Loading...
HomeMy WebLinkAbout26 - Aberdeen Ave SP_GEOTECHNICAL REPORT 971516 Kwan Short Plat Infiltration October 17, 2016 Mr. Curran Kwan Prime Metropolis Properties, Inc. 950 Taraval Street San Francisco, California 94116 Geotechnical Engineering Evaluation Aberdeen Avenue Short Plat and Infiltration 1824 Aberdeen Avenue Renton, Washington NGA Job No. 971516 Dear Mr. Kwan: We are pleased to submit the attached report titled “Geotechnical Engineering Evaluation – Aberdeen Avenue Short Plat and Infiltration – 1824 Aberdeen Avenue – 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 20, 2016. The site is generally situated on level to gently sloping ground that descends from the east to the west. The site is currently occupied by an existing single-family residence and attached carport within the western portion of the property, a detached garage, and a detached shop building in the south-central and eastern portions of the property, respectively. We understand that the proposed development includes removing the existing site structures and subdividing the property to create new western and eastern parcels. We also understand that a single-family residence and associated pavement and underground utilities will be constructed within each new parcel. Specific grading and stormwater plans were not available at the time this letter was prepared. However, we understand that stormwater is likely to be directed into on-site infiltration systems, if feasible. The City of Renton uses the 2009 King County Surface Water Design Manual to determine the design infiltration rate. On-site infiltration testing is required to determine the suitability of stormwater infiltration. For our use in preparing this report, we have been provided with a preliminary plat map titled “Aberdeen Ave. Subdivision,” dated March 24, 2016 and prepared by CG Engineering, Inc., showing the existing and proposed site conditions along with proposed infiltration test locations. We monitored the excavation of four test pits and two infiltration test pits at the site on October 4, 2016. Our explorations indicated that the site is generally underlain by competent native glacial soils, with localized undocumented fill soils encountered up to 4.0 feet in depth. We have concluded that the site is generally compatible with the planned development. We have recommended that any undocumented fill soils be over-excavated and that the new structures be founded on the underlying medium dense or better Geotechnical Engineering Evaluation NGA File No. 971516 Aberdeen Avenue Short Plat and Infiltration October 17, 2016 Renton, Washington Summary - Page 2 _____________________________________________________________________________________________ native soils or structural fill extending to these soils for bearing capacity and settlement considerations. These soils should generally be encountered approximately one to four feet below the existing ground surface, based on our explorations. However, deeper areas of loose soil and/or undocumented fill could also exist within unexplored areas of the site. We also performed on-site infiltration testing based on the 2009 King County Surface Water Design Manual. Based on our field testing, the native granular glacial soils encountered at depth within our explorations within the property are suitable for on-site stormwater infiltration. In the attached report, we have provided recommendations for infiltration system design and installation. We have also included recommendations for general site grading, foundation and slab support, and drainage. 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 TABLE OF CONTENTS NELSON GEOTECHNICAL ASSOCIATES, INC. INTRODUCTION .......................................................................................................................... 1 SCOPE ............................................................................................................................................ 1 SITE CONDITIONS ...................................................................................................................... 2 Surface Conditions.................................................................................................................... 2 Subsurface Conditions .............................................................................................................. 2 Hydrologic 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 Retaining Walls ........................................................................................................................ 8 Structural Fill ............................................................................................................................ 9 Slab-on-Grade ......................................................................................................................... 10 Pavements ............................................................................................................................... 10 Utilities ................................................................................................................................... 11 Site Drainage .......................................................................................................................... 11 CONSTRUCTION MONITORING .......................................................................................... 13 USE OF THIS REPORT ............................................................................................................. 13 LIST OF FIGURES Figure 1 – Vicinity Map Figure 2 – Site Plan Figure 3 – Soil Classification Chart Figures 4 and 5 – Test Pit Logs NELSON GEOTECHNICAL ASSOCIATES, INC. Geotechnical Engineering Evaluation Aberdeen Avenue Short Plat and Infiltration 1824 Aberdeen Avenue Renton, Washington INTRODUCTION This report presents the results of our geotechnical engineering investigation and evaluation of the planned Aberdeen Avenue Residential Development project in Renton, Washington. The project site is located at 1824 Aberdeen Avenue 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. For our use in preparing this report, we have been provided with a preliminary plat map titled “Aberdeen Ave. Subdivision,” dated March 24, 2016 and prepared by CG Engineering, Inc. The site is relatively level to gently sloping from the east to the west and is currently occupied by a single-family residence structure and attached carport, as well as, two detached shop buildings in the south-central and eastern portions of the site. The proposed development plan consists of demolishing the existing structures and subdividing the current property into two, western and eastern parcels. We understand that stormwater is likely to be directed into on-site infiltration systems, if feasible. The existing site layout and the approximate locations of our explorations are 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. Based on our understanding of the planned development and site conditions, the services to be provided by NGA are to: 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. Provide recommendations for earthwork, foundation support, and slabs-on-grade. 4. Provide recommendations for temporary and permanent slopes. 5. Provide recommendations for pavement subgrade. 6. Provide recommendations for site drainage and erosion control. 7. Perform on-site infiltration testing as required. Geotechnical Engineering Evaluation NGA File No. 971516 Aberdeen Avenue Short Plat and Infiltration October 17, 2016 Renton, Washington Page 2 _____________________________________________________________________________________________ NELSON GEOTECHNICAL ASSOCIATES, INC. 8. Perform grain-size sieve analysis on soil samples, if necessary. 9. Provide estimates of the infiltration capacity of the soils based on the 2009 King County Surface Water Design Manual. 10. Provide recommendations for infiltration system installation. 11. Document the results of our findings, conclusions, and recommendations in a written geotechnical report. SITE CONDITIONS Surface Conditions The site consists of a rectangular-shaped parcel covering approximately 0.37-acres. The site is bounded to the north, south, and east by existing residential properties and to the west by Aberdeen Avenue. An existing single-family residential structure and attached carport are located in the western portion of the property. There are two detached garage buildings within the south-central and eastern portions of the site. The site topography is relatively level to gently sloping from the east to the west. Vegetation within the site generally consists of grass, underbrush, and scattered trees. We did not observe surface water on the site during our visit on October 5, 2016. Subsurface Conditions Geology: The geologic units for this site are shown on Geologic map of surficial deposits in the Seattle 30' by 60' quadrangle, Washington by Yount, J.C., Minard, J.P., and Dembroff, G.R., (USGS, 1993). The site is mapped as glacial recessional outwash (Qvr). The recessional outwash deposits are described as a poorly to moderately sorted, stratified mixture of sands and gravels with minor amounts of silt. Our explorations within the property generally encountered fine to medium sand with silt consistent with the description of recessional outwash deposits at depth. Explorations: The subsurface conditions within the site were explored on October 4, 2016 by excavating four test pits and two infiltration test pits to depths ranging from 3.0 to 10.0 feet below the existing ground surface using a mini-trackhoe. The approximate locations of our explorations are shown on the Schematic 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. Geotechnical Engineering Evaluation NGA File No. 971516 Aberdeen Avenue Short Plat and Infiltration October 17, 2016 Renton, Washington Page 3 _____________________________________________________________________________________________ NELSON GEOTECHNICAL ASSOCIATES, INC. 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 Figures 4 and 5. We present a brief summary of the subsurface conditions in the following paragraphs. For a detailed description of the subsurface conditions, the logs of the test pits should be reviewed. Within Test Pit 2 through 3 and Infiltration Pit 1 through 2 we encountered 0.5 to 1.5 of grass and surficial topsoil. Underlying the topsoil we generally encountered medium dense or better, light brown to gray brown, fine to medium sand with gravel, and minor amounts of silt, which we interpreted as native recessional outwash deposits. These explorations terminated within the native recessional outwash deposits at depths in the range of 8.0 to 9.5 feet below the existing ground surface. Within Test Pit 1 and Test Pit 4 we encountered 0.6 to 1.5 feet of grass and surficial topsoil underlain by 2.2 to 3.4 feet of loose to medium dense, fine to medium sand with silt, intermixed with concrete rubble that we interpreted as undocumented fill soils. Underlying the fill in Test Pit 1, we encountered medium dense to dense, light-brown to gray-brown, fine to medium sand with silt and trace gravel that we interpreted as native recessional outwash deposits. Test Pit 4 was terminated at a concrete drain pipe within undocumented fill soils at 3.0 feet below the existing ground surface, while Test Pit 1 was terminated in native recessional outwash soils at approximately 10.0 feet below the existing ground surface. Hydrologic Conditions Groundwater seepage was not encountered in our explorations. If groundwater seepage occurs on this site, we would interpret the water to be perched water. Perched water occurs when surface water infiltrates through less dense, more permeable soils and accumulates on top of a relatively low permeability material. Perched water does not represent a regional groundwater "table" within the upper soil horizons. Perched water tends to vary spatially and is dependent upon the amount of rainfall. We would expect the amount of perched groundwater to decrease during drier times of the year and increase during wetter periods. SENSITIVE AREA EVALUATION Seismic Hazard We reviewed the 2015 International Building Code (IBC) for seismic site classification for this project. Since medium dense or better glacial soils were generally encountered underlying the site at depth, the site conditions best fit the IBC description for Site Class D. Geotechnical Engineering Evaluation NGA File No. 971516 Aberdeen Avenue Short Plat and Infiltration October 17, 2016 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 competent native glacial soils interpreted to underlie the site have a low potential for liquefaction or amplification of ground motion. Erosion Hazard The criteria used for determination of the erosion hazard for affected areas include soil type, slope gradient, vegetation cover, and groundwater conditions. The erosion sensitivity is related to vegetative cover and the specific surface soil types, which are related to the underlying geologic soil units. The Soil Survey of King County Area, Washington, by the Soil Conservation Service (SCS) lists the soils within the project area as Indianola Loamy Sand, 5 to 15 percent slopes. The erosion hazard for these soils is listed as slight. It is our opinion that the site would have a slight erosion hazard for areas where the soils are exposed. It is also our opinion that the erosion hazard for site soils should be low in areas where vegetation is not disturbed. CONCLUSIONS AND RECOMMENDATIONS General It is our opinion from a geotechnical standpoint that the site is generally compatible with the planned development. Our explorations indicated that the site is generally underlain by competent native glacial soils at depth. However, our explorations within the central and eastern portion of the property encountered approximately 3.0 to 4.0 feet of unsuitable undocumented fill soils. 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 soil, or structural fill extending to these soils. The native medium dense or better soils should typically be encountered approximately one to four feet below the existing surface, based on our explorations. We should note that localized areas of deeper unsuitable soils and/or undocumented fill could be encountered at this site. This condition would require additional excavations in foundation, slab, and pavement areas to remove the unsuitable soils. It is our opinion that the native granular glacial soils encountered within the property are suitable for on- site infiltration systems. The subsurface soils encountered at depth are generally fine to medium sands with varying amounts of silt. We also did not observe groundwater in our explorations to the depths explored. We recommend on-site infiltration systems be founded in native recessional outwash deposits. Geotechnical Engineering Evaluation NGA File No. 971516 Aberdeen Avenue Short Plat and Infiltration October 17, 2016 Renton, Washington Page 5 _____________________________________________________________________________________________ NELSON GEOTECHNICAL ASSOCIATES, INC. The storm water management system should be designed in accordance with 2009 King County Surface Water Design Manual. This is further discussed in the Stormwater Infiltration subsection of this report. The surficial soils encountered on this site are considered moisture-sensitive and may 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. This will depend on the moisture content of the soils at the time of construction. NGA should be retained to determine if the on-site soils can be used as structural fill material during construction. 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 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, loose soils and/or undocumented fill from foundation, slab, pavement areas, and other structural areas, to expose medium dense or better native soils. The stripped soil should be removed from the site or stockpiled for later use as a landscaping fill. Based on our observations, we anticipate stripping depths of approximately one to four feet across the site. It should be noted that additional stripping may be required if areas of undocumented fill and/or loose soil are encountered in unexplored areas of the site. Specific plans for protecting existing structures, driveway, and utilities should be devised during the planning phase of the project and should be made clear to contractors during the bidding phase. Geotechnical Engineering Evaluation NGA File No. 971516 Aberdeen Avenue Short Plat and Infiltration October 17, 2016 Renton, Washington Page 6 _____________________________________________________________________________________________ NELSON GEOTECHNICAL ASSOCIATES, INC. After site stripping, if the exposed subgrade is loose, it should be compacted to a non-yielding condition and then proof-rolled with a heavy rubber-tired piece of equipment. Areas observed to pump or weave during the proof-roll test should be reworked to structural fill specifications or over-excavated and replaced with properly compacted structural fill or rock spalls. If loose soils are encountered in the pavement areas, the loose soils should be removed and replaced with rock spalls or granular structural fill. If significant surface water flow is encountered during construction, this flow should be diverted around areas to be developed, and the exposed subgrades should be maintained in a semi-dry condition. If wet conditions are encountered, alternative site stripping and grading techniques might be necessary. These could include using large excavators equipped with wide tracks and a smooth bucket to complete site grading and covering exposed subgrade with a layer of crushed rock for protection. If wet conditions are encountered or construction is attempted in wet weather, the subgrade should not be compacted as this could cause further subgrade disturbance. In wet conditions it may be necessary to cover the exposed subgrade with a layer of crushed rock as soon as it is exposed to protect the moisture sensitive soils from disturbance by machine or foot traffic during construction. The prepared subgrade should be protected from construction traffic and surface water should be diverted around areas of prepared subgrade. The site soils are considered to be moisture-sensitive and may disturb 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. Geotechnical Engineering Evaluation NGA File No. 971516 Aberdeen Avenue Short Plat and Infiltration October 17, 2016 Renton, Washington Page 7 _____________________________________________________________________________________________ NELSON GEOTECHNICAL ASSOCIATES, INC. 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. Temporary shoring may be needed where these recommended inclinations cannot be met due to existing structures or property boundaries. 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 soils, or be supported on structural fill or rock spalls extending to those soils. Medium dense soils should be encountered approximately one to four 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 2015 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. Geotechnical Engineering Evaluation NGA File No. 971516 Aberdeen Avenue Short Plat and Infiltration October 17, 2016 Renton, Washington Page 8 _____________________________________________________________________________________________ NELSON GEOTECHNICAL ASSOCIATES, INC. For foundations constructed as outlined above, we recommend an allowable design bearing pressure of not more than 2,000 pounds per square foot (psf) be used for the design of footings founded on the medium dense or better native soils or structural fill extending to the competent native 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. Retaining Walls Specific grading plans for this project were not available at the time this report was prepared, but retaining walls may be incorporated into project plans. In general, the lateral pressure acting on subsurface retaining walls is dependent on the nature and density of the soil behind the wall, the amount of lateral wall movement which can occur as backfill is placed, wall drainage conditions, and the inclination of the backfill. For walls that are free to yield at the top at least one thousandth of the height of the wall (active condition), soil pressures will be less than if movement is limited by such factors as wall stiffness or bracing (at-rest condition). We recommend that walls supporting horizontal backfill and not subjected to hydrostatic forces, be designed using a triangular earth pressure distribution equivalent to that exerted by a fluid with a density of 35 pcf for yielding (active condition) walls, and 55 pcf for non- yielding (at-rest condition) walls. Geotechnical Engineering Evaluation NGA File No. 971516 Aberdeen Avenue Short Plat and Infiltration October 17, 2016 Renton, Washington Page 9 _____________________________________________________________________________________________ NELSON GEOTECHNICAL ASSOCIATES, INC. 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 subsurface height of the wall, and do not account for surcharge loads. Additional lateral earth pressures should be considered for surcharge loads acting adjacent to subsurface walls and within a distance equal to the subsurface height of the wall. This would include the effects of surcharges such as traffic loads, floor slab loads, slopes, or other surface loads. We could consult with the structural engineer regarding additional loads on retaining walls during final design, if needed. The lateral pressures on walls may be resisted by friction between the foundation and subgrade soil, and by passive resistance acting on the below-grade portion of the foundation. Recommendations for frictional and passive resistance to lateral loads are presented in the Foundations subsection of this report. All wall backfill should be well compacted as outlined in the Structural Fill subsection of this report. Care should be taken to prevent the buildup of excess lateral soil pressures due to over-compaction of the wall backfill. This can be accomplished by placing wall backfill in 8-inch loose lifts and compacting the backfill with small, hand-operated compactors within a distance behind the wall equal to at least 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. Geotechnical Engineering Evaluation NGA File No. 971516 Aberdeen Avenue Short Plat and Infiltration October 17, 2016 Renton, Washington Page 10 _____________________________________________________________________________________________ NELSON GEOTECHNICAL ASSOCIATES, INC. Materials: Structural fill should consist of a good quality, granular soil, free of organics and other deleterious material, and be well graded to a maximum size of about three inches. All-weather fill should contain no more than five-percent fines (soil finer than U.S. No. 200 sieve, based on that fraction passing the U.S. 3/4-inch sieve). Some of the more granular on-site soils may be suitable for use as structural fill depending on the moisture content of the soil during construction. We should be retained to evaluate all proposed structural fill material prior to placement. Fill Placement: Following subgrade preparation, placement of structural fill may proceed. All filling should be accomplished in uniform lifts up to eight inches thick. Each lift should be spread evenly and be thoroughly compacted prior to placement of subsequent lifts. All structural fill underlying building areas and pavement subgrade should be compacted to a minimum of 95 percent of its maximum dry density. Maximum dry density, in this report, refers to that density as determined by the ASTM D-1557 Compaction Test procedure. The moisture content of the soils to be compacted should be within about two percent of optimum so that a readily compactable condition exists. It may be necessary to over- excavate and remove wet soils in cases where drying to a compactable condition is not feasible. All compaction should be accomplished by equipment of a type and size sufficient to attain the desired degree of compaction. Slab-on-Grade Slabs-on-grade should be supported on subgrade soils prepared as described in the Site Preparation and Grading subsection of this report. We recommend that all floor slabs be underlain by at least six inches of free-draining gravel with less than three percent by weight of the material passing Sieve #200 for use as a capillary break. We recommend that the capillary break be hydraulically connected to the footing drain system to allow free drainage from under the slab. A suitable vapor barrier, such as heavy plastic sheeting (6-mil minimum), should be placed over the capillary break material. An additional 2-inch-thick moist sand layer may be used to cover the vapor barrier. This sand layer 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 minimum of six Geotechnical Engineering Evaluation NGA File No. 971516 Aberdeen Avenue Short Plat and Infiltration October 17, 2016 Renton, Washington Page 11 _____________________________________________________________________________________________ NELSON GEOTECHNICAL ASSOCIATES, INC. inches of clean granular pit run or crushed rock. We should be retained to observe the proof-rolling and recommend repairs prior to placement of the asphalt or hard surfaces. Utilities We recommend that underground utilities be bedded with a minimum 12 inches of pea gravel prior to backfilling the trench with on-site or imported material. Trenches within settlement sensitive areas should be compacted to 95% of the modified proctor as described in the Structural Fill subsection of this report. Trenches located in non-structural areas should be compacted to a minimum 90% of the maximum dry density. Site Drainage Infiltration: It is our opinion that the subsurface soils within the site are suitable for typical stormwater infiltration. We recommend that any infiltration systems within this property be designed utilizing the recommended long-term design infiltration rate provided below. We conducted two on-site infiltration tests within the central and eastern portion of the property based on the 2009 King County Surface Water Design Manual to determine a long-term design infiltration rate for the on-site infiltration systems. The test locations are shown as Inf-1 and Inf-2 on the Site Plan in Figure 2. Infiltration Pits 1 and 2 were performed at respective depths of approximately 2.0 feet below the existing ground surface. We installed a six-inch diameter vertical tube into the native soil at the test locations within Infiltration Pit 1 and 2. Approximately two inches of pea gravel was placed over the native soil. We made repeated measurements of the time for the infiltration of six inches of water into the native sand soil. The average infiltration rate from each data set was used for the measured infiltration rate. The measured infiltration rates obtained in the field were 116.03 inches per hour for Infiltration Pit 1 and 61.31 inches per hour for Infiltration Pit 2. We have not applied a factor of safety to the in-place infiltration test results. We referenced Equation 5-11 within Chapter 5.4.1 of the 2009 King County Surface Water Design Manual that applies adequate correction factors to the field measured infiltration rate to generate a long- term design infiltration rate. Correction factors of 0.30, 0.50, and 0.80 were utilized in this equation for Ftesting, Fgeometry, Fplugging, respectively. Using these correction factors along with the lower of the two field measured infiltration rates, we calculated a long-term design infiltration rate of 7.35 inches per hour. We recommend that the on-site infiltration systems exposing the competent granular glacial outwash soils be designed utilizing the 7.35 inch per hour rate. We should be retained during Geotechnical Engineering Evaluation NGA File No. 971516 Aberdeen Avenue Short Plat and Infiltration October 17, 2016 Renton, Washington Page 12 _____________________________________________________________________________________________ NELSON GEOTECHNICAL ASSOCIATES, INC. construction to evaluate the soils exposed in the infiltration systems to verify that the soils are appropriate for infiltration. We recommend that the infiltration trenches extend through any organic soil or fill to expose undisturbed native granular outwash soils. We should be retained to verify the existence of such material at the time of construction. The storm water management systems should be designed in accordance with the King County Stormwater Design Manual. We recommend that any proposed infiltration systems be located 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 in accordance with the City of Renton code. 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 any groundwater in our explorations to the depths explored. Surface Drainage: The finished ground surface should be graded such that stormwater is directed to an appropriate stormwater collection system. Water should not be allowed to stand in any areas where footings, slabs, or pavements are to be constructed. Final site grades should allow for drainage away from the residences. We suggest that the finished ground be sloped at a minimum 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 appropriate discharge system. The overflow water should be dispersed to discharge into an appropriate location. 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. 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 Geotechnical Engineering Evaluation NGA File No. 971516 Aberdeen Avenue Short Plat and Infiltration October 17, 2016 Renton, Washington Page 13 _____________________________________________________________________________________________ NELSON GEOTECHNICAL ASSOCIATES, INC. tightlines leading to an appropriate 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 Curran Kwan with Prime Metropolis Properties, Inc. and his agents, for use in the planning and design of the development on this site only. The scope of our work does not include services related to construction safety precautions and our recommendations are not intended to direct the contractors’ methods, techniques, sequences, or procedures, except as specifically described in our report for consideration in design. There are possible variations in subsurface conditions between the explorations and also with time. Our report, conclusions, and interpretations should not be construed as a warranty of subsurface conditions. A contingency for unanticipated conditions should be included in the budget and schedule. We recommend that NGA be retained to provide monitoring and consultation services during construction to confirm that the conditions encountered are consistent with those indicated by the explorations, to provide recommendations for design changes should the conditions revealed during the work differ from those anticipated, and to evaluate whether or not earthwork and foundation installation activities comply with contract plans and specifications. We should be contacted a minimum of one week prior to construction activities and could attend pre-construction meetings if requested. Within the limitations of scope, schedule, and budget, our services have been performed in accordance with generally accepted geotechnical engineering practices in effect in this area at the time this report was prepared. No other warranty, expressed or implied, is made. Our observations, findings, and opinions are a means to identify and reduce the inherent risks to the owner. o-o-o Geotechnical Engineering Evaluation NGA File No. 971516 Aberdeen Avenue Short Plat and Infiltration October 17, 2016 Renton, Washington Page 14 _____________________________________________________________________________________________ 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. Alex B. Rinaldi, GIT Staff Geologist Lee S. Bellah, LG Project Geologist Khaled M. Shawish Principal LSB:KMS:dy Five Figures Attached LOG OF EXPLORATION DEPTH (FEET) USC SOIL DESCRIPTION ABR:LSB NELSON GEOTECHNICAL ASSOCIATES, INC. FILE NO 971516 FIGURE 4 TEST PIT ONE 0.0 – 0.2 GRASS AND ROOTS 0.2 – 0.6 DARK BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL AND ROOTS (LOOSE TO MEDIUM DENSE, MOIST) (TOPSOIL) 0.6 – 4.0 4.0 – 10.0 SP-SM LIGHT BROWN, FINE TO MEDIUM SAND WITH SILT, CONCRETE RUBBLE, TRACE ROOTS, AND GRAVEL (LOOSE TO MEDIUM DENSE, MOIST) (FILL) LIGHT BROWN TO GRAY BROWN, FINE TO MEDIUM SAND WITH SILT AND TRACE GRAVEL (MEDIUM DENSE TO DENSE, MOIST) SAMPLES WERE COLLECTED AT 3.0, 6.0, 9.0, 10.0 FEET GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED SLIGHT TEST PIT CAVING WAS ENCOUNTERED TEST PIT WAS COMPLETED AT 10.0 FEET ON 10/4/16 TEST PIT TWO 0.0 – 0.2 GRASS AND ROOTS 0.2 – 0.5 DARK BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL AND ROOTS/ORGANICS (LOOSE TO MEDIUM DENSE, MOIST) (TOPSOIL) 0.5 – 6.0 6.0 – 9.0 SP-SM SP-SM LIGHT BROWN, FINE TO MEDIUM SAND WITH SILT, TRACE GRAVEL AND ROOTS (MEDIUM DENSE TO DENSE, MOIST) BROWN-GRAY, FINE TO COARSE SAND WITH SILT, GRAVEL, AND TRACE ROOTS (MEDIUM DENSE TO DENSE, MOIST) SAMPLES WERE COLLECTED AT 2.0, 6.0, AND 9.0 FEET GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED SLIGHT TO MODERATE TEST PIT CAVING WAS ENCOUNTERED FROM 6.0 TO 9.0 FEET TEST PIT WAS COMPLETED AT 9.0 FEET ON 10/4/16 TEST PIT THREE 0.0 – 0.2 GRASS AND ROOTS 0.2 – 0.9 DARK BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, IRON-OXIDE STAINING, AND ROOTS (LOOSE TO MEDIUM DENSE, MOIST) (TOPSOIL) 0.9 – 8.0 8.0 – 9.5 SP-SM SP-SM LIGHT BROWN, FINE TO MEDIUM SAND WITH SILT, TRACE IRON-OXIDE WEATHERING, ROOTS AND GRAVEL (MEDIUM DENSE TO DENSE, MOIST) GRAY-BROWN, FINE TO COARSE SAND WITH SILT, TRACE IRON-OXIDE WEATHERING, ROOTS AND GRAVEL (MEDIUM DENSE TO DENSE, MOIST) SAMPLES WERE COLLECTED AT 2.5, 6.5, AND 9.0 FEET GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED SLIGHT TEST PIT CAVING WAS ENCOUNTERED FROM 0.2 TO 9.5 FEET TEST PIT COMPLETED AT 9.5 FEET ON 10/4/16 LOG OF EXPLORATION DEPTH (FEET) USC SOIL DESCRIPTION ABR:LSB NELSON GEOTECHNICAL ASSOCIATES, INC. FILE NO 971516 FIGURE 5 TEST PIT FOUR 0.0 – 0.2 GRASS AND ROOTS 0.2 – 0.8 DARK BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, IRON-OXIDE STAINING, AND ROOTS (LOOSE TO MEDIUM DENSE, MOIST) (TOPSOIL) 0.8 – 2.5 2.5 – 3.0 LIGHT BROWN, FINE TO MEDIUM SAND WITH SILT, TRACE ROOTS AND GRAVEL (LOOSE TO MEDIUM DENSE, MOIST) (FILL) DRAIN ROCK AND CONRETE DRAIN PIPE NO SAMPLES WERE COLLECTED GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED INFILTRATION PIT CAVING WAS NOT ENCOUNTERED INFIILTRATION PIT COMPLETED AT 3.0 FEET ON 10/4/16 INFILTRATION PIT ONE 0.0 – 0.2 GRASS AND ROOTS 0.2 – 1.0 DARK BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, IRON-OXIDE STAINING, AND ROOTS (LOOSE TO MEDIUM DENSE, MOIST) (TOPSOIL) 1.0 – 8.0 SP-SM LIGHT BROWN, FINE TO MEDIUM SAND WITH SILT, IRON-OXIDE WEATHERING AND TRACE GRAVEL (MEDIUM DENSE TO DENSE, MOIST) SAMPLES WERE COLLECTED AT 3.5 AND 8.0 GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED SLIGHT INFILTRATION PIT CAVING WAS ENCOUNTERED FROM 1.0 TO 8.0 FEET INFIILTRATION PIT COMPLETED AT 8.0 FEET ON 10/4/16 INFILTRATION PIT TWO 0.0 – 0.2 GRASS AND ROOTS 0.2 – 1.5 DARK BROWN, SILTY FINE TO MEDIUM SAND WITH GRAVEL, IRON-OXIDE STAINING, AND ROOTS (LOOSE TO MEDIUM DENSE, MOIST) (TOPSOIL) 1.5 – 7.0 SP-SM LIGHT BROWN, FINE TO MEDIUM SAND WITH SILT, GRAVEL, IRON-OXIDE WEATHERING, AND TRACE ROOTS (MEDIUM DENSE TO DENSE, MOIST) 7.0 – 9.0 SP-SM GRAY-BROWN, FINE TO COARSE SAND WITH SILT AND TRACE GRAVEL (MEDIUM DENSE TO DENSE, MOIST) SAMPLES WERE COLLECTED AT 5.0, 7.0, AND 9.0 GROUNDWATER SEEPAGE WAS NOT ENCOUNTERED SLIGHT INFILTRATION PIT CAVING WAS ENCOUNTERED FROM 1.5 TO 9.0 FEET INFIILTRATION PIT COMPLETED AT 9.0 FEET ON 10/4/16