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Home My WebLink AboutRS_Infiltration_Study_221227_v1 NELSON GEOTECHNICAL ASSOCIATES, INC. November 19, 2021 Binnu Billing VIA Email: binnu.billing@gmail.com Geotechnical Engineering Evaluation Billing Short Plat Infiltration 14103 – 160th Avenue SE Renton, Washington NGA File No. 1308421 Dear Binnu: We are pleased to submit the attached report for a subsurface geotechnical and stormwater infiltration evaluation for the residential short plat project located at 14103 – 160th Avenue SE in Renton, Washington. This report summarizes our observations of the existing surface and subsurface conditions within the site and provides general recommendations for the proposed site development. Our services were completed in general accordance with the proposal signed by you on September 30th, 2021. The property is rectangular in shape and covers approximately 0.86 acres in area. It is currently occupied by a single-family residence in the east-central portion of the site. The property is bordered by 160th Avenue SE to the east, SE 7th Street to the north, and neighboring residential properties on both other sides. Topographically, the site is generally level. We understand the plans for development include a short plat of the property and construction of a residence on the resulting parcel. We performed four test pit explorations throughout the property where one was used for a small-scale pilot infiltration test (PIT). Our explorations indicated that the site was underlain by gray silty fine sand with gravel, which we interpreted to glacial till. 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 soils underlying the site should adequately support the planned structures. Foundations should be advanced through any loose soils down to the competent native bearing glacial material interpreted to underlie the site, for bearing capacity and settlement considerations. These soils should generally be encountered approximately 3.0 – 3.5 feet below the existing ground surface, based on our explorations. However, deeper areas of loose soils or undocumented fill could be encountered in unexplored areas of the site. If encountered, these soils 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. The subsurface soils generally consisted of surficial topsoil/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 till soils at relatively shallow depths. Based on our onsite testing, it is our opinion that stormwater infiltration is not feasible within the site. However, shallow, low-impact design infiltration systems may be feasible at specific areas within the site depending on final layout as determined by the civil engineer. Geotechnical Engineering Evaluation NGA File No. 1308421 Billing Short Plat Infiltration November 19, 2021 Renton, Washington Summary - Page 2 NELSON GEOTECHNICAL ASSOCIATES, INC. In the attached report, we have also provided general recommendations for site grading, slabs-on- grade, and structural fill placement. 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, Khaled M. Shawish, PE Principal Engineer NELSON GEOTECHNICAL ASSOCIATES, INC. TABLE OF CONTENTS INTRODUCTION .......................................................................................................... 1 SCOPE ........................................................................................................................ 1 SITE CONDITIONS ....................................................................................................... 2 Surface Conditions ....................................................................................................... 2 Subsurface Conditions .................................................................................................. 2 Hydrogeologic Conditions ............................................................................................ 3 SENSITIVE AREA EVALUATION .................................................................................... 3 Seismic Hazard ............................................................................................................. 3 Erosion Hazard ............................................................................................................. 4 CONCLUSIONS AND RECOMMENDATIONS .................................................................. 4 General ......................................................................................................................... 4 Erosion Control ............................................................................................................. 5 Site Preparation and Grading ....................................................................................... 5 Temporary and Permanent Slopes............................................................................... 6 Foundations .................................................................................................................. 7 Structural Fill ................................................................................................................ 8 Slab-on-Grade .............................................................................................................. 8 Pavements .................................................................................................................... 9 Utilities ......................................................................................................................... 9 Site Drainage ................................................................................................................ 9 CONSTRUCTION MONITORING ................................................................................. 11 USE OF THIS REPORT ................................................................................................ 11 LIST OF FIGURES Figure 1 – Vicinity Map Figure 2 – Site Plan Figure 3 – Soil Classification Chart Figure 4 – Exploration Logs NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation Billing Short Plat Infiltration 14103 – 160th Avenue SE Renton, Washington INTRODUCTION This report presents the results of our geotechnical engineering investigation and evaluation of the Billing Short Plat project in Renton, Washington. The project site is located at 14103 – 160th Avenue SE in Renton, Washington, 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 site is currently occupied by a single-family residence in the east-central portion of the site. The property is bordered by 160th Avenue SE to the east, SE 7th Street to the north, and neighboring residential properties on both other sides. Topographically, the site is generally level. We understand the plans for development include a short plat of the property and construction of a residence on the resulting parcel. 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 to provide general recommendations for site development. Specifically, our scope of services included the following: 1. A review of available soil and geologic maps of the area. 2. Exploring the subsurface soil and groundwater conditions within the site and prepare the Small-Scale Pilot Infiltration Test (PIT) using trackhoe-excavated test pits. Excavation services were subcontracted by NGA. 3. Providing long-term design infiltration rates based on on-site Small PIT per the 2021 KCSWDM. 4. Providing recommendations for earthwork and foundation support. 5. Providing recommendations for temporary and permanent slopes. Geotechnical Engineering Evaluation NGA File No. 1308421 Billing Short Plat Infiltration November 19, 2021 Renton, Washington Page 2 NELSON GEOTECHNICAL ASSOCIATES, INC. 6. Providing recommendations for subsurface utilities and pavement subgrade preparation. 7. Providing our opinion on stormwater infiltration feasibility. 8. Providing recommendations for infiltration/bioretention system installation, if feasible. 9. Providing general recommendations for site drainage and erosion control. 10. Documenting the results of our findings, conclusions, and recommendations in a written geotechnical report. SITE CONDITIONS Surface Conditions The site consists of a rectangle shaped parcel covering approximately 0.86 acres. The property is currently occupied with an existing single-family residence within the eastern portion of the property. The ground surface within the property is relatively level with a gentle slope on the western portion. The site is generally vegetated with grass, landscaping shrubs and sparse trees. The site is bordered to the north, south, east and west by existing residential properties. We did not observe surface water throughout the site during our visit on October 20th, 2021. Subsurface Conditions Geology: The geologic units for this site are shown on Geologic Map of the Renton Quadrangle, King County, Washington, by Mullineaux, D.R. (USGS, 1965). The site is mapped as glacial till (Qgt). Till is generally described as a compact, non-sorted mixture of clay, silt, sand, pebbles, cobbles, and boulders. Our explorations typically encountered undocumented fill underlain by silty fine to medium sand with varying amounts of gravel, consistent with the description of the glacial till. Explorations: The subsurface conditions within the site were explored on October 20, 2021 by excavating four test pits to approximate depths of 5.5 feet below the existing ground surface using a 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, 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. Geotechnical Engineering Evaluation NGA File No. 1308421 Billing Short Plat Infiltration November 19, 2021 Renton, Washington Page 3 NELSON GEOTECHNICAL ASSOCIATES, INC. At the surface of each exploration, we generally encountered approximately a thin layer of topsoil followed by 2.5 to 3.0 feet of orange-brown to brown, silty, fine to medium sand with gravel, roots, organics, cobbles, and iron-oxide staining which we interpreted as undocumented fill. Underlying the topsoil and fill soils in each exploration, we encountered gray, silty, fine to medium sand with gravel and trace cobbles, which we interpreted as weathered, native glacial till deposits. All test pits were terminated at a depth of 5.5 feet within the glacial till material. Hydrogeologic Conditions We did not encounter ground water seepage within any of our explorations. However, if groundwater was encountered during construction, we would interpret it to be perched water. 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 till 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 1.364 0.51 1.000 1.500 0.909 0.51 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. Geotechnical Engineering Evaluation NGA File No. 1308421 Billing Short Plat Infiltration November 19, 2021 Renton, Washington Page 4 NELSON GEOTECHNICAL ASSOCIATES, INC. 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 glacial till 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 erosion hazard areas include soil type, slope gradient, vegetation cover, and groundwater conditions. The erosion sensitivity is related to the 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 site surface soils were classified using the NRCS classification system as Alderwood gravelly sandy loam, 8 to 15 percent slopes. These soils are listed as having a slight erosion hazard. It is our opinion that the erosion hazard should be low in areas with vegetated, undisturbed soil. CONCLUSIONS AND RECOMMENDATIONS General It is our opinion that the site planned development is feasible from a geotechnical standpoint. Our explorations indicated that the site was generally underlain by competent native, glacial, bearing soils at relatively shallow depths. The native glacial soils encountered at depth should provide adequate support for foundation, slab, and pavement loads. We recommend that the planned structures 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 glacial bearing soils, or structural fill extending to these soils. The medium dense or better native glacial bearing soils should be encountered approximately 3.0 – 3.5 feet below the existing ground 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. Based on the results of our infiltration testing and soil explorations throughout the site, it is our opinion that the onsite native glacial till soils are not suitable for traditional stormwater infiltration. However, shallow, low-impact design infiltration systems may be feasible at specific areas within the site depending on final layout as determined by the civil engineer. Any planned low-impact systems should include an incorporated overflow component directed to an approved point of discharge. This is further discussed in the Site Drainage section of this report. Geotechnical Engineering Evaluation NGA File No. 1308421 Billing Short Plat Infiltration November 19, 2021 Renton, Washington Page 5 NELSON GEOTECHNICAL ASSOCIATES, INC. Erosion Control The erosion hazard for the on-site soils is interpreted to be slight 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 wattles 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 till 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 2.5 to 3.5 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. Geotechnical Engineering Evaluation NGA File No. 1308421 Billing Short Plat Infiltration November 19, 2021 Renton, Washington Page 6 NELSON GEOTECHNICAL ASSOCIATES, INC. The site soils are considered to be moisture-sensitive and will disturb easily when wet. We recommend that construction take place during the drier summer months if possible. However, if construction takes place during the wet season, additional expenses and delays should be expected due to the wet conditions. Additional expenses could include the need for placing a blanket of rock spalls on exposed subgrades, construction traffic areas, and paved areas prior to placing structural fill. Wet weather grading will also require additional erosion control and site drainage measures. Some of the on-site soils may be suitable for use as structural fill, depending on the moisture content of the soil at the time of construction. NGA should be retained to evaluate the suitability of all on-site and imported structural fill material during construction. Temporary and Permanent Slopes Temporary cut slope stability is a function of many factors, including the type and consistency of soils, depth of the cut, surcharge loads adjacent to the excavation, length of time a cut remains open, and the presence of surface or groundwater. It is exceedingly difficult under these variable conditions to estimate a stable, temporary, cut slope angle. Therefore, it should be the responsibility of the contractor to maintain safe slope configurations at all times as indicated in OSHA guidelines for cut slopes. The following information is provided solely for the benefit of the owner and other design consultants and should not be construed to imply that Nelson Geotechnical Associates, Inc. assumes responsibility for job site safety. Job site safety is the sole responsibility of the project contractor. For planning purposes, we recommend that temporary cuts in the upper undocumented fill soils be no steeper than 2 Horizontal to 1 Vertical (2H:1V). Temporary cuts in the competent native glacial soils at depth should be no steeper than 1.5H:1V. If significant groundwater seepage or surface water flow were encountered, we would expect that flatter inclinations would be necessary. We recommend that cut slopes be protected from erosion. The slope protection measures may include covering cut slopes with plastic sheeting and diverting surface runoff away from the top of cut slopes. We do not recommend vertical slopes for cuts deeper than four feet if worker access is necessary. We recommend that cut slope heights and inclinations conform to appropriate OSHA/WISHA regulations. Permanent cut and fill slopes should be no steeper than 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. Geotechnical Engineering Evaluation NGA File No. 1308421 Billing Short Plat Infiltration November 19, 2021 Renton, Washington Page 7 NELSON GEOTECHNICAL ASSOCIATES, INC. Foundations Conventional shallow spread foundations should be placed on medium dense or better native bearing glacial till soils or be supported on structural fill or rock spalls extending to those soils. Medium dense or better bearing soils should be encountered approximately 3.0 – 3.5 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 footing may be extended down to the competent native soils. If footings are supported on structural fill, the fill zone should extend outside the edges of the footing a distance equal to one half of the depth of the 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 2,000 pounds per square foot (psf) be used for the design of footings founded on the medium dense or better native bearing soils or structural fill extending to the competent native bearing material. The foundation bearing soil should be evaluated by a representative of NGA. We should be consulted if higher bearing pressures are needed. Current IBC guidelines should be used when considering increased allowable bearing pressure for short-term transitory wind or seismic loads. Potential foundation settlement using the recommended allowable bearing pressure is estimated to be less than 1-inch total and ½-inch differential between adjacent footings or across a distance of about 20 feet, based on our experience with similar projects. 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. Geotechnical Engineering Evaluation NGA File No. 1308421 Billing Short Plat Infiltration November 19, 2021 Renton, Washington Page 8 NELSON GEOTECHNICAL ASSOCIATES, INC. 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 on-site soils may be suitable for use as structural fill, but this will be highly dependent on the moisture content of these soils at the time of construction. We should be retained to evaluate all proposed structural fill material prior to placement. Fill Placement: Following subgrade preparation, placement of structural fill may proceed. All filling should be accomplished in uniform lifts up to eight inches thick. Each lift should be spread evenly and be thoroughly compacted prior to placement of subsequent lifts. All structural fill underlying building areas and pavement subgrade should be compacted to a minimum of 95 percent of its maximum dry density. Maximum dry density, in this report, refers to that density as determined by the ASTM D-1557 Compaction Test procedure. The moisture content of the soils to be compacted should be within about two percent of optimum so that a readily compactable condition exists. It may be necessary to over- excavate and remove wet soils in cases where drying to a compactable condition is not feasible. All compaction should be accomplished by equipment of a type and size sufficient to attain the desired degree of compaction and should be tested. Slab-on-Grade Slab-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. Geotechnical Engineering Evaluation NGA File No. 1308421 Billing Short Plat Infiltration November 19, 2021 Renton, Washington Page 9 NELSON GEOTECHNICAL ASSOCIATES, INC. 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. Trenches located in non-structural areas should be compacted to a minimum 90 percent of the maximum dry density. The trench backfill compaction should be tested. Site Drainage Infiltration: We conducted one Small PIT within Infiltration Pit One located within the western portion of the site as shown on the attached Site Plan in Figure 2. The test was conducted in accordance with the 2017 City of Renton Surface Water Design Manual. The test was performed within Infiltration Pit One that measured 4.0-feet long by 3.0-feet wide by 5.5-feet deep. The pit was filled with 12-inches of water at the beginning of the day and maintained this level for the soaking period for approximately 6 hours. At this time, the water flow rate into the hole 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. The flow rate for Infiltration Pit One stabilized at 0.052 gallons per minute (3.12 gallons per hour). This equated to an approximate infiltration rate of 0.414 inches per hour. The water was shut off after the steady-state period and monitored at least every 15 minutes for one hour. After 60 minutes, the water level within the pit dropped approximately 0.75 inches, resulting in a measured infiltration rate of 0.75 inches per hour. Geotechnical Engineering Evaluation NGA File No. 1308421 Billing Short Plat Infiltration November 19, 2021 Renton, Washington Page 10 NELSON GEOTECHNICAL ASSOCIATES, INC. In accordance with the Table V-5.1 of the 2016 King County Surface Water Drainage Manual (2016 KCSWDM), correction factors of 0.9, 0.5, and 0.9 for site variability and number of locations tested (CFv), testing method (CFt), and degree of influent control to prevent siltation and bio-buildup (CFm), respectively were applied to the field measured minimum infiltration rate of 0.414 inches per hour, calculated from the steady-state period. A total correction factor of 0.405 was applied to the measured field infiltration rate obtained from the falling head portion of the test to determine the long-term design infiltration rate. Using this correction factor, we calculated a long-term design infiltration rate of 0.17 inches per hour. This does not meet the minimum long-term design infiltration rate of 0.30 inches per hour provided by the 2017 City of Renton Surface Water Design Manual. Based on the results of our infiltration testing and the dense silty nature of the onsite native glacial till soils at depth, it is our opinion that the native glacial till soils at depth within the site are not conducive to traditional stormwater infiltration systems. However, a low-impact stormwater infiltration system may be feasible, depending on final site layout. The project civil engineer should determine possible methods of low- impact stormwater infiltration in conjunction with our calculated long-term design infiltration rate and the 2016 King County Surface Water Drainage Manual utilized by the City of Renton. We did not observe any groundwater during our time on site. It is our opinion that proposed infiltration systems should be able to maintain a minimum separation from the base of the infiltration systems to any impermeable surfaces and/or groundwater table. We recommend that the base of the on-site infiltration systems be terminated in the native deposits. We also recommend that all on-site limited infiltration systems include an overflow component directed into an approved point of discharge, likely within an adjacent street. The stormwater management systems within this site should be sized and designed in accordance with the 2017 City of Renton Surface Water Design Manual. We recommend that any proposed stormwater management systems be located as to not negatively impact any proposed or existing nearby structures and/or properties and also meet all required setbacks from existing property lines, structures, and sensitive areas in accordance with the stormwater manual. 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 structures. 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 structures. Surface water should be collected by permanent catch basins and drain lines and be discharged into an approved discharge system. Geotechnical Engineering Evaluation NGA File No. 1308421 Billing Short Plat Infiltration November 19, 2021 Renton, Washington Page 11 NELSON GEOTECHNICAL ASSOCIATES, INC. 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. Pea gravel is an acceptable drain material. The free-draining material should extend 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 Binnu Billing and associated 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. Geotechnical Engineering Evaluation NGA File No. 1308421 Billing Short Plat Infiltration November 19, 2021 Renton, Washington Page 12 NELSON GEOTECHNICAL ASSOCIATES, INC. 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. 1308421 Billing Short Plat Infiltration November 19, 2021 Renton, Washington Page 13 NELSON GEOTECHNICAL ASSOCIATES, INC. It has been a pleasure to provide service to you on this project. If you have any questions or require further information, please call. Sincerely, NELSON GEOTECHNICAL ASSOCIATES, INC. Jonathan D. Klevan, PE Staff Engineer II Khaled M. Shawish, PE Principal Engineer JDK:CTC:KMS:dy Four Figures Attached 11.19.2021 Not to Scale VICINITY MAP Billing Short Plat Infiltration Vicinity Map Project Site 1 No.Project Number Date By CKRevision Woodinville Office 17311-135th Ave. NE, A-500 Woodinville, WA 98072 (425) 486-1669 / Fax: 481-2510 Wenatchee Office 105 Palouse St. Wenatchee, WA 98801 (509) 665-7696 / Fax: 665-7692www.nelsongeotech.com \\HILL\company\2021 NGA Project Folders\13084-21 Billing SP INF Renton\Drafting\VM.dwg11/2/21 DPN DJOOriginal Figure 1 1308421 Renton, WA Reference: Site plan based on a plan dated July 29, 2021 titled "Topographic Survey Badanhvir Billing," prepared by Site Surveying, Inc.1No.Project NumberDateByCKRevisionWoodinville Office17311-135th Ave. NE, A-500Woodinville, WA 98072(425) 486-1669 / Fax: 481-2510Wenatchee Office105 Palouse St.Wenatchee, WA 98801(509) 665-7696 / Fax: 665-7692www.nelsongeotech.com\\HILL\company\2021 NGA Project Folders\13084-21 Billing SP INF Renton\Drafting\SP.dwgFigure 2130842111/2/21DPNDJOOriginal Billing Short Plat InfiltrationSite PlanSite Plan LEGEND INF-1 Number and approximate location of infiltration test pit Property line TP-1 Number and approximate location of test pit 160th Ave SESE 7th St Scale: 1 inch = 40 feet 0 40 80 INF-1 TP-1 TP-3 TP-2 Existing Residence GW GP GM GC SW SP SM SC ML CL OL MH CH OH PT PEAT ORGANIC CLAY, ORGANIC SILT CLAY OF HIGH PLASTICITY, FAT CLAY SILT OF HIGH PLASTICITY, ELASTIC SILT SILTY SAND SILT ORGANIC SILT, ORGANIC CLAY CLAY CLAYEY SAND POORLY GRADED SAND WELL-GRADED SAND, FINE TO COARSE SAND CLAYEY GRAVEL SILTY GRAVEL POORLY-GRADED GRAVEL WELL-GRADED, FINE TO COARSE GRAVELCLEAN GRAVEL GRAVEL WITH FINES CLEAN SAND SAND WITH FINES INORGANIC ORGANIC INORGANIC ORGANIC HIGHLY ORGANIC SOILS GRAVEL SAND SILT AND CLAY SILT AND CLAY MORE THAN 50 % OF COARSE FRACTION RETAINED ON NO. 4 SIEVE PASSES NO. 4 SIEVE LIQUID LIMIT LESS THAN 50 % 50 % OR MORE LIQUID LIMIT MORE THAN 50 % OF COARSE FRACTION COARSE - GRAINED SOILS FINE - GRAINED SOILS MORE THAN 50 % RETAINED ON NO. 200 SIEVE PASSES NO. 200 SIEVE MORE THAN 50 % MAJOR DIVISIONS GROUP SYMBOL GROUP NAME UNIFIED SOIL CLASSIFICATION SYSTEM NOTES: 1) Field classification is based on visual examination of soil in general accordance with ASTM D 2488-93. 2) Soil classification using laboratory tests is based on ASTM D 2488-93. 3) Descriptions of soil density or consistency are based on interpretation of blowcount data, visual appearance of soils, and/or test data. SOIL MOISTURE MODIFIERS: Dry - Absence of moisture, dusty, dry to the touch Moist - Damp, but no visible water. Wet - Visible free water or saturated, usually soil is obtained from below water table 1 No.Project Number Date By CKRevision Woodinville Office 17311-135th Ave. NE, A-500 Woodinville, WA 98072 (425) 486-1669 / Fax: 481-2510 Wenatchee Office 105 Palouse St. Wenatchee, WA 98801 (509) 665-7696 / Fax: 665-7692www.nelsongeotech.com \\HILL\company\2021 NGA Project Folders\13084-21 Billing SP INF Renton\Drafting\SC.dwgFigure 3 1308421 11/2/21 DPN DJOOriginal Billing Short Plat Infiltration Soil Classification Chart LOG OF EXPLORATION DEPTH (FEET) USCS SOIL DESCRIPTION DPN:DJO NELSON GEOTECHNICAL ASSOCIATES, INC. FILE NO 1308421 FIGURE 4 INFILTRATION TEST PIT ONE 0.0 – 0.5 TOPSOIL 0.5 – 3.5 BROWN TO GRAY-BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, ROOTS, ORGANICS, COBBLES, AND IRON-OXIDE STAINING (LOOSE TO MEDIUM DENSE, MOIST) (FILL) 3.5 – 5.5 SM GRAY, SILTY FINE TO MEDIUM SAND WITH GRAVEL AND TRACE 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 5.5 FEET ON 10/20/2021 TEST PIT ONE 0.0 – 0.3 TOPSOIL 0.3 – 3.0 ORANGE-BROWN TO BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, ROOTS, ORGANICS, COBBLES, AND IRON-OXIDE STAINING (LOOSE TO MEDIUM DENSE, MOIST) (FILL) 3.0 – 5.5 SM GRAY, SILTY FINE TO MEDIUM SAND WITH GRAVEL AND TRACE 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 5.5 FEET ON 10/20/2021 TEST PIT TWO 0.0 – 0.3 TOPSOIL 0.3 – 3.0 ORANGE-BROWN TO BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, ROOTS, ORGANICS, COBBLES, AND IRON-OXIDE STAINING (LOOSE TO MEDIUM DENSE, MOIST) (FILL) 3.0 – 5.5 SM GRAY, SILTY FINE TO MEDIUM SAND WITH GRAVEL AND TRACE 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 5.5 FEET ON 10/20/2021 TEST PIT THREE 0.0 – 0.3 TOPSOIL 0.3 – 3.0 ORANGE-BROWN TO BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, ROOTS, GLASS, ORGANICS, AND COBBLES (LOOSE TO MEDIUM DENSE, MOIST) (FILL) 3.0 – 5.5 SM GRAY, SILTY FINE TO MEDIUM SAND WITH GRAVEL AND TRACE 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 5.5 FEET ON 10/20/2021