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HomeMy WebLinkAboutRS_Geotechnical _Report_210529_V1 NELSON GEOTECHNICAL ASSOCIATES, INC. February 26, 2021 Mr. Long Nguyen 11904 SE 228th Place Kent, Washington 98051 VIA Email: longdnguyen@gmail.com Geotechnical Engineering Evaluation Nguyen Aberdeen Avenue NE Short Plat Residential Development 2309 Aberdeen Avenue NE Renton, Washington NGA File No. 1227220 Dear Mr. Nguyen: We are pleased to submit the attached report titled “Geotechnical Engineering Evaluation – Nguyen Aberdeen Avenue NE Short Plat Residential Development – 2309 Aberdeen Avenue NE – Renton, Washington.” This report summarizes our observations of the existing surface and subsurface conditions within the site and provides general recommendations for the proposed site development. Our services were completed in general accordance with the proposal issued to you on January21, 2021. The property is currently vacant but was previously occupied with an existing single-family residence within the western portion of the property. The ground surface within the property is relatively level to gently sloping down from the west to the east. We understand that the proposed development will include construction of three new single-family residences and associated roadways and underground utilities. We explored the subsurface conditions within the site with eleven trackhoe excavated test pits explorations. The explorations extended to depths in the range of 5.5 to 11.5 feet below the existing ground surface. Our explorations indicated that the site was underlain by surficial undocumented fill soils with competent native glacial outwash 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 native glacial bearing soils underlying the site should adequately support the planned structures. Foundations should be advanced through any loose and/or undocumented fill soils down to the competent glacial bearing material interpreted to underlie the site, for bearing capacity and settlement considerations. These soils should generally be encountered between approximately 2.0 to 6.0 feet below the existing ground surface, based on our explorations. The deepest areas of undocumented fill were generally encountered within the lower very eastern portion of the property. If deeper areas of 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. Geotechnical Engineering Evaluation NGA File No. 1227220 Nguyen Aberdeen Avenue NE Short Plat Residential Development February 26, 2021 Renton, Washington Summary – Page 2 NELSON GEOTECHNICAL ASSOCIATES, INC. Specific grading and stormwater plans have not been finalized at the time this report was prepared. However, we understand that stormwater from the proposed development may be directed into on-site infiltration systems, if feasible. The City of Renton uses the 2016 King County Surface Water Design Manual, as amended by the City of Renton Storm Water Manual to determine the design of infiltration facilities. According to this manual and the City of Renton requirements, on-site infiltration testing consisting of the small Pilot Infiltration Test (PIT) is used to determine the long-term design infiltration rates. Based on the results of our on-site infiltration testing and granular nature of the native glacial outwash soils encountered within the western and central portion of the site, it is our opinion that the onsite native outwash soils within the western and central portion of the property are conducive for traditional stormwater infiltration methods. However, it is our opinion that stormwater infiltration is not feasible within the very eastern portion of the property due to the overall depth of the undocumented fill and presence of perched groundwater encountered at depth within our eastern explorations. This is discussed in more detail in the attached report. In the attached report, we have also provided general recommendations for site grading, slabs-on- grade, structural fill placement, retaining walls, erosion control, and drainage. We should be retained to review and comment on final development plans and observe the earthwork phase of construction. We also recommend that NGA be retained to provide monitoring and consultation services during construction to confirm that the conditions encountered are consistent with those indicated by the explorations, to provide recommendations for design changes should the conditions revealed during 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. Khaled M. Shawish, PE Principal Engineer TABLE OF CONTENTS INTRODUCTION .......................................................................................................... 1 SCOPE......................................................................................................................... 1 SITE CONDITIONS ........................................................................................................ 2 Surface Conditions ........................................................................................................ 2 Subsurface Conditions .................................................................................................. 2 Hydrogeologic Conditions............................................................................................. 3 SENSITIVE AREA EVALUATION ..................................................................................... 4 Seismic Hazard .............................................................................................................. 4 Erosion Hazard .............................................................................................................. 4 CONCLUSIONS AND RECOMMENDATIONS .................................................................. 5 General ......................................................................................................................... 5 Erosion Control ............................................................................................................. 6 Site Preparation and Grading ....................................................................................... 6 Temporary and Permanent Slopes ............................................................................... 7 Foundations .................................................................................................................. 8 Retaining Walls ............................................................................................................. 9 Structural Fill ............................................................................................................... 10 Slab-on-Grade ............................................................................................................. 11 Pavements .................................................................................................................. 11 Utilities ........................................................................................................................ 11 Site Drainage ............................................................................................................... 12 CONSTRUCTION MONITORING .................................................................................. 14 USE OF THIS REPORT ................................................................................................. 14 LIST OF FIGURES Figure 1 – Vicinity Map Figure 2 – Site Plan Figure 3 – Soil Classification Chart Figures 4 through 6 – Exploration Logs Geotechnical Engineering Evaluation Nguyen Aberdeen Avenue NE Short Plat Residential Development 2309 Aberdeen Avenue NE Renton, Washington INTRODUCTION This report presents the results of our geotechnical engineering investigation and evaluation of the planned residential development project in Renton, Washington. The project site is located at 2309 Aberdeen Avenue NE, as shown on the Vicinity Map in Figure 1. The purpose of this study is to explore and characterize the site’s surface and subsurface conditions and to provide geotechnical recommendations for the planned site development. The property is currently vacant but was previously occupied with an existing single-family residence within the western portion of the property. The ground surface within the property is relatively level to gently sloping down from the west to the east. We understand that the proposed development will include construction of three new single-family residences and associated roadways and underground utilities. 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 included the following: 1. Review available soil and geologic maps of the area. 2. Explore the subsurface soil and groundwater conditions within the site with trackhoe excavated test pits. Trackhoe was subcontracted by NGA. 3. Perform grain-size sieve analysis on soil samples, as necessary. 4. Provide recommendations for structure foundations. 5. Provide recommendations for earthwork. 6. Provide recommendations for temporary and permanent slopes. 7. Provide recommendations for retaining walls. Geotechnical Engineering Evaluation NGA File No. 1227220 Nguyen Aberdeen Avenue NE Short Plat Residential Development February 26, 2021 Renton, Washington Page 2 NELSON GEOTECHNICAL ASSOCIATES, INC. 8. Provide recommendations for slab and pavement subgrade preparation. 9. Provide recommendations for utility installation. 10. Provide long-term design infiltration rates based on on-site Pilot Infiltration Testing (PIT) per the 2016 King County Surface Water Design Manual. Location and depth of tests were determined by the civil engineer. Water for the tests was secured by client. 11. Provide recommendations for infiltration system installation. 12. Provide recommendations for site drainage and erosion control. 13. Document the results of our findings, conclusions, and recommendations in a written geotechnical report. SITE CONDITIONS Surface Condition The site consists of a roughly square-shaped parcel covering approximately 0.84 acres. The site is currently vacant but was previously occupied by a single-family residence within the western portion of the site. Topography within the western and central portion of the slopes gently down from the western property line to the eastern portion of the property. The eastern portion of the property is generally level. The site is generally vegetated by grass-covered yard areas, young to mature trees, and landscaping. A horseshoe- shaped gravel driveway is located within the eastern portion of the property. The property is bound to the north, south, and west by single-family residences, and to the east by Aberdeen Avenue NE. We did not observe surface water within the site during our site visit on February 5, 2021. Subsurface Conditions Geology: The geologic units for this site are shown on Preliminary Geologic Map of Seattle and Vicinity, Washington, by Waldron, H.H., Leisch, B.A., Mullineaux, D.R., and Crandell, D.R., (USGS, 1961). The site is mapped as younger gravel (Qyg) with younger sand (Qys) and alluvium (Qa) mapped nearby. The younger gravel and sand deposits are described as fine to coarse sand that contains varying amounts of gravel. The alluvium is described as silt with varying amounts of sand and organic material. Our explorations generally encountered fine to coarse sand with varying amounts of gravel and silt within the western and central portion of the property that we interpreted as younger gravel outwash soils. Our explorations within the very eastern portion of the property generally encountered silt with sand and silty fine to medium sand that we interpreted as native alluvium and younger sand deposits at depth. Geotechnical Engineering Evaluation NGA File No. 1227220 Nguyen Aberdeen Avenue NE Short Plat Residential Development February 26, 2021 Renton, Washington Page 3 NELSON GEOTECHNICAL ASSOCIATES, INC. Explorations: The subsurface conditions within the site were explored on February 5, 2021 by excavating eleven test pits with a mini-trackhoe extending to depths in the range of 5.5 to 11.5 feet below the existing ground surface. The approximate locations of our explorations are shown on the Site Plan in Figure 2. A geologist from NGA was present during the explorations, examined the soils and geologic conditions encountered, obtained samples of the different soil types, and maintained logs of the test pits. 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 Figures 4 through 6. 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 explorations should be reviewed. At the surface of all of our explorations, we encountered approximately 0.5 to 6.0 feet of surficial grass and loose to medium dense dark brown silty fine to medium sand with varying amounts of gravel, organics and debris that we interpreted as surficial topsoil and/or undocumented fill soils. Underlying the surficial topsoil/undocumented fill soils within Test Pit 1, 2, 8, and 9 within the eastern portion of the site, we encountered medium dense/stiff orange-brown to gray-brown silty fine to medium sand and silt with fine sand with varying amounts of gravel and organics that we interpreted as younger sand and alluvium deposits, respectively. Below the surficial topsoil and/or the undocumented fill in Test Pits 3 through 7, 10, and Infiltration Pit 1, we encountered medium dense to dense fine to coarse sand with varying amounts of silt and gravel that we interpreted as native younger gravel soils. All of our test pits were terminated within the native younger gravel and sand soils at depths in the range of 5.5 to 11.5 feet below the existing ground surface. Hydrogeologic Conditions We encountered minor to moderate groundwater seepage within Test Pits 1 and 8 at a depth of 8.0 feet below the existing ground surface. We interpreted this groundwater seepage to be perched groundwater. Perched water occurs when surface water infiltrates through less dense, more permeable soils and accumulates on top of a relatively low permeability material. Perched water does not represent a regional groundwater "table" within the upper soil horizons. Perched water tends to vary spatially and is dependent upon the amount of rainfall. We would expect the amount of perched groundwater to decrease during drier times of the year and increase during wetter periods. Geotechnical Engineering Evaluation NGA File No. 1227220 Nguyen Aberdeen Avenue NE Short Plat Residential Development February 26, 2021 Renton, Washington Page 4 NELSON GEOTECHNICAL ASSOCIATES, INC. 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 at 0.2 sec. (g) Ss Spectral Acceleration at 1.0 sec. (g) S1 Site Coefficients Design Spectral Response Parameters Fa Fv SDS SD1 D 1.473 0.542 1.000 1.500 0.958 0.542 The spectral response accelerations were obtained from the USGS Earthquake Hazards Program Interpolated Probabilistic Ground Motion website (2008 data) for the project latitude and longitude. Hazards associated with seismic activity include liquefaction potential and amplification of ground motion. Liquefaction is caused by a rise in pore pressures in a loose, fine sand deposit beneath the groundwater table. It is our opinion that the medium dense or better glacial outwash deposits interpreted to underlie the site and nearby vicinity 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 King 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 Indianola loamy sand, 5 to 15 percent slopes. The erosion hazard for this material is listed as slight. 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. Geotechnical Engineering Evaluation NGA File No. 1227220 Nguyen Aberdeen Avenue NE Short Plat Residential Development February 26, 2021 Renton, Washington Page 5 NELSON GEOTECHNICAL ASSOCIATES, INC. 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 is generally underlain by competent native glacial till soils at depth throughout the site. The native glacial bearing soils encountered at depth should provide adequate support for foundation, slab, and pavement loads. We recommend that the planned structure be designed utilizing conventional shallow foundations. Footings should extend through any loose soil or undocumented fill soils and be founded on the underlying medium dense or better native glacial till soils, or structural fill extending to these soils. The medium dense or better native glacial bearing soils should typically be encountered approximately 2.0 to 6.0 feet below the existing surface, based on our explorations. In general, the competent native glacial soils were shallower within the western portion of the site and were deepest within the very eastern portion of the site. 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. Based on the results of our on-site infiltration testing and soil explorations throughout the site, it is our opinion that the onsite native granular outwash soils encountered within the western and central portion of the site are conducive for full stormwater infiltration methods. However, due to the deeper undocumented fill soils and presence of perched groundwater within our very eastern explorations, it is our opinion that stormwater infiltration is not feasible within the very eastern portion of the property. This is further discussed in the Site Drainage section of this report. The surficial soils encountered on this site are considered moisture-sensitive and will disturb easily when wet. We recommend that construction take place during the drier summer months, if possible. If construction is to take place during wet weather, the soils may disturb, and additional expenses and delays may be expected due to the wet conditions. Additional expenses could include the need for placing a blanket of rock spalls to protect exposed subgrades and construction traffic areas. Some of the native on- site soils may be suitable for use as structural fill depending on the moisture content of the soil during construction. NGA should be retained to determine if the on-site soils can be used as structural fill material during construction. Geotechnical Engineering Evaluation NGA File No. 1227220 Nguyen Aberdeen Avenue NE Short Plat Residential Development February 26, 2021 Renton, Washington Page 6 NELSON GEOTECHNICAL ASSOCIATES, INC. Erosion Control The erosion hazard for the on-site soils is interpreted to be slight to 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 2.0 to 6.0 feet, depending on the specific locations. In general, the competent native glacial soils were shallower within the western portion of the property and deeper within the eastern portion of the property. 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 Geotechnical Engineering Evaluation NGA File No. 1227220 Nguyen Aberdeen Avenue NE Short Plat Residential Development February 26, 2021 Renton, Washington Page 7 NELSON GEOTECHNICAL ASSOCIATES, INC. 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 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. 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/surficial soils be no steeper than 2 Horizontal to 1 Vertical (2H:1V). Temporary cuts in the competent native 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. Geotechnical Engineering Evaluation NGA File No. 1227220 Nguyen Aberdeen Avenue NE Short Plat Residential Development February 26, 2021 Renton, Washington Page 8 NELSON GEOTECHNICAL ASSOCIATES, INC. Permanent cut and fill slopes should be no steeper than 2H:1V. However, flatter inclinations may be required in areas where loose soils are encountered. Permanent slopes should be vegetated, and the vegetative cover maintained until established. Foundations Conventional shallow spread foundations should be placed on medium dense or better native glacial outwash soils or be supported on structural fill or rock spalls extending to those soils. Native medium dense or better glacial bearing soils should be encountered approximately 2.0 to 6.0 feet below the existing ground surface based on our explorations. We typically encountered deeper areas of surficial undocumented fill soils within our explorations within the eastern portion of the site, and shallower undocumented fill soils within the western portion of the site. Where undocumented fill or less dense soils are encountered at footing bearing elevation, the subgrade should be over-excavated to expose native bearing soil. The over-excavation may be filled with structural fill, or the footings 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 half of the depth of the over-excavation below the bottom of the footing. In case of excessive undocumented fill thickness, deep foundation options may be required. NGA is available to work with the structural engineer to explore those options. 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 2,000 pounds per square foot (psf) be used for the design of footings founded on the medium dense or better native soils or structural fill extending to the competent native bearing material. The foundation bearing soil should be evaluated by a representative of NGA. We should be consulted if higher bearing pressures are needed. Current IBC guidelines should be used when considering increased allowable bearing pressure for short-term transitory wind or seismic loads. Potential foundation settlement using the recommended allowable bearing pressure is estimated to be less than 1-inch total and ½-inch differential between adjacent footings or across a distance of about 20 feet, based on our experience with similar projects. Geotechnical Engineering Evaluation NGA File No. 1227220 Nguyen Aberdeen Avenue NE Short Plat Residential Development February 26, 2021 Renton, Washington Page 9 NELSON GEOTECHNICAL ASSOCIATES, INC. Lateral loads may be resisted by friction on the base of the footing and passive resistance against the subsurface portions of the foundation. A coefficient of friction of 0.35 may be used to calculate the base friction and should be applied to the vertical dead load only. Passive resistance may be calculated as a triangular equivalent fluid pressure distribution. An equivalent fluid density of 200 pounds per cubic foot (pcf) should be used for passive resistance design for a level ground surface adjacent to the footing. This level surface should extend a distance equal to at least three times the footing depth. These recommended values incorporate safety factors of 1.5 and 2.0 applied to the estimated ultimate values for frictional and passive resistance, respectively. To achieve this value of passive resistance, the foundations should be poured “neat” against the native medium dense soils or compacted fill should be used as backfill against the front of the footing. We recommend that the upper one foot of soil be neglected when calculating the passive resistance. Retaining Walls Specific grading plans for this project were not available at the time this report was prepared but retaining walls may be incorporated into project plans. In general, the lateral pressure acting on 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 35 pcf for yielding (active condition) walls, and 55 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. Geotechnical Engineering Evaluation NGA File No. 1227220 Nguyen Aberdeen Avenue NE Short Plat Residential Development February 26, 2021 Renton, Washington Page 10 NELSON GEOTECHNICAL ASSOCIATES, INC. 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 one-half the height of the wall. The thickness of the loose lifts should be reduced to accommodate the lower compactive energy of the hand-operated equipment. The recommended level of compaction should still be maintained. Permanent drainage systems should be installed for retaining walls. Recommendations for these systems are found in the Subsurface Drainage subsection of this report. We recommend that we be retained to evaluate the proposed wall drain backfill material and observe installation of the drainage systems. Structural Fill General: Fill placed beneath foundations, pavement, or other settlement-sensitive structures should be placed as structural fill. Structural fill, by definition, is placed in accordance with prescribed methods and standards, and is monitored by an experienced geotechnical professional or soils technician. Field monitoring procedures would include the performance of a representative number of in-place density tests to document the attainment of the desired degree of relative compaction. The area to receive the fill should be suitably prepared as described in the Site Preparation and Grading subsection prior to beginning fill placement. 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. 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. Geotechnical Engineering Evaluation NGA File No. 1227220 Nguyen Aberdeen Avenue NE Short Plat Residential Development February 26, 2021 Renton, Washington Page 11 NELSON GEOTECHNICAL ASSOCIATES, INC. 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 Pavement subgrade preparation and structural filling where required, should be completed as recommended in the Site Preparation and Grading and Structural Fill subsections of this report. The pavement subgrade should be proof-rolled with a heavy, rubber-tired piece of equipment, to identify soft or yielding areas that require repair. The pavement section should be underlain by a stable subgrade. We should be retained to observe the proof-rolling and recommend repairs prior to placement of the asphalt or hard surfaces. Utilities We recommend that underground utilities be bedded with a minimum 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 percent of the modified proctor as described in the Structural Fill subsection of this report. Trench backfill should be compacted to a minimum of 95 percent 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 percent of the maximum dry density. The trench backfill compaction should be tested. Geotechnical Engineering Evaluation NGA File No. 1227220 Nguyen Aberdeen Avenue NE Short Plat Residential Development February 26, 2021 Renton, Washington Page 12 NELSON GEOTECHNICAL ASSOCIATES, INC. Site Drainage Infiltration: The subsurface soils within our explorations at depth generally consisted of granular glacial outwash soils to the depths explored within the western and central portion of the site and native alluvial soils within the very eastern portion of the property. In accordance with the 2016 King County Surface Water Design Manual (KCSWDM), we conducted one Small PIT within Infiltration Pit 1 within the central portion of the property as shown on the attached Site Plan in Figure 2. Infiltration Pit 1 measured 4.0-feet long by 3.0-feet wide by 8.0-feet deep. Due to the granular nature of the site soils and the maximum capacity of the on-site water source, we were only able to maintain approximately 2.0-inches of standing water within Infiltration Pit 1 instead of the recommended 12-inches. This level was maintained for approximately 6 hours for the pre soak period of the test. After the 6-hour soaking period was completed, the water level was maintained at approximately 2-inches for one hour for the steady-state period with the maximum flow rate of the on-site water source. At this time, the water flow rate into the hole was monitored with a Great Plains Industries (GPI) TM 050 water flow meter. The maximum flow rate of the on-site water source for Infiltration Pit 1 stabilized at 6.67 gallons per minute (400.2 gallons per hour), which equates to an approximate infiltration rate of 53.50 inches per hour. The water was shut off after the steady-state period of the test and was monitored for the falling head portion of the test. After 2 minutes, the water level within the pit had fully infiltrated resulting in an infiltration rate of 60.0 inches per hour. It is our opinion that the native granular outwash soils encountered at depth within the western and central portion of the property are suitable for traditional stormwater infiltration. The subsurface soils within the western and central portion of the property generally consisted of granular outwash sand soils at depth. We have selected the most conservative measured field rate of 53.5 in/hr obtained from the falling head portions of the test in our infiltration pit to be utilized in determining the long-term design infiltration rate for the infiltration systems within the western and central portions of the property. We referenced Equation 5-11 within Chapter 5.2.1 of the (KCSWDM) that applies correction factors to the field measured infiltration rate to generate a long-term design infiltration rate. Correction factors of 0.50, 0.90, and 0.80 were utilized in this equation for Ftesting, Fgeometry, Fplugging respectively, resulting in an infiltration rate of 19.25 inches per hour, to be utilized to design any on-site infiltration systems founded within the native granular outwash soils within the western and central portions of the property. We recommend that the base of the infiltration systems within the western and central portions of the property be terminated within the native granular outwash soils encountered at approximately 0.5 to 6.0 feet below the existing ground surface within the western and central portions of the property. We recommend that stormwater infiltration systems not be located within the eastern portion of the property due to the presence of surficial Geotechnical Engineering Evaluation NGA File No. 1227220 Nguyen Aberdeen Avenue NE Short Plat Residential Development February 26, 2021 Renton, Washington Page 13 NELSON GEOTECHNICAL ASSOCIATES, INC. undocumented fill and perched groundwater encountered at depth within our explorations within the very eastern portion of the property. NGA should be retained to observe infiltration trench excavations. The stormwater manual recommends a three-foot separation between the base of an infiltration system and any underlying bedrock, impermeable horizon, or groundwater. We did not encounter groundwater seepage within our test pit explorations within the western and central portion of the site to a depth of approximately 11.5 feet below the existing ground surface. We recommend that the base of any proposed infiltration systems be located to maintain minimum separation from any groundwater and impermeable horizons in accordance with the stormwater manual and the City of Renton requirements. We also recommend that any proposed 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. 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 recommend that the base of the on-site infiltration systems be extended through the upper undocumented fill and surficial soils and terminated within the native glacial outwash soils encountered at depth throughout the site. We should be retained during construction to evaluate the soils exposed in the infiltration systems to verify that the soils are appropriate for infiltration. 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 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. Geotechnical Engineering Evaluation NGA File No. 1227220 Nguyen Aberdeen Avenue NE Short Plat Residential Development February 26, 2021 Renton, Washington Page 14 NELSON GEOTECHNICAL ASSOCIATES, INC. 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 Mr. Long Nguyen and his agents, for use in the planning and design of the development on this site only. The scope of our work does not include services related to construction safety precautions and our recommendations are not intended to direct the contractors’ methods, techniques, sequences, or procedures, except as specifically described in our report for consideration in design. There are possible variations in subsurface conditions between the explorations and also with time. Our report, conclusions, and interpretations should not be construed as a warranty of subsurface conditions. A contingency for unanticipated conditions should be included in the budget and schedule. We recommend that NGA be retained to provide monitoring and consultation services during construction to confirm that the conditions encountered are consistent with those indicated by the explorations, to provide recommendations for design changes should the conditions revealed differ from those anticipated, and to evaluate whether or not earthwork and foundation installation activities comply with contract plans and specifications. We should be contacted a minimum of one week prior to construction activities and could attend pre-construction meetings if requested. Within the limitations of scope, schedule, and budget, our services have been performed in accordance with generally accepted geotechnical engineering practices in effect in this area at the time this report was prepared. No other warranty, expressed or implied, is made. Our observations, findings, and opinions are a means to identify and reduce the inherent risks to the owner. o-o-o Geotechnical Engineering Evaluation NGA File No. 1227220 Nguyen Aberdeen Avenue NE Short Plat Residential Development February 26, 2021 Renton, Washington Page 15 NELSON GEOTECHNICAL ASSOCIATES, INC. It has been a pleasure to provide service to you on this project. If you have any questions or require further information, please call. Sincerely, NELSON GEOTECHNICAL ASSOCIATES, INC. Lee S. Bellah, LG Project Geologist Khaled M. Shawish, PE Principal LSB:KMS:dy Six Figures Attached LOG OF EXPLORATION DEPTH (FEET) USC SOIL DESCRIPTION DJO:DPN NELSON GEOTECHNICAL ASSOCIATES, INC. FILE NO 1227220 FIGURE 4 INFILTRATION PIT ONE 0.0 – 3.5 GRASS UNDERLAIN BY LIGHT BROWN TO DARK BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, COBBLES, IRON OXIDE WEATHERING, ORGANICS, AND ROOTS (LOOSE TO MEDIUM DENSE, MOIST) (FILL) 3.5 – 5.5 ORANGE-BROWN TO BROWN, SILTY FINE TO MEDIUM SAND WITH ROOTS, IRON OXIDE WEATHERING, GRAVEL, COBBLES, AND TRACE ORGANICS (MEDIUM DENSE, MOIST) (FILL) 5.5 – 10.0 SP-SM GRAY-BROWN TO GRAY, FINE TO COARSE SAND WITH GRAVEL, COBBLES, SILT, ROOTS, AND IRON OXIDE WEATHERING (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 10.0 FEET ON 2/5/21 TEST PIT ONE 0.0 – 3.0 GRASS UNDERLAIN BY BROWN TO LIGHT BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, ROOTS, AND ORGANICS (LOOSE TO MEDIUM DENSE, MOIST) (FILL) 3.0 – 5.0 DARK BROWN TO BLACK, SILTY FINE TO MEDIUM SAND WITH GRAVEL, TRACE ROOTS, AND ORGANICS (LOOSE TO MEDIUM DENSE, MOIST) (FILL) 5.0 – 8.0 ML GRAY TO GRAY-BROWN, SILT WITH FINE SAND, ORGANIC PARTICULATE, IRON OXIDE STAINING (STIFF TO VERY STIFF, MOIST) 8.0 – 10.0 SP-SM GRAY TO GRAY-BROWN, FINE TO MEDIUM SAND WITH SILT (MEDIUM DENSE TO DENSE, MOIST TO WET) SAMPLES WERE COLLECTED AT 5.5 AND 8.5 FEET GROUNDWATER SEEPAGE WAS ENCOUNTERED AT 8.0 TEST PIT CAVING WAS NOT ENCOUNTERED TEST PIT WAS COMPLETED AT 10.0 FEET ON 2/5/21 TEST PIT TWO 0.0 – 3.0 GRASS UNDERLAIN BY BROWN, SILTY FINE TO MEDIUM SAND WITH ROOTS, ORGANICS, AND GRAVEL (LOOSE TO MEDIUM DENSE, MOIST) (FILL) 3.0 – 5.0 DARK BROWN TO BLACK, SILTY FINE TO MEDIUM SAND WITH GRAVEL, ROOTS, ORGANICS, ASPHALT CHUNKS AND TRACE COBBLES (LOOSE TO MEDIUM DENSE, MOIST) (FILL) 5.0 – 10.0 SM ORANGE-BROWN TO BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL (MEDIUM DENSE, MOIST) SAMPLES WERE NOT COLLECTED GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED TEST PIT CAVING WAS NOT ENCOUNTERED TEST PIT WAS COMPLETED AT 10.0 FEET ON 2/5/21 LOG OF EXPLORATION DEPTH (FEET) USC SOIL DESCRIPTION DJO:DPN NELSON GEOTECHNICAL ASSOCIATES, INC. FILE NO 1227220 FIGURE 5 TEST PIT THREE 0.0 – 0.5 GRASS UNDERLAIN BY SILTY, FINE TO MEDIUM SAND WITH ROOTS, ORGANICS, AND GRAVEL (LOOST TO MEDIUM DENSE, MOIST) (FILL) 0.5 – 1.5 SM GRAY-BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL AND IRON OXIDE STAINING (MEDIUM DENSE, MOIST) 1.5 – 10.5 SP BROWN-GRAY FINE TO COARSE SAND WITH GRAVEL AND TRACE SILT (MEDIUM DENSE TO DENSE, MOIST) SAMPLES WERE COLLECTED AT 3.0, 6.0, AND 9.0 FEET GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED SLIGHT TEST PIT CAVING WAS ENCOUNTERED BETWEEN 4.0 AND 10.0 FEET TEST PIT WAS COMPLETED AT 10.5 FEET ON 2/5/21 TEST PIT FOUR 0.0 – 0.5 GRASS UNDERLAIN BY DARK BROWN SILTY FINE TO MEDIUM SAND WITH GRAVEL, ROOTS, AND ORGANICS (LOOST TO MEDIUM DENSE, MOIST) (FILL) 0.5 – 3.0 SM BROWN-GRAY, SILTY FINE TO MEDIUM SAND WITH GRAVEL (MEDIUM DENSE, MOIST) 3.0 – 10.0 SP BROWN-GRAY, FINE TO COARSE SAND WITH GRAVEL AND TRACE SILT (MEDIUM DENSE TO DENSE, MOIST) SAMPLES WERE COLLECTED AT 5.0 AND 9.0 FEET GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED TEST PIT CAVING WAS ENCOUNTERED BETWEEN 5.0 AND 10.0 FEET TEST PIT WAS COMPLETED AT 10.0 FEET ON 2/5/21 TEST PIT FIVE 0.0 – 1.5 GRASS UNDERLAIN BY DARK BROWN TO ORANGE BROWN, SILTY FINE TO COARSE SAND WITH GRAVEL, TRACE COBBLES, AND IRON OXIDE WEATHERING (LOOSE TO MEDIUM DENSE, MOIST) (FILL) 1.5 – 11.0 SP GRAY TO ORANGE-GRAY, FINE TO COARSE SAND WITH GRAVEL, ROOTS, IRON-OXIDE WEATHERING, COBBLES, AND TRACE SILT (MEDIUM DENSE TO DENSE, MOIST) SAMPLES WERE COLLECTED AT 5.0 AND 9.0 FEET GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED SLIGHT TEST PIT CAVING WAS ENCOUNTERED AT 1.5 FEET TEST PIT WAS COMPLETED AT 11.0 FEET ON 2/5/21 TEST PIT SIX 0.0 – 1.0 GRASS UNDERLAIN BY DARK BROWN TO ORANGE-BROWN, SILTY FINE TO COARSE SAND WITH GRAVEL, TRACE COBBLES, AND IRON OXIDE WEATHERING (LOOSE TO MEDIUM DENSE, MOIST) (FILL) 1.0 – 3.0 GRAY TO GRAY-BROWN, SILTY FINE SAND WITH ROOTS, ORGANICS, AND GRAVEL (LOOSE TO MEDIUM DENSE, MOIST) (FILL) 3.0 – 5.5 DARK BROWN TO BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, TRACE ROOTS, ORGANICS, IRON OXIDE WEATHERING, AND TRACE COBBLES (LOOSE TO MEDIUM DENSE, MOIST) (FILL) 5.5 – 11.5 SP GRAY TO ORANGE-GRAY, FINE TO COARSE SAND WITH GRAVEL, ROOTS, IRON OXIDE WEATHERING, COBBLES, AND TRACE SILT (MEDIUM DENSE TO DENSE, MOIST) SAMPLES WERE COLLECTED 2.0, 5.0, AND 10 FEET GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED TEST PIT CAVING WAS NOT ENCOUNTERED TEST PIT WAS COMPLETED AT 11.5 FEET ON 2/5/21 LOG OF EXPLORATION DEPTH (FEET) USC SOIL DESCRIPTION DJO:DPN NELSON GEOTECHNICAL ASSOCIATES, INC. FILE NO 1227220 FIGURE 6 TEST PIT SEVEN 0.0 – 3.0 GRASS UNDERLAIN BY LIGHT BROWN TO BROWN, SILTY FINE TO MEDIUM SAND WITH ROOTS, ORGANICS, AND GRAVEL (LOOSE TO MEDIUM DENSE, MOIST) (FILL) 3.0 – 6.0 ORANGE-BROWN TO BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, TRACE COBBLES, IRON OXIDE WEATHERING, AND ROOTS (LOOSE TO MEDIUM DENSE, MOIST) (FILL) 5.5 – 11.5 SP GRAY TO ORANGE-GRAY, FINE TO COARSE SAND WITH GRAVEL, ROOTS, IRON OXIDE WEATHERING, COBBLES, AND TRACE SILT (MEDIUM DENSE TO DENSE, MOIST) SAMPLES WERE COLLECTED 2.0, 5.0, AND 10 FEET GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED TEST PIT CAVING WAS NOT ENCOUNTERED TEST PIT WAS COMPLETED AT 11.5 FEET ON 2/5/21 TEST PIT EIGHT 0.0 – 4.5 GRASS UNDERLAIN BY DARK BROWN TO GRAY-BROWN, SILTY FINE TO MEDIUM SAND WITH ROOTS, GRAVEL, ORGANICS, AND IRON OXIDE STAINING (LOOSE TO MEDIUM DENSE, MOIST) (FILL) 4.5 – 7.0 SM BROWN TO GRAY-BROWN, SILTY FINE TO MEDIUM SAND WITH TRACE GRAVEL, IRON OXIDE WEATHERING AND TRACE ROOTS (MEDIUM DENSE TO DENSE, MOIST TO WET) 7.0 – 10.5 SM GRAY TO GRAY-BROWN, SILTY FINE TO MEDIUM SAND WITH TRACE IRON OXIDE STAINING (MEDIUM DENSE TO DENSE, MOIST) SAMPLES WERE COLLECTED 6.0 AND 10.0 FEET GROUNDWATER SEEPAGE WAS ENCOUNTERED AT 8.0 FEET TEST PIT CAVING WAS NOT ENCOUNTERED TEST PIT WAS COMPLETED AT 10.5 FEET ON 2/5/21 TEST PIT NINE 0.0 – 4.5 GRASS UNDERLAIN BY DARK BROWN TO GRAY-BROWN, SILTY FINE TO MEDIUM SAND WITH ROOTS, GRAVEL, ORGANICS, AND IRON OXIDE STAINING (LOOSE TO MEDIUM DENSE, MOIST) (FILL) 4.5 – 8.0 SM ORANGE-BROWN TO BROWN, SILTY FINE TO MEDIUM SAND WITH TRACE GRAVEL, IRON OXIDE WEATHERING AND TRACE ROOTS (MEDIUM DENSE TO DENSE, MOIST TO WET) 8.0 – 10.5 SM GRAY TO GRAY-BROWN, SILTY FINE TO MEDIUM SAND WITH TRACE IRON OXIDE STAINING (MEDIUM DENSE TO DENSE, MOIST) SAMPLE WAS COLLECTED 10.0 FEET GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED SLIGHT TEST PIT CAVING WAS ENCOUNTERED AT 8.0 FEET TEST PIT WAS COMPLETED AT 10.5 FEET ON 2/5/21 TEST PIT TEN 0.0 – 3.0 GRASS UNDERLAIN BY ORANGE BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, TRACE COBBLES, AND IRON OXIDE WEATHERING (LOOSE TO MEDIUM DENSE, MOIST) (FILL) 3.0 – 5.5 SP GRAY TO ORANGE-GRAY, FINE TO COARSE SAND WITH GRAVEL, ROOTS, IRON OXIDE WEATHERING, COBBLES, AND TRACE SILT (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 5.5 FEET ON 2/5/21