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HomeMy WebLinkAboutRS_Geotech_Report_250108_v1GEOTECHNICAL ENGINEERING STUDY For WILLIAMS PROPERTY - NEW ADDITION 13411, SE 151ST STREET RENTON, WA 98058 Prepared For JAMES WILLIAMS & PATTY THUMANN 13411, SE 151ST STREET RENTON, WA 98058 Prepared By P.O. BOX 1419, ISSAQUAH, WASHINGTON 98027 PGE PROJECT NUMBER 24-754 June 28, 2024 June 28, 2024 Client: Jamie Williams & Patty Thumann 13411, SE 151st St. Renton, WA 98058 Re: Williams Property Geotechnical Engineering Study 13411, SE 151st St. Renton, WA 98058 PGE Project No. 24-754 Mrs. Patty: As per the request, Pacific Geo Engineering, LLC (PGE) has completed the geotechnical engineering study for the subject property in Renton, Washington, which is shown on Vicinity Map, Figure 1. The study includes soil investigation and development of geotechnical engineering recommendations pertinent to the geotechnical aspect of the proposed new building addition. This geotechnical engineering study report summarizes the results of our evaluations and the recommendations. This study is completed in accordance with the mutually agreed upon scope of services described in our proposal no. 24-04-818, dated April 24th, 2024, which was authorized on May 16, 2024. The scope of services was developed based on the preliminary understanding of the proposed new addition obtained from Mrs. Patty Thumann. Our scope of services is planned to obtain as much subsurface information as possible within the time and budgetary constraints of the project. The primary purposes of our limited geotechnical study were to perform site reconnaissance, explore and characterize subsurface soil and groundwater conditions in the site, perform laboratory testing of native soil, and review of available local geologic maps and geotechnical literatures, and to use the data and the information obtained from the above as a basis for formation of our geotechnical recommendations for the proposed new addition. Our recommendations are provided for the design and construction of the proposed new building addition, allowable bearing capacity value, slab-on-grade floor, site preparation, grading and earthwork operations, overexcavations, and fill placement and compaction. Also, recommendations are developed for site drainage and erosion control measures. We have also evaluated the site’s susceptibility to liquefaction under seismic conditions and the infiltration feasibility of the native soil for considering a below grade infiltration system in the site. The feasibility study was performed following the guidelines provided in the 2021 King County King County Surface Water Design Manual (KCSWDM). ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 2 of 41 1.0 Scope of Services Based on the scope of this geotechnical study delineated in the contract agreement, the following items are accomplished - field exploration, laboratory soil testing, engineering evaluation of the field and laboratory data, grading recommendations, infiltration feasibility, and foundation recommendations. The scope of our work did not include any wetland study, or any environmental analysis or evaluation to find the presence of any hazardous or toxic materials in the soil, surface water, groundwater, or air in or around this site. 1.1 Engineering Evaluation The results from the field and laboratory tests were evaluated and engineering analyses were performed to develop the design information and the engineering recommendations for the geotechnical aspect of the proposed development, which are provided in this report. Subsurface Conditions  Descriptions of the soil and the groundwater conditions;  Soil Test Pit Log;  Depth to water table and any sign of high water table, if encountered;  Laboratory soil index property test results.  Native soil Classification as per USCS system; General Site Development & Earthwork & Grading  Grading and earthwork including site preparation, and fill placement and compaction;  Use of on-site soils as structural fills;  Imported structural fill requirements;  Underground utility structure trench backfilling and pipe bedding;  Temporary and permanent excavation slopes;  Site drainage including permanent subsurface drainage systems and temporary groundwater control measures, if necessary:  Dry and wet weather construction:  Erosion control measurements. ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 3 of 41 Structure  Foundation type recommendation - conventional shallow spread footings;  Allowable bearing capacity value for supporting the proposed footings and the residence;  Estimated total and differential settlements of the footings;  Frictional and passive values for the resistance of lateral forces;  Subgrade preparation for spread footings;  Slab-on-grade for the proposed building, and the subgrade preparation for slab-on-grade;  Modulus of subgrade reaction for the design of the slab-on-grade floor;  Seismic design recommendations, including the site co-efficient as per ASCE7-16 Standard & 2018 IBC. Geologic Hazard Mitigations  Geologic hazards evaluation: erosion, seismic, and landslide;  Liquefaction potential evaluation of native soil;  Erosion control measures. 2.0 Proposed Development The existing residence and the proposed new building addition to be built in the backyard are shown on Aerial Site View, Figure 2. The new addition is marked as ‘B’ in the figure. Based on our on-site conversation with the owner the proposed addition will have a two-story building with a garage floor level and a floor above the garage. There are now two large trees located at the proposed addition area, which will be cut to build the new addition. The part of the southern portion of subject property is located within FEMA designated 100 year floodplain adjacent to the northern bank of the Cedar River. The northerly extent of FEMA 100 year floodplain line as per the King County iMap is shown on Site & Exploration Plan, Figure 3. The proposed new addition will be built beyond the northerly floodplain line near the existing residence. Based on our current understanding of the grading plan, we assume that the new building addition will be built almost at the current grades of the property without any significant grade changes within the proposed new development area. We assume that there will be some amount of cut depths will be required to remove the topsoils, heavy root zone, and the tree root bulbs, to achieve the final native subgrades described later on in this report as ‘competent’ native subgrades, and to achieve the footing embedment depth of minimum 18 inches below the current grades. Some fills may need to be placed to backfill the void areas to be created due to the overexcavation of the topsoils, heavy root zone, and the root bulbs. Based on the ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 4 of 41 findings from our site exploration, approximately, 6 inches of overexcavation depth may be required below the current grades to remove the topsoils. The overexcavation depths to be required to remove the heavy root zone and the tree root bulbs should be decided on-site during the grading operation in the site. The void areas to be created due to the overexcavation will be backfilled either with the native granular soils or new imported structural fills. The final footing subgrades , and the new fills if needed, must bear directly over the final ‘competent’ native subgrades. The slab-on-grade floor will be placed on new fill pad to be placed to backfill the void areas. The new fills must also bear over the final ‘competent’ native subgrades. Based on our experience with similar type of residence, we anticipate that wall loads will be in the range of 2 to 3 kips per lineal foot, isolated column loads in the range of 40 to 60 kips, and slab -on-grade floor loads of 150 pounds per square foot (psf). The conclusions and recommendations contained in this report are based upon our current understanding of the proposed development. We recommend that PGE should be allowed to review the final design grades and the actual features of the proposed development, and the final construction plan to verify that the geotechnical recommendations provided in this report are incorporated into the final construction documents. PGE’s review of the final plan would also allow re-evaluating the recommendations, and if necessary, to modify the recommendations before the construction begins. We believe this would be helpful for the project’s speedy completion and success. 3.0 Surface and Subsurface Features 3.1 Site Location The subject property is located at 13411, SE 151st Street, Renton, as shown in Figure 1. The site has a single parcel number assigned as 222305-9112. The property is almost a rectangular shape land, which is bounded by single family residences on the east and west, by SE 151st St on the north, and by Cedar River on the south. The site has an access via a concrete driveway from the SE 151st Street. 3.2 Site Descriptions The subject property is located within a region dominated by densely populated single family residences. An existing single-story, single family residence is located at the front area of the property close to the SE 151st Street. The property has vegetations comprised of mostly landscape grasses, and scattered small to large trees. The existing site features with contours and the northerly extent of FEMA 100 year floodplain line as per King County iMap are shown on Figure 2 and 3. The property is almost a level ground with minor downgrade slope towards the Cedar River. Based on Figure 3, approximate site elevation is 78 feet near the SE 151st Street which, gradually slopes down to ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 5 of 41 76 feet and then to 74 feet near the northern bank of the Cedar River. The part of the southern portion of subject property is located within a FEMA designated 100 year floodplain as shown in Figure 3. The floodplain area within the property is covered with landscape grasses. The existing residence is located near the SE 151st Street, which is beyond the FEMA 100 year floodplain line. The proposed new addition will also be built beyond the line. 4.0 Field Investigation Soil investigation was performed at one location in the proposed new building area at the backyard of the property as shown on Figure 3. The test pit location was selected by PGE’s engineer. The test pit location plotted on Figure 3 should be considered accurate only to the degree implied by the measuring methods. The test pit was advanced at the site on May 21, 2024, using an excavating machine hired by PGE. The test pit was excavated up to approximately 8 feet depth below the grades. The specific number and location of the test pit was selected in relation to the existing and proposed site features, accessibility, underground utility conflicts, purpose of evaluation, budget considerations, and after the consultation and approval of the owner. An experienced geotechnical engineer from PGE logged the subsurface conditions in the test borings and visually-manually classified the soil samples in the field according to the methods presented in ASTM D-2488-93 (based on the soil samples' density/consistency, moisture condition, grain size, and plasticity estimations) and the 'Key to Exploration Logs' figure in Appendix A, and observed pertinent site features. The final exploration log was prepared with our observation and interpretation of the test boring drilling, and visual examination of the samples in the field. The soils were classified according to the methods presented on the Figure 'Key to Exploration Logs' in Appendix A. This figure also provides a legend explaining the symbols and abbreviations used in the soil exploration logs. The soil logs indicate the depth where the soils change. It should be noted that the indicated stratification lines on the logs represent the approximate boundaries between soil types. The actual transitions of varying soil strata may be more gradual in the field. 5.0 Laboratory Testing Laboratory tests were conducted on several selected representative soil samples to evaluate the general physical properties and the engineering characteristics of the soils encountered. The bulk samples were visually-manually classified in the laboratory following the procedure described in ASTM D-2488-17 (based on the soil samples' density/consistency, moisture condition, grain size, and plasticity estimations), ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 6 of 41 and later on the soil samples' classifications were supplemented by laboratory tests data in ac cordance with the procedure described in ASTM D-2487-17. Moisture content tests were conducted on the samples in accordance with ASTM D-2216-10 procedures. One (1) Sieve Analysis test (Grain size distributions) was performed on one selected sample in accordance with ASTM D-6913 procedure. The results of the moisture content tests and the amount of percentages of minus #200 sieve passed are provided in the test pit logs, Appendix A. The grain-size distributions of the soil obtained from the Sieve Analysis tests are shown in the laboratory test report B-1 in Appendix B. 6.0 Site Soil and Groundwater Conditions Topsoils The explored area was covered with landscape grass, which is underlain by top soils for approximately 6 inches thickness below the current grades. Native Soils Native soils were observed below the topsoils, which were granular in nature consisted of sand with gravel, cobble, and boulder, designated as SP as per USCS soil classification. Hydrogeologic Condition During the month of May, when our soil investigation was done, no groundwater or perched water seepage was observed within the exploration depth. No sign of mottling (oxidized soils) was either noticed in the soil. Typically, mottling signs are indicative of accumulation of seasonal perched groundwater above the underlying denser deposit. Perched water is defined when stormwater permeates through the upper, less denser soils, and accumulates on top of the underlying denser, less permeable soils, like glacial till, which is very typical in the Puget Sound area. Typically, perched water presents in a spatial manner above the glacial till. It is to be noted that fluctuations in the perched water amount and level, and groundwater level may be expected due to the seasonal variations in the amount of rainfall, surface runoff, and other factors not apparent at the time of our explorations. Typically, the perched water and groundwater level rise higher and the flow rate increases during the wet winter months. The possibility of the fluctuations and the presence of perched water and the ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 7 of 41 ground water should be considered when considering any underground infiltration system in this site for managing the stormwater runoff of the proposed development. The preceding discussion on the subsurface conditions of the site is intended as a general review to highlight the major subsurface stratification features and material characteristics. For more complete and specific information at individual test pit location, please review the Soil Test Pit Log (Figure A-1) in Appendix A. The log includes soil descriptions, stratification, and location of the samples, and the laboratory test results. It should be noted that the stratification lines shown on the logs represent the approximate boundaries between various soil strata; actual transitions may be more gradual or more severe. The subsurface explorations made as part of the site evaluation indicate the subsurface conditions at test pit location only and as such the actual subsurface conditions may vary in other areas of the site. The actual nature and extent of such variation would not become evident until additional explorations are performed in the site or until construction activities have begun. 7.0 Regional Geology The site is in the Puget Sound Lowland, a north-south trending structural and topographic depression lying between Olympic Mountains on the west and Cascade Mountains on the east. The lowland depression experienced successive glaciation and nonglaciation activities over the time of Pleistocene period. During the most recent Fraser glaciation, which advanced from and retreated to British Columbia between 13,000 and 20,000 years ago, the lowland depression was buried under about 3,000 feet of continental glacial ice. During the successive glacial and nonglacial intervals, the lowland depression, which is underlain by Tertiary volcanic and sedimentary bedrock, was filled up above the bedrocks to the present-day land surface with Quaternary sediments, which consisted of Pleistocene glacial and nonglacial sediments. The glacial deposits include concrete-like lodgement till, lacustrine silt, fine sand and clay, advance and recessional outwash composed of sand or sand and gravel, and some glaciomarine materials. The nonglacial deposits include largely fluvial sand and gravel, overback silt and clay deposits, and peat attesting to the sluggish stream environments that were apparently widespread during nonglacial times. 7.1 NRCS Map - USDA Soil Unit As per The site is underlain by Pilchuck Loamy Fine Sand (Pc), which is made up of gravelly and sandy alluvium soils with excessively drained characteristic. This soil unit is described as Hydrologic Soil Group ‘A’. The soil observed in the test pit is consistent with this mapped soil unit. ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 8 of 41 8.0 Geologic Hazards As per the City of Renton Municipal Code, the potential geologic hazards i.e., the landslide, seismic, flood, and erosion in the subject property are evaluated, which are discussed in the following subsections. 8.1 Landslide Hazard Based on the topography of the site, the site is almost a level flat ground hence the potential of landslide hazard in this site is considered nil. 8.2 Seismic Hazard Liquefaction Potential Earthquake-induced geologic hazards may include liquefaction, lateral spreading, slope instability, and ground surface fault rupture. Liquefaction is a phenomenon, which takes place due to the reduction or complete loss of soil strength due to an increase in pore water pressure in soils during the seismic vibrations induced by a major earthquake event. Liquefaction primarily affects geologically recent deposits of loose, fine-grained sands and granular silts that are below the groundwater table. Based on our review of the soil and groundwater conditions in the test pit, it is our opinion that the on-site soils are not prone to liquefaction because of the absence of any groundwater table in the test pit and the presence of denser native granular soil containing gravel, cobble, and boulder at shallower depth in the explored area of the site. Therefore, potential for widespread liquefaction and its associated hazards over the site during a seismic event is none. Therefore, subsurface conditions do not warrant additional mitigation techniques relating to liquefaction hazards. While the site is relatively near the Seattle Fault zone, no evidence of ground fault rupture was observed in the subsurface explorations or our site reconnaissance. Therefore, the potential for ground surface is also low. Regional Seismicity The site is located in the Puget Sound region of western Washington, which is seismically active. Seismicity in this region is attributed primarily to the interaction between the Pacific, Juan de Fuca and North American plates. The Juan de Fuca plate is subducting beneath the North American plate at the Cascadia Subduction Zone (CSZ). This produces both intercrustal (between plates) and intracrustal (within a plate) earthquakes. In the following sections we discuss the design criteria and potential hazards associated with the regional seismicity. Provided the design criteria listed below are followed, the proposed structure should have no greater seismic risk damage than other appropriately designed structures in the Puget Sound area. ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 9 of 41 Seismic Design Parameters As per the WA State Interactive Geologic Information Portal, Seismic Map, the NEHRP Seismic Site Class is mapped as Site Class ‘C’, which is described as very dense deposit. According to the SEA OSHPD Seismic Design Maps, and as per the 2016 ASCE 7-16 code standards, Table 20.3-1, for the seismic Site Class ‘C’, the following seismic design parameters should be used for the structural design of the building. Table 1 - Seismic Design Parameters Spectral Response Acceleration (SRA) and Site Coefficients Short Period (0.2 sec) Maximum Considered Earthquake (MCE) Ss = 1.387g Site Response Coefficient (Site Class D) Fa = 1.2 Adjusted Spectral Response Acceleration (SRA) of MCE SMS = Ss x Fa = 1.664g Design SRA SDS = 2/3 x SMS = 1.109g Spectral Response Acceleration (SRA) and Site Coefficients One Second Period (1 sec) Maximum Considered Earthquake (MCE) S1 = 0.473g Site Response Coefficient (Site Class D) Fv = 1.5 Adjusted Spectral Response Acceleration (SRA) of MCE SM1 = S1 x FV = 0.71g Design SRA SD1 = 2/3 x SM1 = 0.473g Peak Ground Acceleration The mapped peak ground acceleration (PGA) for this site is 0.59g. To account for site class, the PGA is multiplied by a site amplification factor (FPGA) of 1.2. The resulting site modified peak ground acceleration (PGAM) is 0.708g. 8.3 Flood Hazard The part of the southern portion of subject property is located within FEMA designated 100 year floodplain adjacent to the northern bank of the Cedar River, which is shown on Flood Hazard Map, Figure 4. The northerly extent of FEMA 100 year floodplain line as per the King County iMap is shown on Figure 3, which shows that the proposed new addition will be built beyond the northerly floodplain line near the existing residence, which is not a floodplain area as per Figure 4. ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 10 of 41 8.4 Erosion Hazard Typically, uncontrolled surface water with runoff over unprotected site surfaces during construction activities is considered the single most important factor that impacts the erosion potential of a site. The erosion process may be accelerated significantly when factors such as soils with high fines, sloped surface, and wet weather combines together. Taking into consideration the factors such as the presence of near-surface native soil with fines less than 5 percent, level grades of the subject property, and assuming that the proposed construction will take place during dry summer period it is our opinion that the potential for erosion hazard of the site soils is not a limiting factor for the proposed development. This possibility will be further reduced if appropriate erosion control measures are installed and maintained as recommended below. These measurements must be kept in place and to be maintained throughout the earthwork and the grading activities. Though special mitigations are not necessary, a temporary erosion and sediment control (TESC) plan should be created and implemented during site construction. It is our opinion that implementation of a relatively basic erosion control plan will prevent off site sediment transport. The proper use of “best management practices” (BMPs) should be utilized during development of the building to minimize the potential for erosion and sediment off of the property due to clearing, grading and construction traffic. Implementation of a TESC plan will likely be a requirement of the clearing and grading or building permit. City of Renton will perform TESC inspections during construction to verify compliance with the TESC plan and permit conditions. Erosion Control Measures & Mitigations All erosion sediment control measures must conform to the City of Renton or King County requirements. As a minimum, we recommend implementing the following erosion and sediment control Department of Ecology (DOE) best Management Practices (BMPs) prior to, during, and immediately after clearing and grading activities at the site.  Mass grading activities and the earthwork should be completed within the dry summer period since the near surface site soils containing some silts may pose some erosion related problems.  Limit disturbance to areas where construction is imminent. If possible, site clearing and grading should be performed in stages, with successive stages not being cleared until erosion control measures for the previous stages are in place.  Determine staging areas for temporary stockpiles of excavated soils as part of the excavation planning. ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 11 of 41  Provide temporary cover for denuded areas including cut slopes and soil stockpiles during periods of inactivity. From October 1 to April 30, no soil shall remain un-stabilized for more than 48 hours. From May 1 to September 13, no soil shall remain un-stabilized for more than seven days. Temporary cover may consist of straw mulch or plastic sheeting that is securely anchored to the ground surface. Plastic covering should be placed and anchored, as specified in BMP C123 provided in Chapter 4.1 of the Stormwater Management Manual for Western Washington. Mulching should be conform to the guide lines outlined in the BMP C121 provided in Chapter 4.1 of the Stormwater Management Manual for Western Washington  Establish permanent covers for exposed areas that will not be worked for period of 30 days or more by seeding in conjunction with a mulch cover or appropriate hydroseeding. Seeding should conform to the specifications outlined in BMP C120 provided in Chapter 4.1 of the Stormwater Management Manual for Western Washington.  Measurements such as the control of surface water must be maintained during construction.  Vegetation clearing must be kept very limited in this site to reduce the exposed surface areas. It is recommended that following the clearing of the vegetations, grading the open exposed areas should be covered with mulch or hydroseed.  No disturbance or removal of the existing vegetations, tress, and undergrowths should be made beyond the proposed construction area.  Temporary erosion and sedimentary control (TESC) plan, as a part of the Best Management Practices (BMP) must be developed and implemented as well. The TESC plan should include the use of geotextile barriers (silt fences) along any down-slope, straw bales to de-energize downward flow, controlled surface grading, limited work areas, equipment washing, storm drain inlet protection, and sediment traps. The TESC plan may need to be reviewed and modified periodically to address the changing site conditions during ongoing progress of the construction and the weather.  A permanent erosion control plan is to be implemented following the completion of the construction. Permanent erosion control measurements such as establishment of landscaping, replantation of trees and groundcover vegetations as soon as feasible in areas that are necessarily disturbed by earthwork activities, control of downspouts and surface drains, control of sheet flow over the excavation slope, prevention of discharging water over the excavation slope and at the toe of the slope are to be implemented following the completion of the construction.  Install temporary or permanent tightline pipes, where necessary and practical, to convey stormwater from above slope to appropriate downslope facilities on flatter terrain.  Install permanent stormwater runoff diversion systems, such as swales, curbs, berms, or pipes, to prevent flow directly over any final slope grades.  We recommend that completed graded-areas be restricted from traffic or protected prior to wet weather conditions. The graded areas may be protected by paving, placing asphalt -treated base, a layer of free-graining material such as pit run sand and gravel or clean crushed rock material containing less than 5 percent fines, or some combination of the above. ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 12 of 41 Containment  Install a silt fence along the downhill side of the construction area that will be disturbed. The silt fence should be placed before cleaning and grading is initiated and should conform to the specifications outlined in BMP C233 provided in Chapter 4.2 of the Stormwater Management Manual for Western Washington.  Construct interceptor dikes and shallow drainage swales to intercept surface water flow and route the flow away from the construction tare to be stabilized and approved point of controlled discharge. Some small detention ponds with pipe slope drains may be incorporated with the swales in order to collect and transport the runoff to the discharge point.  Provide on-site sediment retention for collected runoff. Runoff should not flow freely over the top of the slope or off the site.  The on-site contractor should perform daily review and maintenance of all erosion and sedimentation control measures at the site to ensure their proper working order. Provided the recommended erosion and sedimentation control BMP’s are properly implemented and maintained, it is our opinion that the planned development will not increase the potential for erosion at the site or on adjacent properties. 9.0 Executive Summary Based on this study, the subject site is considered suitable for the proposed development, if the geotechnical recommendations provided in this report are properly understood and interpreted, and strictly implemented during the design and construction phases of the proposed development. It should be noted that if the subsurface conditions are found to be different in the unexplored areas of the site than what it is found in the explored areas then the recommendations provided in this report may need to be revisited and altered, to incorporate the changes if to be found on the subsurface conditions. This may calls for possible changes in the final design of the project as well. A contingency plan should be in place by the owner considering the above scenario. Based on the soil conditions encountered in the test pit, the footings of the new building addition should be installed on the ‘competent’ native subgrades to be consisted of native soil deposit. A ‘competent’ native subgrade is described when the final native subgrade is ‘redensified’, and ‘compacted and proofrolled’ adequately in a thorough manner to firm, unyielding, and stable conditions following the procedures described later on in Section 10.1.3, 'Subgrade Preparation' of this report. We recommend that the topsoils, root zone, and root bulbs from the new building footprint area must be completely removed to achieve the final ‘competent, native subgrades. During the removals, the overexcavation depths below the current grades may vary, which should be determined by PGE’s geotechnical engineer. The final ‘competent’ native subgrades must be verified ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 13 of 41 by PGE’s on-site geotechnical engineer prior to placing the forms and rebars of the footings and placing new structural fills. The footings for the new building addition can be comprised of perimeter wall strip footings with interior isolated column spread footings. The final ‘competent’ native subgrades will be able to support the footings and to provide an allowable bearing capacity value of 2000 psf for supporting the footings, and keeping the total settlement and differential settlement of the footings within the allowable limits of 1 inch or less and ½ inch or less, respectively We recommend that the slab-on-grade floor is to be bearing on the new fill pad to be placed in the void areas to be created after the removals of the topsoil, root zone, and root bulbs. The fills must be backfilled with either on-site, native granular soils, or new, imported structural fills, which must be adequately compacted prior to placing the floor slab on top of the fill pad. The new fill thickness may vary based on the final floor elevation and the thickness of the existing fills. The final fill thickness requirement should be decided by the PGE’s on- site geotechnical engineer. The new structural fills must be placed after the final native subgrades should be ‘redensified’ and ‘proofrolled’ adequately to prepare the final native subgrades as ‘competent’ native subgrades, and to be accepted by the PGE’s on-site geotechnical engineer. The other hard surfaces such as paved driveway, patios, or walkways should be placed over the final native ‘competent’ subgrades similar to the footings and the new fills. The structural fills to be used for filling up the void areas may be new, imported, structural fill materials or the native granular material. The structural fills must be consisted of clean, crushed rock or crushed gravel, and sand that is fairly well graded between coarse and fine as described later on in Section 10.1.6, ‘Structural Fills’ of this report. The native soils are consisted of granular soils having very low fines content hence considered as suitable for reuse as structural fills. However, the soils contain larger-size particles such as large gravel, cobble, and boulder, which will pose problem during the compaction of the soils. The native soils can be used as structural fills if the larger size particles, larger gravel, cobble and boulder, are removed from the native soils. The new fills must be placed and compacted as per the recommendations provided later on in Section 10.1.7, 'Fill Placement and Compaction Requirements' of this report. The new structural fills must be compacted adequately to firm and unyielding condition to achieve 95% or more of fills’ dry density value to be determined from the ASTM Test Designation D-1557 (Laboratory Modified Proctor) method. A single or double-drum heavy duty vibratory roller should be used to perform the redensification, proofrolling, final native subgrade preparation, and fill compaction. Alternatively, a walk-behind, heavy-duty, vibratory plate compactor (similar to TMG-PC330K Reversible Plate Compactor with 14HP Kohler Engine) can also be used to perform the above activities. The vibratory plate compactor produces much less vibrations ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 14 of 41 than the big drum roller. Therefore, if vibration is an issue for the neighbors and their residences’ stability then we recommend that the vibratory plate compactor will be suitable for this site. The horizontal limits of the fill placement under any load-bearing structure should extend laterally beyond the each side of the fill pad for a horizontal distance equal to the depth of the fill pad. This is to avoid the loading from the structure (which is assumed exerts pressure through an imaginary line at 1H:1V inclination or at 450 angle from below the footings) to pass through the fill thickness instead the loading line to pass below the fill thickness. The depths and the degree of the competence of the final native subgrades may vary across the site, which must be verified and approved on-site by the PGE’s on-site geotechnical engineer. The redensification of the final native subgrades, proofrolling and preparation of the final native subgrades as ‘competent’ native subgrades, and the fill placement and compaction must be monitored and approved by the on-site geotechnical engineer prior to placing the new fills, and the footings, slab-on-grade floor, concrete paved driveway, side-walk, and concrete patio directly above new fills. The heavy-root, organic reach topsoil of approximately 6 inches thickness must be removed completely from the proposed development area prior to start the cut and fill operations in this site. The topsoils cannot be used structural fills and be stockpiled for later use in the landscaping areas. In our opinion, the combination of geological factors such as the presence of native sand with gravel, cobble, and boulder with its measured infiltration rate obtained from our infiltration test , and the absence of perched water seepage within the test pit depth explored, are considered to be feasible for a below grade ‘full-infiltration’ system like a drywell in this site. A detail of the infiltration feasibility study in this site and the recommendation for final design infiltration rate are provided later on in Section 11.0 of this report. 10.0 Conclusion & Recommendations 10.1 Site Preparation Preparation of the site should involve clearing, stripping, subgrade preparation and proofrolling, cutting, filling, excavations, and drainage installations. The following paragraphs provide specific recommendations on these issues. 10.1.1 Clearing and Grubbing Initial site preparation for construction of the proposed new addition building, driveway, parking area, any other load-bearing structure, and placing new fills on the final native subgrades should include ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 15 of 41 stripping of vegetation and topsoil from the construction area. Based on the topsoil thickness encountered at our test pit location, we anticipate topsoil stripping depths of about 6 inches, however, thicker layers of topsoil may be present in unexplored portions of the site. It should be realized that if the stripping operation takes place during wet winter months, it is typical a greater stripping depth might be necessary to remove the near-surface moisture-sensitive silty soils disturbed during the stripping; therefore, stripping is best performed during dry weather period. Stripped vegetation debris should be removed from the site. Stripped organic topsoil will not be suitable for use as structural fill but may be used for future landscaping purposes. 10.1.2 Overexcavation Once the clearing of the vegetations and the topsoils from the proposed development area will be completed, the overexcavation of the heavy root zone, tree root bulbs, unsuitable native soils if there will be any underneath the proposed construction area can be initiated to establish the desired final ‘competent’ native subgrades for bearing the proposed new building addition and the new structural fills. The overexcavation depth must be a minimum of 18 inches below the current or the final site grade to achieve the footing embedment depth requirement below the above grades. The overexcavation in the proposed new construction area must be verified by PGE’s on-site geotechnical engineer. The overexcavation should be performed using smooth-edged bucket to limit the disturbances of the potential final native subgrades. 10.1.3 Subgrade Preparation Redensification After the clearing of the vegetations and topsoils, and following the completion of the overerxcavation upto the final ‘competent’ native subgrades, we recommend that all final native subgrades that are supposed to be supporting the load-bearing structure should be redensified to enhance the in-situ density of the final native subgrades, improving their bearing capacity hence reducing their potentials of undergoing excessive settlement. Typically, the redensification is effective for the upper one to two feet of soil below the final native subgrades. The depth of the in-situ density increase depends on the compaction equipment to be used. Typically, the redensification of the final native subgrades is done using a big, heavy- weight, double-drum, vibratory roller. The redensification is achieved by having the compaction equipment make several passes as to be found necessary by the on-site geotechnical engineer. One pass is considered to consist of a passage of the compactor in each direction, forwards and backwards, over the same strip of subgrade. The redensification process should be carried out over the whole of the excavated “at grade” footing subgrade and slab-on-grade areas, and any other load-bearing structures such as the new fill pad and sidewalk. ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 16 of 41 Proofroling Any exposed subgrades that are intended to provide direct support for new construction and/or require new fills should be adequately proofrolled to evaluate their conditions and to identify the presence of any isolated soft and yielding areas and to verify that stable subgrades are achieved to support the proposed structures, and any new fills. Proofrolling should be done with a loaded dump truck or a front -end loader or a big vibratory double-drum roller under the supervision of the PGE’s on-site geotechnical engineer. If it is found by the on-site geotechnical engineer that the soil is too wet near the subgrade to be proofrolled or it not feasible to proofroll the subgrade, then an alternative method (i.e., visual evaluation and probing with a 1/2- inch diameter steel T-probe) can be used by the geotechnical engineer to identify the presence of any isolated soft and yielding areas and to verify that stable subgrades are achieved to support the proposed structures and any new fills. If any subgrade area is found in soft and moist conditions, ruts and pumps excessively, and cannot be stabilized in place by compaction the affected soils should be over -excavated completely to firm and unyielding suitable bearing materials, and to be replaced with ne w structural fills to desired final native subgrade levels. If the depth of overexcavation to remove unstable soils becomes excessive, a geotextile fabric, such as Mirafi 500X or equivalent in conjunction with structural fills may be considered to achieve a firm bearing final subgrades to support the proposed structures and any new fills. Any final native subgrades and foundation bearing surfaces should not be exposed to standing water. If water is present in the final native subgrades or in the base of the footing excavation, it must be removed completely to bring the subgrades into dry condition before placing any new fills and formwork and rebars. Protection of exposed soil, such as placing a 6-inch thick layer of crushed rock or a 3- to 4-inch layer of lean- mix concrete, could be used to limit disturbance to bearing surfaces. If the base of an overerxcavated area is excessively soft and wet and needs stabilization then we recommend considering a 6 to 12-inch layer of ballast rock or quarry spalls should be placed to form a base on which the structural fill needs to be placed and compacted to achieve the final grade. Ballast rock should meet the requirements for Class B Foundation Material in Section 9-03.17 and quarry spalls should meet the requirements in Section 9-13.1(5) of the 2024 WSDOT Standard Specifications. The ballast rock or quarry spalls should be pushed into the subgrade with the back of a backhoe bucket or with the use of a large - vibratory steel drummed roller without the use of vibration. Such decision should be made the on-site geotechnical engineer during the actual construction of the project. ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 17 of 41 10.1.4 Backfilling of Test Pit and Tree Root Bulb Holes The loosely backfilled soils in the area of exploratory test pit should be overexcavated completely to the firm native soils and backfilled with adequately compacted new, imported structural fills to the final grades, following the procedures described later on in Section 10.1.7, 'Fill Placement and Compaction Requirements' of this report. The new, imported structural fills should be granular materials like sand and gravel meeting the requirements provided later on in Section 10.1.6, ‘Structural Fills’ of this report. Prior to placing the new fills the final native subgrades at the bottom of the overexcavated areas must be proofrolled adequately to firm and unyielding conditions as recommended earlier in Section 10.1.3, ‘Subgrade Preparation’ of this report and accepted by PGE’s on-site geotechnical engineer prior to placing new fills. Similar procedures described above should be followed after the holes to be created due to the removals of the tree root bulbs. 10.1.5 Reuse of Native Soils as Structural Fills The ability to use native soils as structural fills, to be obtained during the mass grading activities, will depend on the factors such as the quality of the native soils, i.e., the presence of excessive roots and organics, fines content, larger-size particles, moisture content, soil types and their gradation, and the prevailing weather conditions during the time of the construction i.e., dry or wet weather. The weather plays a significant role in determining if the native soils can be compacted adequately during the wet weather period, especially when the native soils content higher percentages of fines. Typically, native soils containing unsuitable materials such as the excessive roots and organics are not considered suitable for use as structural fills. If the native soil below the topsoils contains percentages of fines greater than the typical ‘imported structural fills’ that contains 5% or lesser fines, then the native soil is to be considered as moisture insensitive soils. The percentage of fines content in the native soil can be determined from running a sieve analysis test on the native soil sample. Typically, when the fines content (that portion passing the U.S. No. 200 sieve) of soil increases, the soil becomes increasingly sensitive to small changes in moisture content, which makes the soils’ compaction more difficult or impossible. Soils containing more than about 5 percent fines by weight cannot be consistently compacted to the recommend degree when the moisture content is more than about 2 percent above or below the optimum. Especially, if the soils with higher fines content are used during the wet weather period, typically between October and May, significant reduction in the soils strength and support capabilities occur. Also, when these soils become wet they may be slow to dry and thus significantly retard the progress of grading and compaction activities. Therefore, the native soil can be used as structural fills during the wet weather period based on the percentage of fines present in the native soil. However, irrespective of the ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 18 of 41 percentage of fines content the native soil can be used as borrow materials for general filling purposes during the dry season, provided the optimum moisture content of the soils can be maintained during the compaction. In addition to the higher percentage of fines, if the native soils are found in excessively over the optimum moisture content, then the soils would pose problems during their compaction. This may require moisture conditioning of the native soils prior to their placement and compaction. Other criteria that is to be considered critical prior to use native soils as structural fills is the presence of significant amount of larger-size particles such as larger size gravels, and cobbles and boulders. Typically, this type of soil is not considered suitable to use as structural fills, since the larger-size gravels, and cobbles and the boulders pose problems during the compaction of the fills. Therefore, the native soils if considered to be used as structural fills or borrow materials then the larger-size gravels, and cobbles and boulders must be removed from the native soils. This can be accomplished either by screening the native soils on-site using a screener machine or by selectively handpicking the larger-size particles, whichever methodology is feasible and economical. The PGE’s on-site geotechnical engineer should inspect the final product to verify that the final structural fills do not contain larger size particles. The final fills should contain a maximum of 2 to 3-inch particle diameter for being able to be adequately compacted. Based on the above criteria, the native soil is considered acceptable as ‘structural fills’ because of its low fines content, however it contains significant amount of larger size particles hence will pose problem during its compaction. As it is recommended, the native soils can be used if the larger size particles are removed. The suitability of using the native soils should be verified and approved by the on-site geotechnical engineer prior to their use. If the native soils cannot be used after the inspection and asked by PGE’s on-site geotechnical engineer to discard then imported new structural fills are to be brought in to the site for backfilling purposes. We recommend that a contingency plan should be in place in the project budget if the native soils are to be exported out, or new structural fills are to be imported into the site, or on-site screening of the native soils is to be required. 10.1.6 Structural Fill General Requirements Typically, excavated native soils containing topsoil, unsuitable materials such as excessive roots and organics, wood debris and pieces, trash, left over construction debris are not considered suitable for use as structural fills, and should be properly disposed offsite. ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 19 of 41 If the native soils are found unsuitable for using as structural fills then we recommend that imported structural fill should be used for backfilling purposes. The workability of material for use a s structural fill will depend on the gradation and moisture content of the soil. Structural fill is defined as non-organic soil, free of any debris and deleterious materials, and well-graded and free-draining granular material, such as sand and gravel or crushed rock with a maximum particle size of 3 inches for any individual particle and less that 5 percent fines by weight based on the minus ¾-inch fraction. We recommend that washed crushed rock or select granular fill, as described below, be used for structural fill during wet weather. If prolonged dry weather prevails during the earthwork phase of construction, materials with somewhat higher fines content may be acceptable. Weather and site conditions should be considered when determining the type of impo rt fill materials purchased and brought to the site for use as structural fill. Frozen material should not be used as structural fills. All materials should be approved by the project geotechnical engineer prior to use. A sample of each fill material type should be submitted to the project geotechnical engineer for evaluation and approval prior to use. A typical gradation for structural fill is presented in the following table. Table 2 - Structural Fill U.S. Standard Sieve Size Percent Passing by Dry Weight 3 inch 100 ¾ inch 50 –100 No. 4 25 – 65 No. 10 10 – 50 No. 40 0 – 20 No. 200 5 Maximum* * Based on the ¾ inch fraction. ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 20 of 41 WSDOT Structural Fills For reference purpose, the following table provides the specifications for various types of structural fills that can be considered in this site for use as new, imported structural fills. Table 3 - WSDOT 2024 Structural Fills Specifications Fill Type Recommended Materials Structural Fill 9-03.9(1) Ballast 9-03.9(3) Crushed Surfacing Base Course 9-03-12(1)A Gravel Back fill for Foundation Class A 9-03.14(1) Gravel Borrow Common Fill Section 9-03.14(3) Common Borrow Free-draining Granular Fill 9-03.9(2) Permeable Ballast 9-03.12(2) Gravel Backfill for Walls 9-03.12(4) Gravel Backfill for Drains For most applications, we recommend that structural fill consist of material similar to ‘Gravel Borrow’ or ‘Select Borrow’ as described in Section 9-03.14(1) or Section 9-03.14(2), respectively, of the WSDOT 2024 Standard Specifications. Select Granular Fill Imported materials with gradation characteristics similar to WSDOT 2024 Standard Specification 9- 03.9 (Aggregates for Ballast and Crushed Surfacing), or 9-03.14 (Gravel Borrow) is suitable for use as select granular fill, provided that the fines content is less than 5 percent (based on the minus ¾-inch fraction) and the maximum particle size is 6 inches. Other Fill Materials Other materials may also be considered suitable for use as structural fill provided they are approved by the project geotechnical engineer. Such materials typically used include clean, well -graded sand and gravel (pit-run); clean sand; various mixtures of gravel; crushed rock; controlled-density-fill (CDF, it should meet the requirements in Section 2-09.3(1)E of the WSDOT 2024 Standard Specifications); and lean-mix concrete (LMC). Recycled asphalt, concrete, and glass, which are derived from pulverizing the parent materials also potentially useful as structural fill in certain applications. These materials must be thoroughly ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 21 of 41 crushed to a size deemed appropriate by the geotechnical engineer (usually less than 2 inches). The structural fills should have a maximum 2 to 3-inch particle diameter. Pipe Bedding Trench backfill to be placed beneath, adjacent to, and for at least 2 feet above utilities line should consist of well-graded granular material with a maximum particle size of 1 inch and less than 10 percent by dry weight passing the US Standard No. 200 Sieve, and should meet the standards of ‘Gravel Backfill for Pipe Zone Bedding’ described in Section 9-03.12(3) of the 2024 WSDOT Standard Specifications. Trench backfill must be free of debris, organic material and rock fragments larger than 1 inch. Trench backfills We recommend that the trench backfills to be placed 2 feet above the pipe and upto the final pavement subgrade level should be consisted of materials similar to ‘Gravel Borrow’ described in Section 9- 03.14(1) or ‘Select Borrow’ as described in Section 9-03.14(2), of the 2024 WSDOT Standard Specifications. Trench backfill must be free of debris, organic material and rock fragments larger than 3 inch. Stabilization Material Stabilization rock should consist of pit or quarry run rock that is well -graded, angular, crushed rock consisting of 4- or 6-inch-minus material with less than 5 percent passing the US Standard No. 4 Sieve. The material should be free of organic matter and other deleterious material. WSDOT SS 9-13.(15) - Quarry Spalls can be used as a general specification for this material with the stipulation of limiting the maximum size to 6 inches. 10.1.7 Fill Placement and Compaction Requirements Generally, quarry spalls, controlled density fills (CDF), lean mix concrete (LMC) do not require special placement and compaction procedures. In contrast, clean sand, crushed rock, soil mixtures and recycled concrete should be placed under special placement and compaction procedures and specifications described here. The structural fills under structural elements should be placed in uniform loose lifts not exceeding 12 inches in thickness for a big, heavy-weight, double-drum, vibratory roller or a big, heavy-duty, hand-guided, walk-behind, vibratory plate compactor (similar to TMG-PC330K Reversible Plate Compactor with 14HP Kohler Engine). A regular, walk-behind vibratory plate compactor can be used when the loose fill thickness will be kept within 4 to 6 inches. No heavy compaction equipment such as hoe pack or big vibratory roller should be used to compact the backfills to be placed behind the footing stem walls, within the horizontal distance equal to the heights of ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 22 of 41 the walls. Use of the heavy compaction equipment will impose excess surcharge load on the walls, which may cause permanent lateral instability to the walls. We recommend that the fills behind the footing stem walls should be placed in 4 inches lifts and to be compacted with a hand held smaller and lighter compaction equipment. Each lift of fills whether 12 inches or 4 inches or 6 inches should be compacted to a minimum of 95 percent of the fill’s maximum dry density as to be determined in the laboratory by ASTM Test Designation D-1557 (Modified Proctor) method, or to the applicable minimum City or County standard, whichever is the more conservative. The fill should be moisture conditioned such that its final moisture content at the time of compaction should be at or near (typically within about 2 percent) of its optimum moisture content, as determined by the ASTM Test Designation D-1557 (Modified Proctor) method. This should help enhance the compatibility of the materials and avoid the risks involved with wet, moisture sensitive soils. Fills should not be placed on frozen subgrades. If the fill materials are on the wet side of optimum, they can be dried by relatively inexpensively periodic windrowing and aeration or by intermixing lime or cement powder to absorb excess moisture. An ordinary Portland cement powder can be used in this regard. In using concrete we have found that the hydration of the cement not only results in water absorption, but also develops some “concrete-like” strength within the soil and cement matrix. In our experience the soil cement matrix can sometimes generates a compressive strength in excess of two thousand (2,000) psi. If this option is selected, we recommend that for a preliminary estimation purpose, the cement powder may be intermixed at a rate of about 3% by weight of the soil. The actual cement content should be decided during the mass grading activity de pending on the wet weather, soils’ natural moisture content, and the soil types. This form of soil treatment is not suitable for any type soils that are considered as free-daring backfills. The compacted structural fill pad should extend outside all foundations and other load bearing structures elements for a minimum distance equal to the thickness of the fill pad. Because of the sensitivity of this project we recommend that any and all structural fills and /or load bearing backfills be tested for determining the in-place density and the water content of the fills as per the Nuclear Density Gauge method (ASTM D6938). This test results will help to verify that the backfills have the achieved the appropriate degree of compaction and the moisture content. We recommend that compaction of the fills be tested periodically throughout the fill placement. A field compaction testing program should be prepared by the contractor with the assistance from the project geotechnical engineer. If field density tests indicate that the last lift of compacted fills has not been achieved the required percent of compaction or the surface is pumping and weaving under loading, then the fill should be scarified, moisture-conditioned to near optimum moisture content, re-compacted, and re-tested prior to placing additional lifts. ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 23 of 41 We recommend that a minimum of one test be performed for one hundred (100) square feet of compacted or backfill surface area or for every one hundred (100) cubic feet of fill or backfill, whichever generates the greater number of compaction tests. We also recommend that to verify the compaction of the fill pad in both horizontal and vertical directions, when the fill thickness will be more than one foot, the compaction test locations and the elevations should be spaced in both directions. In this manner it should be possible to show with a reasonable degree of accuracy the “density profile” through the backfill. This is an important element of the QA/QC program of the project in the event there is a problem with the fill or backfill performance and subsequent litigation. 10.1.8 Dry Weather Construction Since the near surface native soils have some fines content, we prefer the proposed construction should be completed during the dry season to mitigate any erosion related issues that may otherwise arise during the construction activities in the wet season. Erosion particularly happens, when uncontrolled surface runoff is allowed to flow over unprotected excavation areas of the site during the wet winter months. 10.1.9 Wet Weather Construction If the construction takes place during the wet weather, the near surface soils, which is anticipated as to be moisture sensitive, will be found susceptible to degradation and disturbed when get wet. Therefore, it may be necessary to adopt some remedial measures to enhance the subgrade conditions in this site if the construction takes place in the winter. The contractor should include a contingency in the earthwork budget for this possibility. The appropriate remedial measure is best determined by the geotechnical engineer during the actual construction of the project. The following remedial measures may be considered in this regard:  The earth contractor must use reasonable care during site preparation and excavation so that the subgrade soils are remained firm, unyielding, and stable.  Removal of the affected soil that is already wet exposing suitable bearing subgrades and replacing with imported free-draining materials as structural fills that can be compacted.  Aeration of the surficial materials during favorable dry weather by methods such as scarifying or windrowing repeatedly and expose to sunlight to dry near optimum moisture content prior to placement and compaction  Chemical modification of the subgrades with admixtures like hydrated lime or Portland cement, depending on the soil type.  Limiting the size of areas that are stripped of topsoil and left exposed.  Limiting construction traffic over unprotected soils. ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 24 of 41  Sloping excavated surfaces to promote runoff.  Limiting the size and type of construction equipment used.  Providing gravel or quarry spalls “working mat” over areas of protected subgrade.  Removing wet surficial soil prior to commencing fill placement each day.  Sealing the exposed ground surface by rolling with a smooth drum compactor or rubber-tired roller at the end of each day.  Providing upgradient perimeter ditches or low earthen berms and using temporary sumps to collect runoff and prevent water from ponding and damaging exposed subgrades.  Mechanical stabilization with a coarse crushed aggregate (such as sand and gravel, crushed rock, or quarry spalls) compacted into the subgrade, possibly in conjunction with a geotextile fabric, such as Mirafi 500X.  In the event earthwork takes place during the wet season, we recommend that special precautionary measurements should be adopted to minimize the impact of water and construction activities on the moisture sensitive soils.  It is recommended that earthwork be progressed part by part in small sections to minimize the soil’s exposure to wet weather. Traversing of construction equipment can cause considerable disturbance to the exposed subgrades, therefore, should be restricted within the specific drive areas. This will also prevent excessive widespread disturbance of the subgrades. Construction of a new working surface from an advancing working surface could be used to avoid trafficking the exposed subgrade soils.  Any excavations or removal of unsuitable soils should be immediately followed by the placement of backfill or concrete in footings.  At the end of each day, no loose on-site soils and exposed subgrades be left uncompacted or properly tamped, which will help seal the subgrade and thereby to minimize the potential for moisture infiltration into the underlying layers of fills or subgrades.  In case site filling must proceed during wet weather the contractor should include a contingency in the earthwork budget for the possibility of using imported clean, granular fill. For general structural fill purposes, we recommend that using well-graded sand and gravel, such as ‘Ballast’ or ‘Gravel Borrow’ per 2024 WSDOT Standard Specifications 9-03.9(1) and 9-03.14(1), respectively. Alternatively, ‘free- draining’ soil similar to the one described earlier in the Structure Fill Table may also be considered suitable as filling material for the wet weather construction. This type of fill refers to soils that have a fines content of 5 percent or less (by weight) based on the minus ¾-inch soil fraction. 10.1.10 Subgrade Degradation Prevention The near surface alluvium deposit (silt) containing high percentage of fines when will be used as subgrades will be susceptible to degradation during the wet weather conditions. To protect against subgrade degradation due to construction traffic we recommend a ‘working mat’ be placed over final prepared ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 25 of 41 subgrades. We recommend this ‘working mat’ consists of 12 inches thick free draining materials consist of crushed rocks or quarry spalls, possibly in conjunction with a geotextile fabric, such as Mirafi 500X placed underneath the crushed rocks or quarry spalls layer. Construction traffic should be limited to these ‘working mat’ areas. The stabilization materials can be as per the requirements recommended later on in Section 10.1.7, ‘Stabilization Materials’. 10.1.11 Site Drainage Surface Drainage The final site grades of the finished development must be such that surface runoff will flow by gravity away from the building and other structure, such as the pavement and sidewalks, using sloped and drainage gradients towards the local stormwater collection system. We recommend providing a minimum drainage gradient of about 2% for a minimum distance of about 10 feet from the building perimeter. Surface water should not be allowed to pond and soak into the ground surface near buildings or paved areas during or after constructions. A combination of using controlled surface drainage and capping of the building surroundings by concrete, asphalt, or low permeability silty soils will help minimize or preclude surfac e water infiltration around the perimeter of the building and beneath the garage basement floor slab. Paved areas should be graded to direct runoff to catch basins and or other collection facilities. Collected water should be directed to the on-site drainage facilities by means of properly sized smooth walled PVC pipe. Interceptor ditches or trenches or low earthen berms should be installed along the upgrade perimeters of the site to prevent surface water runoff from precipitation or other sources entering in to the lower area of the lot. It should be noted that surface water runoff from precipitation flows as a sheet flow over slope is considered to be the primary cause of surficial sloughing and triggering slope failure. Therefore, the surface drainage system should be designed in such a way that stormwater runoff over the finished lot must not create any sheet flow over the sloped areas of the site, instead, the stormwater runoff must be collected in drain pipes to discharge in approved discharge points at the toe of the slope. Surface drainage system and the water collection facilities should be designed by a professional civil engineer. Footing Excavation Drain Water must not be allowed to pond in the foundation excavations or on prepared subgrades either during or after construction. If due to the rainfall, runoff, seasonal fluctuations, groundwater seepage is encountered within footing depths, we recommend that the bottom of excavation should be sloped toward one corner to facilitate removal of any collected rainwater, groundwater, or surface runoff, and then direct the water to ditches, and to collect it in prepared sump pits from which the water can be pumped and discharged into an approved storm drainage system. Water handling needs will typically be lower during the summer and early fall months. ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 26 of 41 Footing Drain Footing drains should be used where (1) crawl spaces or basements will be below a structure, (2) a slab below the outside grade, and (3) the outside grade does not slope downward from a building. To reduce the potential for groundwater and surface water to seep into the interior spaces of the building we recommend that an exterior footing drain system be constructed around the perimeter of the building footings as shown in Figure 5, ‘Floor Slab, Footing, & Footing Drain’ of this report. The drains must be laid with a gradient sufficient to promote positive flow by gravity to a controlled point of approved discharge. The foundation drains should be tightlined separately from the roof drains to this discharge point. Footing drains should consist of at least 6-inch diameter, heavy-walled, perforated PVC pipe or equivalent. The pipe should be surrounded by at least 6 inches of free-draining gravel over the pipe and 3 inches of free-draining gravel below the pipe. The free-draining material may consist of open-graded drain rocks consisted of ¾” minus washed gravels should be wrapped up by a non-woven geotextile filter fabric (Mirafi 140N) to limit the ingress of fines into the gravel and the pipe. The free-draining material should contain less than 2 percent by weight passing the U.S. Standard No. 200 sieve (based on a wet sieve analysis of that portion p assing the U.S. Standard No. 4 sieve). The drains should be located along the outside perimeter of the spread footings or the footing stem walls. Also, the invert of the footing pipe should be placed at approximately the same elevation as the bottom of the footing or 12 inches below the adjacent floor slab grade, whichever is deeper, so that water will not seep through walls or floor slabs. The footing drains should discharge to an approved drain system and include cleanouts to allow periodic maintenance and inspection. Downspout or Roof Drain These should be installed once the building roof in place. They should discharge directly in tightlines to a positive, permanent stormwater collection system. Under no circumstances connect these tightlines to the perimeter footing drains. The drain is shown in Figure 5 of this report. 10.1.12 Temporary Excavations As we understand from the project plan that the proposed site development is likely to involve some overexcavation of approximately 8 feet depth below the current grades if a drywell infiltration system is to be planned in this site for managing the stormwater runoff from the proposed development. The inclination of the overexcavation embankment should be made as per the recommendations provided below. As a general rule, all temporary soil excavations in excess of 4 feet in height and less than 20 feet in depth, the side slopes should be adequately sloped back or properly shored in accordance with Safety Standards for Construction Work Part N, WAS 296-155-657 to prevent sloughing and collapse. As for the current ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 27 of 41 estimation purposes, in our opinion, the side slopes in the native soils (OSHA soil Type C - Sand) should be laid back at a minimum slope inclination of 1.5:1 (Horizontal:Vertical). However, estimation of the proper inclination of excavation side slopes should be made on-site after inspecting the soil and groundwater conditions, which will be revealed during the actual construction in the site. It should be recognized that slopes of the above gradients do ravel and require occasional maintenance. All temporary exposed slopes and excavations should be protected as soon as possible using appropriate methods to prevent erosion to occur during periods of wet weather. This can be achieved by installing a durable reinforced plastic membrane, jute matting, or other erosion control mats with proper anchorage to the ground. In addition, we recommend that experienced personnel of the contractor should regularly check the slope condition to notice if any signs of raveling or sloughing off is underway to prevent any catastrophic slope failure. All temporary soil cuts greater than 4 feet in height, if cannot be sloped back because of the limited horizontal distance to be available between the top of the excavation line and the property line, a properly shoring system is to be considered to prevent sloughing and collapse of the slope. Any excavation side inclinations will assume that the ground surface behind the cut slopes is level, that surface loads from equipment and materials are kept a sufficient distance away from the top of the slope. If these assumptions are not valid, we should be contacted for additional recommendations. Flatter slopes may be required if soils are loose or caving and/or water, are encountered along the slope faces. If such conditions occur and the excavation cannot stand by itself, or the excavation slope cannot be flattened because of the space limitations between the excavation line and the boundary of the property, temporary shoring may be considered. The shoring will assist in preventing slopes from failure and provide protection to field personnel during excavation. Because of the diversity available of shoring stems and construction techniques, the design of temporary shoring is most appropriately left up to the contractor engaged to complete the installation. We can assist in designing the shoring system by providing with detailed shoring design parameters including earth-retaining parameters, if required. Where sloped embankments are used, the top of the slopes should be barricaded to prevent vehicles and storage loads within 10 feet of the top of the slopes. Greater setbacks may be necessary when considering heavy vehicles, such as concrete trucks and cranes. If the temporary construction embankments are to be maintained during the rainy season, berms are suggested along the top of the slopes to prevent runoff water from entering the excavation and eroding the slope faces. All temporary slopes should be protected from surface water runoff. The owner and the contractor should be aware that in no case should the excavation slopes be greater than the limits specified in local, state, and federal safety regulations, particularly, the Occupational Safety ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 28 of 41 and Health Administration (OSHA) regulations in the “Construction Standards for Excavations, 29 CFR, part 1926, Subpart P, dated October 31, 1989” of the Federal Register, Volume 54, the United States Department of Labor. As mentioned above, we also recommend that the owner and the contractor should follow the local and state regulations such as WSDOT Section 2-09.3(3) B, Washington Industrial Safety and Health Act (WISHA), Chapter 49.17RCW, and Washington Administrative Code (WAC) Chapter 296-115, Part N. These documents are to better insure the safety of construction worker entering trenches or excavation. It is mandated by these regulations that excavations, whether they are for utility trenches or footings, be constructed in accordance with the guidelines provided in the above documents. We understand that these regulations are being strictly enforced and, if they are not closely followed, both the owner and the contractor could be liable for substantial penalties. Stability of temporary excavations is a function of many factors including the presence of, and abundance of groundwater and seepage, the type and density of the various soil strata, the depth of excavation, surcharge loadings adjacent to the excavation, and the length of time and weather conditions while the excavation remains open. It is exceedingly difficult under these unknown and variable circumstances to pre- establish a safe and maintenance-free temporary excavation slope angle at this time of the study. We therefore, strongly recommend that all temporary, as well as permanent, cuts and excavations in excess of 4 feet be examined by a representative of PGE during the actual construction to verify that the recommended slope inclinations are appropriate for the actual soil and groundwater seepage conditions exposed in the cuts. If the conditions observed during the actual construction are different than anticipated during this study then, the proper inclination of the excavation and cut slopes or requirements of temporary shoring should be determined depending on the condition of the excavations and the slopes. The above information is provided solely for the benefit of the owner and other design consultants, and under no circumstances should be construed to imply that PGE assumes responsibility for construction site safety or the contractor’s activities; such responsibility is not being implied and should not be inferred. Therefore, the contractor is solely responsible for designing and constructing stable, temporary excavations and should shore, slope, or bench the sides of the excavations as required to maintain stability of both the excavation sides and bottom. The contractor’s “responsible person”, as defined in 29 CFR Part 1926, should evaluate the soil exposed in the excavations as part of the contractor’s safety procedures. We expect that the excavation can be completed using conventional equipments such as bulldozers or backhoes. 10.1.13 Utility Support and Backfill Based on the soils encountered at the site within the exploration depths, the majority of the soils appear to be adequate for supporting utility lines; however, softer soils may be encountered at isolated locations, where, ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 29 of 41 it should be removed to a depth that will provide adequate support for the utility. A major concern with utility lines is generally related to the settlement of the trench backfill along utility alignments and pavements. The trench backfill settlement causes misalignment of the utility lines and breaking apart of the joints. Therefore, it is important that each section of utility be adequately supported on proper bedding material and properly backfilled. We recommend that the on-site geotechnical engineer should evaluate the final subgrades of the bottom of the utility trench to verify if the subgrade is competent to support the utility lines and the backfills, or the subgrades need some proofrolling and recompaction, or require overexcavation of unsuitable loose fills and replacement with suitable structural fills. We recommend that if needed the bottom grades of the utility trench must be adequately proofrolled and compacted to firm and unyielding conditions. A layer of geo-grid such as Mirafi 500X or equivalent should be placed on the proofrolled subgrades prior to placing the bedding materials and laying the utility lines. This should be decided on-site by the geotechnical engineer on-site based on the observed subgrade conditions at the bottom of the trench. It is recommend that utility trenching, installation, and backfilling conform to all applicable Federal, State, and local regulations such as WISHA and OSHA for open excavations. Pipe Bedding & Pipe Zone Trench backfill to be placed beneath, adjacent to, and for at least 2 feet above utilities line should consist of well-graded granular material with a maximum particle size of 1 inch and less than 10 percent by dry weight passing the US Standard No. 200 Sieve, and should meet the standards of ‘Gravel Backfill for Pipe Zone Bedding’ described in Section 9-03.12(3) of the 2024 WSDOT Standard Specifications. Trench backfill must be free of debris, organic material and rock fragments larger than 1 inch. The bedding materials should be hand tamped to ensure support is provided around the pipe haunches. Trench backfill should be carefully placed and hand tamped to about 12 inches above the crown of the pipe before any heavy compaction equipment is brought into use. In order to reduce the potential for damaging the utilities, heavy compaction equipment should not be permitted to operate directly over utilities until a minimum of two (2) feet of backfill will be placed. In general, pipe bedding should be placed in loose lifts not exceeding 6 inches in thickness and compacted to at least 90 percent of the fills’ maximum dry density value as to be determined by the laboratory Modified Proctor (ASTM D1557) test method. The fill materials within the pipe bedding and pipe zone, their thicknesses and compactions should be suitable for the utility system and materials installed, and in accordance with any applicable manufacturers' recommendations or local building department. Pipe bedding materials should be placed on relatively undisturbed native soil. Based on our field explorations, we anticipate relatively coarse-grained soils comprised of poorly graded gravel with cobbles. Some overexcavation and removal of cobbles should be anticipated at the pipe invert elevation to maintain a uniform grade for the utility installation. Where overexcavation is needed, additional pipe bedding materials should be placed to restore the grade. ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 30 of 41 Trench Backfills We recommend that the backfills to be placed 2 feet above the pipe and upto the final pavement subgrade level should be consisted of materials similar to ‘Gravel Borrow’ described in Section 9-03.14(1) or ‘Select Borrow’ as described in Section 9-03.14(2), of the 2024 WSDOT Standard Specifications. Where excavations occur in the wet, alternative such as ‘Select Granular Fill’ described earlier in Section 10.1.7, Structural Fills’ should be considered. Trench backfill must be free of roots, debris, organic matter and rock fragments larger than 3 inches. Other materials may be appropriate depending on manufacturer specifications and/or local jurisdiction requirements. For site utilities located within the City of Renton, bedding and backfill should be completed in accordance with the city specifications. As a minimum, 5/8 inch pea gravel or clean sand may be used for bedding and backfill materials. The trench backfills to be placed 2 feet above the pipe and upto the final pavement subgrade level should be compacted to 95 percent of the fills’ maximum dry density value as to be determined by the laboratory Modified Proctor (ASTM D-1557) test method. The backfill should be placed in lifts not exceeding 4 inches if compacted with hand-operated equipment or 8 inches if compacted with heavy equipment. Catch basins, utility vaults, and other structures installed flush with the pavement should be designed and constructed to transfer wheel loads to the base of the structure. The utility trenches should not be left open for extended periods to prevent water entry, accumulation, and softening of the subgrade. Should soft soils be encountered at the bottom of the trench, it should be overexcavated and replaced with select fills. As an alternative to undercutting, a Geotextile fabric or crushed rock may be used to stabilize the trench subgrade. Where water is encountered in the trench excavations, it should be removed prior to fill placement. Alternatively, quarry spalls or pea gravel could be used below the water level if allowed by the local authority or the project specifications. 10.2 Construction Monitoring Since this project involves so many aspects of geotechnical engineering related construction activities such as the identification of the existing fills, removals of the existing fills, overexcavations, excavation inclination, final native subgrades preparation and proofrolling, fill placement and compaction of fills, slab-on- grade-floor installation, footing embedment depth, and verification of the allowable bearing capacity value, we recommend that PGE’s on-site geotechnical engineer should inspect all the above activities. A list of inspection items are provided later on in Section 13.0, ‘Geotechnical Special Inspection’ of this report. It is recommended that the above construction activities be monitored by a representative from our firm since we have the prior knowledge, familiarity, and better understanding with our recommendations. ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 31 of 41 10.3 Foundation Recommendations As mentioned earlier, the footings of the proposed new building addition can be supported on conventional shallow spread footings and continuous strip footings to be placed directly over the final native subgrades consisted of denser native soil deposit encountered at approximately 2 feet depth below the existing grades. As recommended earlier in 9.0, ‘Executive Summary’ of this report, the final native subgrades must be prepared as ‘competent’ native subgrades by thoroughly and adequately ‘redensification’ and ‘proofrolling’ of the native subgrades to firm and unyielding conditions, prior to placing the footings. Allowable Bearing Capacity The footings placed directly above the final ‘competent’ native subgrades may be designed for an allowable net bearing capacity value of 2000 psf. The “net allowable bearing pressure” refers to the pressure that can be imposed on the soil at foundation level resulting from the total of all dead loads plus the long - term live loads, exclusive of the weight of the footing or any backfill placed above the footing, i.e., these loadings can be ignored in calculating footing sizes. For short-term loads, such as wind and seismic (earthquake), a 1/3 increase in the above net allowable capacity can be used. We recommend that continuous footings have a minimum width of 18 inches and individual column footings a minimum width of 24 inches. All exterior footings should bear at least 18 inches below the final adjacent finish grade to provide adequate confinement of the bearing materials and frost protection. Settlement Based on our settlement potential evaluation of the shallow foundation options, we anticipate that properly designed and constructed foundations supported on the recommended bearing materials should experience total settlement of less than 1 inch for the allowable bearing pressures presented above. Differential settlement could be on the order of ¼ to ½ inch between similarly loaded foundations over a distance of 50 feet of continuous footings. This estimation was done without the aid of any laboratory consolidation test data, but on the basis of our experience with similar types of structures and subsoil conditions. The soil response to applied stresses caused by building and other loads is expected to be predominantly elastic in nature with most of the settlements occurring during construction as loads are applied; however, due the fines content of the site soils, the estimated settlements could occur over a longer time, and disturbance of the foundation subgrades during construction could result in larger settlements than predicted. ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 32 of 41 Lateral Load Resistance Lateral foundation loads can be resisted by friction between the foundation base and the underlying supporting soil, and by passive earth pressure acting on the face of the embedded portion of the fo undation below the grades. For frictional resistance, a coefficient of 0.35 can be used. For passive earth pressure, the available resistance can be computed using an equivalent fluid pressure of 300 pcf, which includes a factor of safety of 1.5. This value assumes the foundation must be poured "neat" against the undisturbed native soils or structural fill placed and compacted as described earlier in Section 10.1.8, 'Fill Placement and Compaction Requirements' of this report. The passive earth pressure and friction components may be combined provided that the passive component does not exceed two-thirds of the total resistance. Footing Subgrade Inspection We recommend that PGE representative examine the bearing materials prior to placing forms or rebar. Variations in the quality and strength of the potential bearing soils can occur with depth and distance away from the test pits. Therefore, a careful evaluation of the bearing material and the design bearing capacity value as recommended in this report must be verified at the proposed footing locations at the time of footing construction. 10.4 Slab-on-grade Floor Slab-on-grade floor for the new building addition should not be placed over topsoils, uncontrolled existing fills, loose and yielding native soils, or any soils containing heavy roots. The slab-on-grade floor should bear directly on structural fill pad of a minimum of 12 inches of compacted thickness, which must be adequately compacted and to be placed on ‘redensified’ and adequately ‘proofrolled’ final native subgrades, considered to be as ‘competent’ native subgrades as described earlier in Section 9.0, ‘Executive Summary’ of this report. The ‘competent’ native subgrade is described as the subgrade that is compacted to firm and unyielding conditions, which will be evidenced during the proofrolling. The fills thickness may vary based on the final floor elevation and the depth of the acceptable final native subgrades to be determined by the PGE’s geotechnical engineer. After the final slab subgrade preparation is completed, the slab should be provided with a capillary break to retard the upward wicking of ground moisture beneath the flo or slab. The capillary break would consist of a minimum of 6-inch thick clean, free-draining sand or pea gravel. The structural fill requirements specified in Section 10.1.7, Structural Fill, could be used as capillary break materials except that there sho uld be no more than 2 percent of fines passing the no. 200 sieve. Alternatively, ‘Gravel Backfill for Drains’ per 2023 WSDOT Standard Specifications 9-03.12(4) can be used as capillary break materials. This layer should ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 33 of 41 be placed and compacted to an unyielding condition. Where moisture by vapor transmission is undesirable, we recommend the use of a vapor barrier such as a layer of durable plastic sheeting (such as Crossstuff, Moistop, or Visqueen) between the capillary break and the floor slab to prevent th e upward migration of ground moisture vapors through the slab. This is particularly importance where moisture migration through the slab is an issue, such as where adhesives are used to anchor carpet or tile to the slab. During the casting of the slab, care should be taken to avoid puncturing the vapor barrier. At owner’s or architecture’s discretion, the membrane may be covered with 2 inches of clean, moist sand as a ‘curing course’ to guard against damage during construction and to facilitate uniform curing of the overlying concrete slab. The addition of 2 inches of sand over the vapor barrier is a non -structural recommendation. A cross-sectional view of the slab-on-grade floor showing the above features is provided in Figure 5, Footing, Slab-on-grade, Footing Drain of this report. The final slab subgrade consisted of adequately compacted, new imported structural fills, a modulus of subgrade reaction value of about 150 pounds per cubic inch (pci) can be used to estimate slab deflections, which could arise due to elastic compression of the subgrades 11.0 Infiltration Potential Evaluation As a part of the scope of this geotechnical study the permeability characteristic of the native soil was evaluated to assess the feasibility of using a below grade infiltration system in this site for managing the stormwater runoff from the proposed new building addition. To achieve this, the surface and subsurface conditions in the proposed new infiltration system area was observed as a basis for determining a site-specific measured infiltration rate and assessing the feasibility of the subsurface soil to support the infiltration system. Specifically the scope of services includes the following:  Reviewing the available geologic, hydrogeologic, and geotechnical data for the site area;  Exploring surface and subsurface conditions by reconnoitering the site and monitoring the excavation of two test pits at the site;  Performing one small-scale pilot infiltration test (PIT) in one of the test pits in accordance with the 2021 King County Surface Water Design Manual (KCSWDM);  Performing laboratory sieve analysis test per ASTM method to determine the grain-size distribution of the native soil;  Describing surface and subsurface conditions, including soil type and depth to groundwater if encountered;  Providing our opinion about the feasibility of on-site infiltration in accordance with the 2021 KCSWDM, including a design infiltration rate based on the measured infiltration rate from our in- situ infiltration testing; ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 34 of 41  Preparing this written Soils Report in accordance with the section C.2.3.1 of 2021 KCSWDM, summarizing our site observations and conclusions and our recommendations and design criteria - along with the supporting data. King County has adopted per the King County Code Chapter 2.98 the 2021 KCSWDM for design of stormwater management system. We evaluate the feasibility of stormwater infiltration at the site using the procedure for a soil report, investigation, and infiltration rates testing in the 2021 KCSWDM. We have assumed that, if infiltration facilities are planned in this site, they will serve less than about 1 acre of tributary area. The 2021 KCSWDM states that groundwater mounding analysis is not required for infiltration facilities serving less than 1 acre of tributary area provided that a minimum 3-foot separation is maintained between the bottom of the facility and restrictive layer such as seasonal high groundwater level or any low permeability stratum (i.e., recessional lacustrine deposits and glacial till). Infiltration Test The infiltration test was conducted using the procedure outlined for small-scale Pilot Infiltration Test (PIT) in Reference 6-A of 2021 KCSWDM manual. The test was conducted in the test pit TP-1 at approximately 4 feet (48 inches) depth below the current grades in the native, light gray, gravelly sand deposit. The test pit was measured approximately 4 feet (length) by 3 feet (width) with an area encompassing about 12 square feet. As per the manual guideline, the test includes three phases: a pre-soaking period of 6 hours, a constant or steady-head phase, and a falling-head phase. Following the test depth reached at 4 feet, a measuring staff gauge with 1/8-inch divisions was placed at the base of the test pit to monitor the water level drop during the testing. Then water was poured into the test pit using a garden hose which was sourced from an outlet of the exiting residence. The water was poured via a rigid plastic pipe of 6-inch diameter, on a splash board placed at the base of the test pit, to minimize the side-wall erosion or excessive disturbance, turbulence, and scouring at the test pit bottom. Excessive erosion and bottom disturbance will result in clogging of the infiltration receptor and yield lower than actual infiltration rates. Following the completion of the test pit preparation, an attempt was made to fill up the test pit with pouring water for an almost hour and half to build up a 12 inches head of water above the bottom of the test pit. However, it was never achieved hence the test had to terminate. We believe that the native soil consisted of gravely sand caused the water to drain fast and as a result the head was never built up. During the test, instantaneous flow rate readings were taken and based on that the actual infiltration rate was determined as 8 inches per hour. After the test was abandoned, the bottom of the test pit was advanced further below to almost 8 feet depth below the grade to determine the type of sediment that accumulated at the bottom of the test depth, and to observe if there is any presence of geological features like restrictive layer (silt, clay deposit or glacial till), heavy mottling signs, groundwater, and mounding within the ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 35 of 41 maximum exploration depth. None of these features were noticed and the gravelly sand deposit was observed continued upto the bottom of the test pit. Final Design Infiltration Rate The actual infiltration rate obtained from our field PIT test is representative of the measured (unfactored) infiltration rate of the soil test at the test depth at the test pit location, TP-1. The section 5.2.1 of 2021 KCSWDM recommends that correction factors be applied to the measured infiltration rates to estimate the long-term deign infiltration rate. Different correction factors are applied depending on the facility type. The correction factors account for the number of infiltration tests done in relation to the size of the infiltration area, site soil variability, test method, and other factors. The foll owing equations are used to determine the final design infiltration rate. Ksat Design = Ksat Initial x CFT, where, Ksat initial and Ksat design are the field measured and the final design infiltration rates respectively, and Total Correction Factor, CFT = CFg x CFt x CFm The table below summarizes the correction factors and the total correction factors that, in our opinion, are suitable for the design of the system. Correction factors were selected based on our project understanding, observed soil conditions, criteria provided in the manual, and our local experience assisting in the design of stormwater infiltration facilities. Table 5 - Infiltration Rate Test Pit No. Test Depth, Inches Soil Description @ Test Depth Field Measured Uncorrected Infiltration Rate (in/hr) Correction Factors Applied Corrected Infiltration Rate (in/hr) Ksat - Initial (Field) Rate CFg CFt CFm CFT Ksat - (Design Rate) TP-1 48 Lt. Gray, Sand w/Gravel 8 1.0 0.5 0.9 0.45 3.6 In/hr – inches/hour CFg – Correction factor for geometry - 1.0* CFt – Correction factor for test method uncertainty - for small scale PIT 0.5 CFm – Correction factor for long-term conductivity loss due to plugging - for medium sand 0.9 CFT – Total correction factor ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 36 of 41 *Based on the final geometry of the infiltration facility, the CFg correction factor need to be adjusted by the project civil engineer. The above final design infiltration rate is applicable to facility that will be located in proximity to the infiltration test location, at or near the test depth, in the same soil tested, and in no groundwater condition within the test pit depth. Should any of the above criteria is found to be different then PGE should be contacted to perform additional in-situ PIT at the proposed location of the infiltration facility. A contingency plan is to be kept in place if such situation occurs. Performance/Verification Testing – Field percolation performance verification testing shall be conducted as per section 5.2.1 ‘Performance Testing’ of 2021 KCSWDM to demonstrate that the final design infiltration rate recommended above is valid, which is critical to ascertain the infiltration facility would perform as designed. In addition to the verification testing, we recommend that the soil and groundwater conditions should be verified at the proposed infiltration system location prior to installing the system. The verification is critical to find out if groundwater, perched water seepage, signs of mottling, and restrictive layer (like till) are present at the infiltration facility. Also, the verification should be made to ascertain if the vertical separation requirement below the bottom of the facility is available. PGE should observe construction of the infiltration system. Due to the possibility of the variability of the native soils across the depth and the horizontal extent of the site, the soils may change, hence their infiltration rates. In the event, the soil condition and the infiltration rate are found to be different during the performance testing than the soil conditions and the final design infiltration rate obtained during this study, the infiltration system may need to be relocated and redesigned from the original plan to reflect the actual soil condition and the infiltration rate to be encountered during the performance testing. A contingency plan is to be kept in place if such situation occurs. Infiltration Feasibility It is our opinion that the native, gravelly sand (USCS Soil Classification: SP) encountered below the topsoils is considered suitable as a ‘full-infiltration’ receptor as defined in the 2021 KCSWDM section C.2.3 for stormwater to be generated by the new impervious surfacing for the proposed new addition. Our PIT test was performed on this soil at 4 feet depth below the grade. A sieve analysis test was performed on this material and the result is shown in Figure B-1 of this report. The gravelly sand is described as ‘Sand’ (USCS Soil Class: SP) report. There was no groundwater, perched water seepage, signs of mottling, restrictive layer (like till), and mounding was noticed within the maximum exploration depth of 8 feet. Based on these geological factors, in our opinion, the sand deposit can support a ‘full-infiltration’ system such as a drywell according to sections C.2.3 and C.2.3.2 of the 2021 KCSWDM. For designing the ‘limited-infiltration’ ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 37 of 41 drywell system within the King County Urban Growth Area, section C.2.3.4 specifies for each 1,000 square feet of new impervious area surface area, the gravel filled drywell should have a minimum volume of 360 cubic feet in soils classified as sand. We recommend that any proposed infiltration system be placed as to not negatively impact any proposed or existing nearby structure and also meet all required setbacks from the existing property lines, structures, and sensitive areas as discussed in the drainage manual. The various aspects of the design requirements described in the 2021 KCSWDM should be incorporated into the system design. The 2021 KCSWDM requires a minimum 3 feet or more of permeable soil beneath below grade infiltration trench or drywell bottom for their proper functioning. In the test pit location we observed that the permeable soil extends upto the maximum exploration depth of approximately 8 feet depth below the existing grade. We believe that the observed permeable soil is likely to e xtend further below the test pit depth, which must be verified during the actual installation of the infiltration system. W e expect that if the proposed drywell is designed properly will be able to maintain the minimum 3 feet of permeable soil requirement below the system. Based on the minimum depth requirement the final size and the bottom elevation of the drywell or the infiltration trench should be determined. The vertical separation depth is required for providing space for the stormwater in the drywell to infiltrate within the soil in the minimum depth requirement. Additional Study We recommend that an additional study is to be performed by a hydro -geologist to determine if there will be any impact of the highest water level in Cedar River (typically occurs in the wet winter months) on the proposed infiltration system. The result of the study will provide if the groundwater condition observed in our test pit in the month of May would remain same due to the impact of the highest water level in the river. 12.0 Additional Services Additional services described below can be performed by PGE in the event the project requires such services. These services will be performed upon written authorization of the client or the civil engineer, and with additional cost to perform such services, under a separate contract between PGE and the client. 12.1 Design Phase Engineering Services As the geotechnical engineer of record for the proposed development, at owner’s option, PGE can perform a review of the final project plans and specifications to verify that the geotechnical recommendations of this report have been properly interpreted and incorporated into the project final design and specifications, and that the impact of the final site grades, the proposed building and its footing, and any other structure. ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 38 of 41 12.2 Construction-time Testing and Inspection As the geotechnical engineer of record for the proposed development, at owner’s option, PGE can provide geotechnical consultation, material testing, and construction monitoring servi ces during the construction of the project recommended earlier in Section 10.2, ‘Construction Monitoring’ of this report. These services are important for the project to confirm that the earthwork and the general site development are in compliance with the general intent of design concepts, specifications, and the geotechnical recommendations presented in this report. Also, participation of PGE during the construction will help PGE engineers to make on-site engineering decisions in the event that any variations in subsurface conditions are encountered or any revisions in design and plan are made. PGE can assist the owner before construction begins to develop an appropriate monitoring and testing plan to aid in accomplishing a fast and cost-effective construction process. 13.0 Geotechnical Special Inspection The construction of the proposed development in this site involves several aspects of the geotechnical engineering that are considered to be critical for the successful completion of the project and continue that throughout the project life. Therefore, PGE recommends that the following geotechnical special inspection services to be performed during the construction of the proposed development. According to PGE, the following items should be considered as a minimum but not limited to.  A professional geotechnical engineer should be retained to provide geotechnical consultation, material testing, and construction monitoring services during the construction of the project.  A pre-construction meeting should be held on-site to discuss the geotechnical aspects of the development and the special inspection services to be performed during the construction.  The site preparation activities including but not limited to stripping, cut and filling, final subgrade preparation for foundation, floor slab, paved driveway, and retaining wall be monitored by a geotechnical engineer or his representative under the engineer’s supervision.  A list of the possible items that require special geotechnical inspection and approval by the geotechnical engineer is as follows:  Stripping of topsoils.  Removal of loose, native soils, and the uncontrolled, existing fills.  Compaction and proofrolling of any exposed native subgrades that are intended to provide direct support for any load bearing structure such as new fill pad, slab-on-grade floor, footing, retaining wall, and paved driveway.  Any structural fills to be used in this site, and structural fills placement and its compaction. ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 39 of 41  Temporary or permanent excavation inclinations, and excavation stability.  Backfilling and its compaction, and drainage behind retaining walls.  The footing bearing materials, bearing capacity value, and the embedment depth of the footings prior to placing forms and rebars.  Subgrade preparation for soil supported slab-on-grade floors.  Subgrade preparation for paved driveways.  Compaction of CSBC, CSTC, and laying of concrete pavement in driveway.  Site drainage.  Installation of drainage system such as footing excavation drain and footing drain, and daylighting of such drains and downspout or roof drains.  Bedding and the backfilling materials, and backfilling of utility lines.  Performing field verification percolation test at the proposed drywell location.  Observing the construction of drywell system.  Any other items specified in the approved project plans to be prepared by other consultants relevant to the geotechnical aspect of the project. 14.0 Report Limitations The conclusions and recommendations presented in this report are based on our soil investigation, the laboratory test results, geological literature review, and our engineering evaluation. The study was performed using a mutually agreed-upon scope of work between PGE and the client. It should be noted that PGE cannot take the responsibility regarding the accuracy of the information provided in the project plan prepared by other consultants. If any of the information considered during this study is not correct or if there are any revisions to the plans for this project, PGE should be notified immediately of such information and the revisions so that necessary amendment of our geotechnical recommendations can be made. If such information and revisions are not notified to PGE, no responsibility should be implied on PGE for the impact of such information and the revisions on the project. Variations in subsurface (soil and groundwater) conditions may reveal during the construction of the proposed below grade infiltration system. The nature and the extent of the subsurface variations may not be evident until construction occurs. If any subsurface conditions are encountered at the site that are different from those described in this report, we should be notified immediately to review the applicability of our recommendations if there are any changes in the project scope. This report may be used only by the client and for the purposes stated, within a reasonable time from its issuance. Land use, site conditions (both off and on-site), or others factors including advances in our understanding of applied science, may change over time and could materially affect our findings. Therefore, ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 40 of 41 this report should not be relied upon after 24 months from its issuance. PGE should be notified if the project is delayed by more than 24 months from the date of this report so that we may review to determine that the conclusions and recommendations of this report remain applicable to the changed conditions. The scope of our work does not include services related to construction safety precautions. Our recommendations are not intended to direct the contractors' method, techniques, sequences or procedures, except as specifically described in our report for consideration in design. Additionally, the scope of our work specifically excludes the assessment of environmental characteristics, particularly those involving hazardous substances. This report including its evaluation, conclusions, specifications, recommendations, or professional advice has been prepared for planning and design purposes for specific application to the proposed project in accordance with the generally accepted standards of local practice at the time this report was written. No warranty, express or implied, is made. This report is the property of our client Patty Thumann, and has been prepared for the exclusive use of our client and its authorized representatives for the specific application to the proposed development at the subject site in Renton, Washington. It is the client's responsibility to see that all parties to this project, including the civil engineer, designer, contractor, subcontractor, future homeowner, etc., are made aware of this report in its entirety. The use of information contained in this report for bidding purposes should be done at the contractor's option and risk. Any party other than the client who wishes to use this report shall notify PGE of such intended use and for permission to copy this report. Based on the intended use of the report, PGE may require that additional work be performed and that and updated report be reissued. Noncompliance with any of these requirements will release PGE from any liability resulting from the use this report. ___________________________________________________________________________________________________________________________ Proposed New Addition Geotechnical Engineering Study 13411, SE 151st St Renton, WA 98058 PGE Project No. 24-754 June 28, 2024 Page 41 of 41 Closure We trust the information presented in this report is sufficient for your current needs. We appreciate the opportunity to provide the geotechnical services at this phase of the project and look forward to continued participation during the design and construction phase of this project. Should you have any questions or concerns, which have not been addressed, or if we may be of additional assistance, please do not hesitate to call us at 425-218-9316. Respectfully submitted, Santanu Mowar, P.E. D:\Geotechnical\2024-proj\24-754 Attachments: Figure 1 Vicinity Map Figure 2 Aerial Site View Figure 3 Site & Exploration Plan Figure 4 Flood Hazard Map Figure 5 Footing, Slab-on-grade, Footing Drain Details Figure 6 Notes Appendix A Soil Test Pit Log Appendix B Laboratory Test Report N Site Figure 1 – Vicinity Map N Figure 2 – Aerial Site View Proposed Addition Area Figure 3 – Site & Exploration Plan TP-1 N Figure 4 – Flood Hazard Map FOOTING, SLAB-ON-GRADE, & FOOTING DRAIN Conceptual only (not a construction drawing) Footing Wall Slope backfill w/ minor slope C J D K A Floor Level F Concrete slab-on-grade B 6" min. gravel on top 3" min. gravel at bottom H G E L Vapor Barrier Capillary Break Layer Gravel Base Footing must be placed on native subgrade, to be prepared as ‘competent’ subgrade, which must be verified on-site by PGE’s geotechnical engineer. Footings must not be placed over topsoils, and underlying native soil containing root zone Drain Rocks Drain Pipe Mirafi 140N Footing Excavation Slope Compacted Backfills Roof Drain New fill pad under the slab must be placed over ‘competent’ native subgrade G C New imported structural fill pad below slab IActual fill thk. below the slab-on-grade floor should be decided based on final floor elev., removal depths of topsoil, root zone, and tree rootball, and depth of final native ‘competent’ subgrade to be encountered, which must be verified on-site by PGE’s geotechnical engineer. Fills must be placed & compacted as per Curing Sand Layer A Figure 5 Not to Scale Project No. –24-754 Project – Patty Thumann Site – New Addition 13411, SE 151st St., Renton, WA 98058 FOOTING, SLAB-ON-GRADE, & FOOTING DRAIN Final native subgrades supporting the footings, the slab-on-grade floor, or the new structural fill pad directly, must be thoroughly ‘redensified’ and then adequately ‘proofrolled’ to firm & unyielding conditions to prepare the final native subgrades as ‘competent’ native subgrades, prior to placing the footings, slab-on-grade floor, and new fill pad. The allowable bearing capacity value of 2000 psf to be verified on-site by PGE’s geotechnical engineer @ the final footing subgrades or at the top of the newly placed compacted fill pad, prior to placing rebars and forms. Topsoils and tree rootbulbs, and any unsuitable native soil containing heavy root zone must be removed completely prior to the preparation of the final ‘competent’ native subgrades. Excavation face slope should be determined based on the actual soil and groundwater conditions to be exposed during the construction. Non-woven Geotextile Filter Fabric -Mirafi 140 N must wrap around the drain rocks to be placed around the footing drain pipe and the vertical drainage layer to be installed against the wall, to prevent migration of fines into the drain rocks. A B C D K Capillary Break layer – min. 6" thk, of free-draining 5/8-inch crushed rocks containing no more than 2% fines or pea-gravel. Slab-on-grade floor should be placed directly on a capillary break layer in unheated areas e.g., garage, storage rooms. E H L NOTES:- F G Drain rocks -the drain pipe must be enveloped by drain rocks consisted of ¾” minus washed gravel (free draining). Stormwater roof drain,must be tightlined and must not be connected to footing drain. Pipe should be sloped towards approved discharge point so that no backflow should occur into the pipe. J Curing Sand Layer -as an additional layer can be placed above the vapor barrier or plastic membrane to guard the membrane against damage during construction and to facilitate uniform curing of the overlying concrete slab. Vapor Barrier – a durable 10 to 15-mil. plastic membrane be placed over capillary break layer as a vapor retarder. Backfill compaction -void areas to be created by overexcavation of topsoils, rootzone, and tree rootbulbs, for achieving footing embedment depth of minimum 18 inches below the current grades, and below the slab-on-grade floor must be backfilled with approved new structural fills. The fills must be compacted to 95% of fills’ max. dry density value (to be determined as per the laboratory Mod. Proctor Test ASTM D1557). The fills to be placed should be compacted with care within the horizontal distance equal to the height of the footing wall to avoid over compaction and overstressing the footing wall & the footing. No heavy compaction equipment such as vibratory roller or hoe-pac be used to compact the fills because of these equipment will impose excess surcharge loading on the wall, causing a lateral instability to the footing wall & the footing. Fills must be placed in 12 inch thick loose lifts and to be compacted with a walk-behind, hand-held, big, heavy-duty vibratory plate compactor (similar to TMG-PC33OK Reversible Plate Compactor with 14 HP Kohler Engine) close to the footing wall. The new imported structural fills should be used as per the recommendations provided in PGE’s geotechnical report 24-754. Gravel base -Min. 6" thk compacted (95% or more), which must be extended 6" beyond both sides of the footing. Wall Footing Drain - 6" minimum diameter, perforated or slotted rigid concrete, metal, or plastic pipe with tight plastic joints, with a positive gradient (~2%) towards thee discharge ends sufficient to generate gravity flow w/out backflow to occur into the pipe, and provided with accessible cleanouts at regular intervals. The pipe must be taken to final discharge point (approved). The pipe must be placed as low as possible, at least 6 inches below footing or crawl space. Perforations (¼” max. diameter) to be in lower half of pipe, with lower quadrant segment un-perforated to facilitate water flow. Slotted pipe to have 1/8" maximum width slots. Must NOT be tied to roof downspout or perimeter footing grain lines. The drain pipe must be enveloped w¾” minus washed gravels (free draining), which then be wrapped around with Mirafi 140N to prevent migration of fines into the din rocks and clog the pipe. Fill Thickness Under Slab -Actual fill thk. should be decided by the on-site geotechnical engineer based on the final floor elev. and the depth of the final native ‘competent’ subgrade to be encountered, which must be verified on-site by PGE’s geotechnical engineer. Fills must be placed & compacted as per the recommendations provided in I A M PGE’s Geotechnical Report -In addition to the above recommendations the designer and the contractor of the project must read PGE’s geotechnical report no. 24-754, for additional recommendations and better understanding of the above recommendations. Figure 6 Not to Scale Project No. –24-754 Project – Patty Thumann Site – New Addition 13411, SE 151st St., Renton, WA 98058 Appendix A Soil Test Pit Log Figure A-1 Not to Scale TEST PIT –1 0 ft 1 ft 2 ft 3 ft 4 ft 5 ft 6 ft 0 ft Soil Layer Descriptions Laboratory Test ResultsSample Depth Sample Nos.Moist. Content - #200 Sieve Soil Layer Depth USCS Soil Class Test Pit Depth4 ft4 ft Test Pit Width Surface Elev. Ft.Date of Excavation Test Pit Depth Water/Seepage Depth Mottling Depth Ground Cover Cave in Depth Notes - Landscape grass 8 ft None Top Soils – Brn., Sandy Silt w/ organics & root V. Moist, Soft 0 – 0.5 ft 2 1 None 05/21/2024 Test Pit Location See site plan Permeability None 2 Project No. –24-754 1 0.5 ft –8 ft 7 ft 8 ft Field Logging by ASTM D5434-12 Soil Sampling by ASTM D-75-19 Visual-Manual Soil Identification by ASTM D2488-17 9 ft 10 ft Lt. Gray, Sand w/ small to large size Gravel, Cobble, Boulder Moist, Loose upto 2 ft & Med. dense below this depth SP S-1 @ 5 ft 14.8 %4.9 % (Sieve Test Graph B-1) Infiltration test performed @ 4 ft below grade; Final design infiltration rate K ~ 3.6 in/hr Project – Patty Thumann Site – New Addition 13411, SE 151st St., Renton, WA 98058 KEY TO EXPLORATION LOG Sample Descriptions: Classification of soils in this report is based on visual field and laboratory observations, which include density/consistency, moisture condition, grain size, and plasticity estimates, and should not be construed to imply field or laboratory testing unless presented herein. Visual-manual classification methods in accordance with ASTM D-2488-17 were used as an identification guide. Where laboratory data available, soil classifications are in general accordance with ASTM D2487-17. Soil density/consistency in borings is related primarily to the Standard Penetration Resistance values. Soil density/consistency in test pits is estimated based on visual observations of excavations. Undrained shear strength = ½ unconfined compression strength. RELATIVE DENSITY OR CONSITENCY VS. SPT N-VALUE COARSE GRAINED SOILS: SAND OR GRAVEL FINE GRAINED SOILS: SILT OR CLAY Density N (Blows/ft.) Approx. Relative Density (%) Consistency N (Blows/ft.) Approx. Undrained Shear Strength (psf) Very Loose 0 – 4 0- 15 Very Soft 0 – 2 <250 Loose 4 – 10 15 – 35 Soft 2 – 4 250 –500 Medium Dense 10 – 30 35 – 65 Medium Stiff 4 – 8 500 – 1000 Dense 30 – 50 65 – 85 Stiff 8 – 15 1000 – 2000 Very Dense >50 85 – 100 Very Stiff Hard 15 – 30 > 50 2000 – 4000 > 4000 MOISTURE CONTENT DEFINITIONS Dry Absence of moisture, dusty, dry to the touch Moist Damp but no visible water Wet Visible free water, from below water table DESCRIPTIONS FOR SOIL STRATA AND STRUCTURE General Thickness or Spacing Structure General Attitude Parting < 1/16 in Pocket Erratic, discontinuous deposit of limited extent Near Horizontal 0 - 10 deg Seam 1/16 - 1/2 in Lens Lenticular deposit Low Angle 10 - 45 deg Layer ½ - 12 in Varved Alternating seams of silt and clay High Angle 45 - 80 deg Stratum > 12 in Laminated Alternating seams Near Vertical 80 - 90 deg Scattered < 1 per ft Interbedded Alternating Layers Numerous > 1 per ft Fractured Breaks easily along definite fractured planes Slickensided Polished, glossy, fractured planes Blocky, Diced Breaks easily into small angular lumps Sheared Disturbed texture, mix of strengths Homogeneous Same color and appearance throughout Appendix B Laboratory Test Reports Particle Size Distribution Report PE R C E N T F I N E R 0 10 20 30 40 50 60 70 80 90 100 GRAIN SIZE - mm. 0.0010.010.1110100 % +3"Coarse % Gravel Fine Coarse Medium % Sand Fine Silt % Fines Clay 0.0 24.8 19.7 11.5 23.8 15.3 4.9 6 i n . 3 i n . 2 i n . 1½ i n . 1 i n . ¾ i n . ½ i n . 3/ 8 i n . #4 #1 0 #2 0 #3 0 #4 0 #6 0 #1 0 0 #1 4 0 #2 0 0 TEST RESULTS Opening Percent Spec.*Pass? Size Finer (Percent)(X=Fail) Material Description Atterberg Limits (ASTM D 4318) Classification Coefficients Date Received:Date Tested: Tested By: Checked By: Title: Date Sampled:Location: TP-1 Sample Number: S-1 Depth: @ 5 ft below grade Client: Project: Project No:Figure Lt. Gray, Poorly graded Sand w/ Gravel 3 inch 1.5 inch 1 inch 3/4 inch 1/2 inch 3/8 in #4 #10 #20 #40 #60 #80 #100 #140 #200 100.0 89.0 82.5 75.2 69.2 65.2 55.5 44.0 28.8 20.2 15.8 12.1 9.9 6.8 4.9 NP NV NP SP A-1-a 40.9296 28.8254 6.6044 3.0512 0.9170 0.2320 0.1513 43.65 0.84 05-21-24 05-23-24 Sraboni Santanu Mowar, PE Principal 05-21-24 Patty Thuman New Building Addition @ 13411, SE 151st St, Renton, 98058 24-754 PL=LL=PI= USCS (D 2487)=AASHTO (M 145)= D90=D85=D60= D50=D30=D15= D10=Cu=Cc= Remarks *(no specification provided) B-1