROUNDWATER SEEPAGE WAS ENCOUNTERED AT 2.7 FEET TEST PIT CAVING WAS NOT ENCOUNTERED TEST PIT WAS COMPLETED AT 5.5 FEET ON 2/5/2020 DJO:CTC NELSON GEOTECHNICAL ASSOCIATES, INC. FILE NO 1154419 FIGURE 4