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Home My WebLink AboutEx_08_Geotech_ReportSouth Sound Geotechnical Consulting August 3, 2020 LSB Petroleum, LLC 11706 - 164th Ave SE Renton, WA 98059 Attention: Mr. Lakhpal Brar Subject: Geotechnical Engineering Report Brar Short Plat 8054 S. 132nd Street Renton, Washington SSGC Project No. 20058 Ms. Brar, South Sound Geotechnical Consulting (SSGC) has completed a geotechnical assessment for the planned short plat on the above addressed property in Renton, Washington. Our services have been completed in general conformance with our proposal P20050 (dated June 23, 2020) and authorized per signature of our agreement for services. Our scope of services included completion of five test pits, one infiltration test, laboratory testing, engineering analyses, and preparation of this report. PROJECT INFORMATION The project property is on the north side of S. 132nd Street and encompasses approximately 0.75 acres. It is rectangular with the long axis trending in a north-south direction. A single-family residence currently occupies the southern portion. One shop/garage structure is in the northwestern portion. The property is on a south-facing slope with elevation change on the order of 30 feet (+/-) per Google satellite imagery. Proposed development includes short plating the property into four individual lots. We understand the existing residence will remain as one of the lots. Conventional spread footing foundations are anticipated for support of new structures with concrete slab-on-grade garage floors. SUBSURFACE CONDITIONS Subsurface conditions were characterized by completing four test pits and one infiltration test on the site on July 8, 2020. Test holes were advanced to final depths between 6 and 10 feet below existing ground surface. Approximate locations of the test pits and infiltration test site are shown on Figure 1, Exploration Plan. Logs of these explorations are provided in Appendix A. A summary description of observed subgrade conditions is provided below. RECEIVED 01/13/2021 AMorganroth PLANNING DIVISION DocuSign Envelope ID: DE8E6DBB-4A7F-41D7-9A73-AA7FD64486D1 Geotechnical Engineering Report SSGC Brar Short Plat – S. 132nd St Renton, WA SSGC Project No. 20058 August 3, 2020 2 Soil Conditions Fill was observed at the surface in two of the test pits and extended to depths of about 1 to 1.5 feet. It consisted of loose silt, sand, and gravel and likely sourced from the property during construction of existing improvements. Topsoil was below the fill (and below the surface in the remaining excavations) and ranged in thickness from about 6 inches to 1 foot at the test locations. Native soil below the topsoil consisted of an upper silty sand with variable gravel. This soil was in a loose condition and ranged in thickness from about 6 inches to 2 feet, where present. An interpreted weathered glacial till was below the upper soil and consisted of silty sand with gravel and occasional cobbles in a loose to medium dense condition. This weathered zone extended to depths between 3 and 46feet. Dense glacial till was below the weathered zone and extended to the termination depths of the explorations. Groundwater Conditions Groundwater was not observed in the test holes at the time of excavation. However, mottling of the upper native silty sand and weathered till layer implies perched groundwater conditions during the wetter seasons of the year. The presence of dense glacial till can create perched groundwater in the upper soils. Groundwater levels will vary throughout the year based on seasonal precipitation and on- and off-site drainage patterns. Geologic Setting Soils within the development area have been classified by the NRCS in the Soil Survey of King County, Washington. Surface soils are mapped as Alderwood gravelly sandy loam. This soil is described as forming in glacial till. Native soils observed in the test holes appear to consist of a thin layer of alluvium over glacial till, conforming to the mapped soil type. GEOTECHNICAL DESIGN CONSIDERATIONS Development for the proposed plat is considered feasible based on observed soil conditions in the test pits. Properly prepared native soils can be used for support of conventional spread footing foundations, floor slabs, and pavements. Infiltration to assist in stormwater control will be difficult at this site. The presence of dense glacial till at fairly shallow depth will limit infiltration to lateral flow through the upper silty sand and weathered till horizons. Infiltration systems will be limited to shallow dispersion facilities as allowed by the city. Recommendations presented in the following sections should be considered general and may require modifications at the time of construction. They are based upon the subsurface conditions observed in the test pits and the assumption that finish site grades will not be substantially different than existing grades. It should be noted subsurface conditions across the site can vary from those depicted on the exploration logs and can change with time. It should be expected that fill of unknown type and thickness may be DocuSign Envelope ID: DE8E6DBB-4A7F-41D7-9A73-AA7FD64486D1 Geotechnical Engineering Report SSGC Brar Short Plat – S. 132nd St Renton, WA SSGC Project No. 20058 August 3, 2020 3 present due to historic uses of this site. Therefore, proper site preparation will depend upon the weather and soil conditions encountered at the time of construction. We recommend SSGC review final plans and further assess subgrade conditions at the time of construction, as warranted. General Site Preparation Site grading and earthwork should include procedures to control surface water runoff. Grading the site without adequate drainage control measures may negatively impact site soils, resulting in increased export of impacted soil and import of fill materials, t hereby potentially increasing the cost of the earthwork and subgrade preparation phases of the project. Site grading should include removal (stripping) of fill and topsoil or very loose or soft soils in building and pavement areas. Topsoil/fill extended to depths ranging from about 6 inches to 2 feet in the observed test holes, but may be deeper in other areas. Final stripping depths can only be determined at the time of construction. Subgrades should consist of firm, undisturbed native soils following stripping. General Subgrade Preparation Subgrades in building footprints and pavement areas should consist of firm, undisturbed native soils. We recommend exposed subgrades in building and conventional pavement areas are proofrolled using a large roller, loaded dump truck, or other mechanical equipment to assess subgrade conditions following stripping. Proofrolling efforts should result in the upper 1 foot of subgrade soils in building and conventional pavement areas achieving a compaction level of at least 95 percent of the maximum dry density (MDD) per the ASTM D1557 test method. Wet, loose, or soft subgrades that cannot achieve this compaction level should be removed (over-excavated) and replaced with structural fill. The depth of over-excavation should be based on soil conditions at the time of construction. A representative of SSGC should be present to assess subgrade conditions during proofrolling. Grading and Drainage Positive drainage should be provided during construction and maintained throughout the life of the development. Surface water should not be allowed to flow into construction excavations or fill areas. Structural Fill Materials The suitability of soil for use as structural fill will depend on the gradation and moisture content of the soil when it is placed. Soils with higher fines content (soil fraction passing the U.S. No. 200 sieve) will become sensitive with higher moisture content. It is often difficult to achieve adequate compaction if soil moisture is outside of optimum ranges for soils that contain more than about 5 percent fines. DocuSign Envelope ID: DE8E6DBB-4A7F-41D7-9A73-AA7FD64486D1 Geotechnical Engineering Report SSGC Brar Short Plat – S. 132nd St Renton, WA SSGC Project No. 20058 August 3, 2020 4 Site Soils: Topsoil and existing fill are not considered suitable for use as structural fill. Native soils contain sufficient fines (silt and clay) that make them moisture sensitive and difficult to use as structural fill. These soils would have to be moisture conditioned within optimal moisture content to use as structural fill. Optimum moisture is considered within about +/- 2 percent of the moisture content required to achieve the maximum dry density (MDD) per the ASTM D-1557 test method. If moisture content is higher or lower than optimum, soils would need to be dried or wetted prior to placement as structural fill. Import Fill Materials: We recommend imported structural fill placed during dry weather consist of material which meets the specifications for Gravel Borrow as described in Section 9-03.14(1) of the 2018 Washington State Department of Transportation (WSDOT) Specifications for Road, Bridge, and Municipal Construction manual (Publication M 41-10). Gravel Borrow should be protected from disturbance if exposed to wet conditions after placement. During wet weather, or for backfill on wet subgrades, import soil suitable for compaction in wetter conditions should be provided. Imported fill for use in wet conditions should conform to specifications for Select Borrow as described in Section 9-03.14(2), or Crushed Surfacing per Section 9-03.9(3) of the 2018 WSDOT M-41 manual, with the modification that a maximum of 5 percent by weight shall pass the U.S. No. 200 sieve for these soil types. Structural fill placement and compaction is weather-dependent. Delays due to inclement weather are common, even when using select granular fill. We recommend site grading and earthwork be scheduled for the drier months of the year. Frozen soil is not suitable as structural fill. Structural Fill Placement We recommend structural fill is placed in lifts not exceeding about 10 inches in loose measure. It may be necessary to adjust lift thickness based on site and fill conditions during placement and compaction. Finer grained soil used as structural fill and/or lighter weight compaction equipment may require significantly thinner lifts to attain required compaction levels. Granular soil with lower fines contents could potentially be placed in thicker lifts (1 foot maximum) if they can be adequately compacted. Structural fill should be compacted to attain the recommended levels presented in Table 1, Compaction Criteria. DocuSign Envelope ID: DE8E6DBB-4A7F-41D7-9A73-AA7FD64486D1 Geotechnical Engineering Report SSGC Brar Short Plat – S. 132nd St Renton, WA SSGC Project No. 20058 August 3, 2020 5 Table 1. Compaction Criteria Fill Application Compaction Criteria* Footing areas 95 % Upper 2 feet in pavement areas, flatwork, and utility trenches 95 % Below 2 feet in pavement areas, flatwork, and utility trenches 92 % Utility trenches or general fill in non-paved or -building areas 90 % *Per the ASTM D 1557 test method. Trench backfill within about 2 feet of utility lines should not be over-compacted to reduce the risk of damage to the line. In some instances, the top of the utility line may be within 2 feet of the surface. Backfill in these circumstances should be compacted to a firm and unyielding condition. We recommend fill procedures include maintaining grades that promote drainage and do not allow ponding of water within the fill area. The contractor should protect compacted fill subgrades from disturbance during wet weather. In the event of rain during structural fill placement, the exposed fill surface should be allowed to dry prior to placement of additional fill. Alternatively, the wet soil can be removed. We recommend consideration is given to protecting haul routes and other high traffic areas with free-draining granular fill material (i.e. sand and gravel containing less than 5 pe rcent fines) or quarry spalls to reduce the potential for disturbance to the subgrade during inclement weather. Structural fill placed on sloping ground should be constructed using a benched (stairstep) methodology. Benches should be cut level or with a slight downward incline into the slope in firm native soil. Benches should be wide enough to accommodate a minimum 20-ton vibratory roller and be a maximum of about two feet high. Earthwork Procedures Conventional earthmoving equipment should be suitable for earthwork at this site. Earthwork may be difficult during periods of wet weather or if elevated soil moisture is present. Excavated site soils may not be suitable as structural fill depending on the soil moisture content and weather conditions at the time of earthwork. If soils are stockpiled and wet weather is anticipated, the stockpile should be protected with securely anchored plastic sheeting. If stockpiled soils become wet and unusable, it will become necessary to import clean, granular soils to complete wet weather site work. Wet or disturbed subgrade soils should be over-excavated to expose firm, non-yielding, non-organic soils and backfilled with compacted structural fill. We recommend the earthwork portion of this project be completed during extended periods of dry weather. If earthwork is completed during the wet season (typically October through April) it may be necessary to take extra measures to protect subgrade soils. DocuSign Envelope ID: DE8E6DBB-4A7F-41D7-9A73-AA7FD64486D1 Geotechnical Engineering Report SSGC Brar Short Plat – S. 132nd St Renton, WA SSGC Project No. 20058 August 3, 2020 6 If earthwork takes place during freezing conditions, we recommend the exposed subgrade is allowed to thaw and re-compacted prior to placing subsequent lifts of structural fill. Alternatively, the frozen soil can be removed to unfrozen soil and replaced with structural fill. The contractor is responsible for designing and constructing stable, temporary excavations (including utility trenches) as required to maintain stability of excavation sides and bottoms. Excavations should be sloped or shored in the interest of safety following local and federal regulations, including current OSHA excavation and trench safety standards. Temporary excavation cuts should be sloped at inclinations of 1.5H:1V (Horizontal:Vertical) or flatter, unless the contractor can demonstrate the safety of steeper cut slopes. It should be noted outwash soils have the tendency to cave into open excavations. Shoring may be necessary for deeper utility trenches on this site. Permanent cut and fill slopes should be inclined at grades of 2H:1V, or flatter. A geotechnical engineer and accredited materials testing firm should be retained during the construction phase of the project to observe earthwork operations and to perform necessary tests and observations during subgrade preparation, placement and compaction of structural fill, and backfilling of excavations. Foundations Foundations can be placed on firm native soils or on a zone of structural fill above prepared subgrades as described in this report. The following recommendations are for conventional spread footing foundations: Bearing Capacity (net allowable): 3,000 pounds per square foot (psf) for footings supported on firm native soils or structural fill over native subgrades prepared as described in this report. Footing Width (Minimum): 16 inches (Strip) 24 inches (Column) Embedment Depth (Minimum): 18 inches (Exterior) 12 inches (Interior) Settlement: Total: < 1 inch Differential: < 1/2 inch (over 30 feet) Allowable Lateral Passive Resistance: 325 psf/ft* (below 12 inches) Allowable Coefficient of Friction: 0.40* *These values include a factor of safety of approximately 1.5. The net allowable bearing pressures presented above may be increased by one-third to resist transient, dynamic loads such as wind or seismic forces. Lateral resistance to footings should be ignored in the upper 12-inches from exterior finish grade unless restricted. DocuSign Envelope ID: DE8E6DBB-4A7F-41D7-9A73-AA7FD64486D1 Geotechnical Engineering Report SSGC Brar Short Plat – S. 132nd St Renton, WA SSGC Project No. 20058 August 3, 2020 7 Foundation Construction Considerations All foundation subgrades should be free of water and loose soil prior to placing concrete , and should be prepared as recommended in this report. Concrete should be placed soon after excavating and compaction to reduce disturbance to bearing soils. Should soils at foundation level become excessively dry, disturbed, saturated, or frozen, the affected soil should be removed prior to placing concrete. We recommend SSGC observe foundation subgrades prior to placement of concrete. Foundation Drainage Ground surface adjacent foundations should be sloped away to facilitate drainage. We recommend footing drains are installed around perimeter footings. Footing drains should include a minimum 4- inch diameter perforated rigid plastic or metal drain line installed along the exterior base of the footing. The perforated drain lines should be connected to a tight line pipe that discharges to an approved storm drain receptor. The drain line should be surrounded by a zone of clean, free-draining granular material having less than 5 percent passing the No. 200 sieve or meeting the requirements of section 9-03.12(2) “Gravel Backfill for Walls” in the 2018 WSDOT (M41-10) manual. The free- draining aggregate zone should be at least 12 inches wide and wrapped in filter fabric. The granular fill should extend to within 6 inches of final grade where it should be capped with compacted fill containing sufficient fines to reduce infiltration of surface water into the footing drains. Alternately, the ground surface can be paved with asphalt or concrete. Cleanouts are recommended for maintenance of the drain system. On-Grade Floor Slabs On-grade floor slabs should be placed on native soils or structural fill prepared as described in this report. We recommend a modulus subgrade reaction of 175 pounds per square inch per inch (psi/in) for native soil or compacted granular structural fill over properly prepared native soil. We recommend a capillary break is provided between the prepared subgrade and bottom of slab. Capillary break material should be a minimum of 4 inches thick and consist of compacted clean, free- draining, well graded coarse sand and gravel. The capillary break material should contain less than 5 percent fines, based on that soil fraction passing the U.S. No. 4 sieve . Alternatively, a clean angular gravel such as No. 7 aggregate per Section 9-03.1(4) C of the 2018 WSDOT (M41-10) manual could be used for this purpose. We recommend positive separations and/or isolation joints are provided between slabs and foundations, and columns or utility lines to allow independent movement where needed. Backfill in interior trenches beneath slabs should be compacted in accordance with recommendations presented in this report. DocuSign Envelope ID: DE8E6DBB-4A7F-41D7-9A73-AA7FD64486D1 Geotechnical Engineering Report SSGC Brar Short Plat – S. 132nd St Renton, WA SSGC Project No. 20058 August 3, 2020 8 A vapor retarder should be considered beneath concrete slabs that will be covered with moisture sensitive or impervious coverings (such as tile, wood, etc.), or when the slab will support equipment or stored materials sensitive to moisture. We recommend the slab designer refer to ACI 302 and/or ACI 360 for procedures and limitations regarding the use and placement of vapor retarders. Seismic Considerations Recommended seismic parameters and values in Table 2 are based on the 2015 International Building Code (IBC). Table 2. Seismic Parameters PARAMETER VALUE 2015 International Building Code (IBC) Site Classification1 D Ss Spectral Acceleration for a Short Period 1.46 S1 Spectral Acceleration for a 1-Second Period 0.546g Fa Site Coefficient for a Short Period 1.00 Fv Site Coefficient for a 1-Second Period 1.50 1 Note: In general accordance with 2015 International Building Code, Section 1613.3.1 for risk categories I,II,III. IBC Site Class is based on the estimated characteristics of the upper 100 feet of the subsurface profile. Ss, S1, Fa, and Fv values based on the OSHPD Seismic Design Maps website. Liquefaction Soil liquefaction is a condition where loose, typically granular soils located below the groundwater surface lose strength during ground shaking, and is often associated with earthquakes. The Seattle Hazard Explorer website does not show the site in a liquefaction prone area. Native soils consist of principally dense to very dense glacially consolidated materials at relatively shallow depth. The risk of liquefaction at this site is considered low for the design level earthquake. Lateral Earth Pressures We anticipate retaining walls may be required in portions of the development. Below grade or retaining walls will be subject to lateral earth pressures. Subgrade walls are typically designed for “active” or “at- rest” earth pressure conditions. Active earth pressure is commonly used for design of free-standing cantilever retaining walls and assumes lateral movement at the top of the wall of around 0.002H to 0.004H, where H is the height of the wall. The at-rest condition assumes no wall movement. DocuSign Envelope ID: DE8E6DBB-4A7F-41D7-9A73-AA7FD64486D1 Geotechnical Engineering Report SSGC Brar Short Plat – S. 132nd St Renton, WA SSGC Project No. 20058 August 3, 2020 9 We anticipate most walls will retain native outwash or new structural fill to accommodate final site grades. The following recommended earth pressures (Table 3) should be applied as a triangular distribution starting at the top of the wall (for active and at-rest) and bottom of wall (for passive) and assume: ▪ Backfill behind walls is level and no surcharge loads will be applied; ▪ Drainage is provided behind the wall to prevent the development of hydrostatic pressures. Table 3. Lateral Earth Pressures Soil Type Earth Pressure Coefficient* Equivalent Fluid Pressure (pcf)* Loose Native Soils (0 to 4 feet) Active: 0.36 At-rest: 0.53 Passive: 2.80 Active: 40 At-rest: 55 Passive: 300 Dense Glacial Till (Below 4 feet) Active: 0.25 At-rest: 0.38 Passive: 350 Active: 30 At-rest: 45 Passive: 350 * A factor of safety of about 1.5 should be applied to these values. Additional lateral pressure should be added to these values to model surcharges such as adjacent structures, sloped backfill behind the wall, traffic, construction equipment, or seismic loads. We recommend an active seismic pressure of 5H psf (where H is the height of the subgrade wall) and an at- rest seismic pressure of 8H. The effects of other surcharge loads should be accounted for as appropriate. Wall Backfill Backfill behind retaining walls should consist of granular material that satisfies the criteria of Section 9-03.12(2) “Gravel Backfill for Walls” per the 2018 WSDOT (M 41-10) manual, or as approved by the engineer. Wall backfill should be placed in lifts not exceeding 8 inches and compacted with hand-operated compaction equipment. Compaction of wall backfill should be between 90 to 92 percent of the maximum dry density (MDD) per the ASTM D1557 test method within 3 feet of the back of the wall. At a distance greater than 3 feet behind the back of the wall, backfill can be compacted using conventional rollers, with backfill compacted to at least 92 percent of the MDD (ASTM D1557). Wall Drainage Drainage should be provided behind subgrade walls to reduce the potential for hydrostatic pressure developing against the wall and to reduce the risk of groundwater from entering subgrade floors. We recommend a minimum 12-inch wide zone of free draining granular soil (WSDOT Section 9- 03.12(4), or as approved by the design engineer) is placed directly behind the wall. Alternatively, DocuSign Envelope ID: DE8E6DBB-4A7F-41D7-9A73-AA7FD64486D1 Geotechnical Engineering Report SSGC Brar Short Plat – S. 132nd St Renton, WA SSGC Project No. 20058 August 3, 2020 10 an approved drainage mat can be used behind the wall. A perforated rigid plastic drainpipe at least 6-inches in diameter should be installed behind the base of the wall within 6-inches of the bottom of the footing. The drain line should be surrounded with the free-draining granular soil zone and sloped to provide flow to an approved storm water receptor. The granular fill zone should extend to within 1 foot of final grade of the wall, where it should be capped with compacted low permeable fill containing sufficient fines to reduce infiltration of surface water into the drainage zone. A filter fabric (such as Mirafi 140N, or other approved material) should be placed between native soils and the granular drain material to limit siltation into the drainage zone. Cleanouts are recommended for maintenance of the drain system. Infiltration Characteristics We understand stormwater control will use infiltration facilities. General assessment of infiltration potential of native soils was performed by completing one small-scale Pilot Infiltration Test (PIT) in the proposed storm tract per the 2016 King County Surface Water Design Manual. Test PIT-1 was completed in the upper native silty sand. Results of the infiltration test are presented in Table 3. Table 3. Infiltration Test Results Infiltration Test No. Depth of Test from Surface (feet) Soil Type Field Infiltration Rate (in/hr) Corrected Infiltration Rate (in/hr) Correction Factors* (Fg/Ft/Fp) PIT-1 2 Alluvium/Weathered Till 3 1.2 (1.0/0.5/0.8) * Correction Factors from the 2017 City of Renton Surface Water Design Manual. Correction factors applied to the field rate per equation 5-11 of the County Manual. The tested infiltration rate from PIT-1 is considered appropriate for the soil tested. However, the presence of dense glacial till below the alluvium/weathered till will form a barrier to vertical groundwater flow. We interpret the infiltration rate to be reflective of mostly horizontal flow through the upper soils. Infiltration facilities planned in the upper 3 to 4 feet of site soils should be restricted to shallow dispersion systems as the dense glacial till is considered a barrier which should be accounted for in design . An infiltration rate of 1.2 inches per hour is recommended for dispersion systems in the upper silty sand and weathered till. No infiltration should be accounted for in the dense (unweathered) glacial till. Cation Exchange Capacity (CEC) and organic content test were completed on samples from the two test holes in the storm track area to assess treatment characteristics of the upper outwash soil. Test results are summarized in Table 4. DocuSign Envelope ID: DE8E6DBB-4A7F-41D7-9A73-AA7FD64486D1 Geotechnical Engineering Report SSGC Brar Short Plat – S. 132nd St Renton, WA SSGC Project No. 20058 August 3, 2020 11 Table 4. CEC and Organic Content Results Test Location, Depth Soil Type CEC Results (milliequivalents) CEC Required* (milliequivalents) Organic Content Results (%) Organic Content Required* (%) PIT-1, 2 feet Alluvium 10 ≥ 5 2.79 ≥1.0 *Per the 2017 City of Renton Surface Water Design Manual. CEC and organic test results satisfy City criteria on the tested soil. Conventional Pavement Sections Subgrades for conventional pavement areas should be prepared as described in the “Subgrade Preparation” section of this report. Subgrades below pavement sections should be graded or crowned to promote drainage and not allow for ponding of water beneath the section. If drainage is not provided and ponding occurs, the subgrade soils could become saturated, lose strength, and result in premature distress to the pavement. In addition, the pavement surfacing should also be graded to promote drainage and reduce the potential for ponding of water on the pavement surface. Minimum recommended pavement section for private driveways are presented in Table 5. Pavement sections in public right-of-ways (S. 132nd Street) should conform to City of Renton requirements for the road designation. Table 5. Preliminary Pavement Sections Traffic Area Minimum Recommended Pavement Section Thickness (inches) Asphalt Concrete Surface1 Portland Cement Concrete Aggregate Base Course2 Subbase Aggregate3 Driveways 2 - 4 12 1 1/2 –inch nominal aggregate hot-mix asphalt (HMA) per WSDOT 9-03.8(1) 2 Crushed Surfacing Base Course per WSDOT 9-03.9(3) 3 Native granular soils compacted to 95% of the ASTM D1557 test method, or Gravel Borrow per WSDOT 9-03.14(1) or Crushed Surfacing Base Course WSDOT 9-03.9(3) Conventional Pavement Maintenance The performance and lifespan of pavements can be significantly impacted by future maintenance. The above pavement sections represent minimum recommended thicknesses and, as such, periodic maintenance should be completed. Proper maintenance will slow the rate of pavement deterioration and will improve pavement performance and life. Preventative maintenance consists of both localized maintenance (crack and joint sealing and patching) and global maintenance (surface DocuSign Envelope ID: DE8E6DBB-4A7F-41D7-9A73-AA7FD64486D1 Geotechnical Engineering Report SSGC Brar Short Plat – S. 132nd St Renton, WA SSGC Project No. 20058 August 3, 2020 12 sealing). Added maintenance measures should be anticipated over the lifetime of the pavement section if any existing fill or topsoil is left in-place beneath pavement sections. Geologic Hazard Area Discussion The City of Renton’s COR Map for critical areas identifies the south-facing slope as having inclinations between 15 and 25 percent which satisfies criteria per Title IV, Chapter 3 of the City of Renton Municipal Code (RMC) as a regulated slope. The slope is not mapped as a high erosion hazard. Landslide Hazard Native soils on site slopes consist of a relatively thin layer of alluvium, over weathered glacial till, over dense to very dense glacial till. No evidence of recent landslide activity was apparent on site slopes or near the property at the time of our fieldwork. In addition, we are unaware of active landslides within 300 feet of the property. The proposed development should not adversely impact slope stability on the site or on adjacent properties. Erosion Hazard Native glacial soils on the site mapped by the USDA are considered to have a moderate potential to erosion. We observed no evidence of excessive erosion on the property, including the west- facing slope. Regarding planned development, it is our opinion that Best Management Practices (BMP) for erosion control (silt fencing, straw bales, etc) can be utilized such that the risk of off - site transport of sediment is limited during construction. Additional erosion control measures may be necessary if earthwork is scheduled during the wetter seasons. All erosion control provisions should follow City of Renton regulations to reduce the risk of off-site transport of sediments. Exposed soils following construction should be vegetated as soon as possible. REPORT CONDITIONS This report has been prepared for the exclusive use of Mr. Lakhpal Brar and his agents for specific application to the project discussed, and has been prepared in accordance with generally accepted geotechnical engineering practices in the area. No warranties, either express or i mplied, are intended or made. The analysis and recommendations presented in this report are based on observed soil conditions and test results at the indicated locations, and from other geologic information discussed. This report does not reflect variations that may occur across the site, or due to the modifying effects of construction or weather or other natural events. The nature and extent of such variations may not become evident until during or after construction. If variations appear, we should be immediately notified so that further evaluation and supplemental recommendations can be provided. DocuSign Envelope ID: DE8E6DBB-4A7F-41D7-9A73-AA7FD64486D1 DocuSign Envelope ID: DE8E6DBB-4A7F-41D7-9A73-AA7FD64486D1 N South Sound Geotechnical Consulting P.O. Box 39500 Lakewood, WA 98496 (253) 973-0515 Figure 1 – Exploration Plan Brar Short Plat Renton, WA SSGC Project #20058 Approximate Test Pit Location PIT - 1 TP - 1 PIT - 1 Approximate Infiltration Test Location Scale: NTS Base map from plan titled “Brar Short Plat - Plot Plan”, by Kaul Design Architecture, PLLC, undated. Legend TP-1 PIT-1 TP-2 TP-3 TP-4 DocuSign Envelope ID: DE8E6DBB-4A7F-41D7-9A73-AA7FD64486D1 Geotechnical Engineering Report SSGC Brar Short Plat – S. 132nd St Renton, WA SSGC Project No. 20058 August 3, 2020 A-1 Appendix A Field Exploration Procedures and Exploration Logs DocuSign Envelope ID: DE8E6DBB-4A7F-41D7-9A73-AA7FD64486D1 Geotechnical Engineering Report SSGC Brar Short Plat – S. 132nd St Renton, WA SSGC Project No. 20058 August 3, 2020 Field Exploration Procedures Our field exploration for this project included four test pits and one infiltration test completed on July 8, 2020. The approximate locations of the explorations are shown on Figure 1, Exploration Plan. The exploration locations were determined by pacing from site features. Ground surface elevations referenced on the logs were inferred from topography from Google Earth satellite imagery. Exploration locations and elevations should be considered accurate only to the degree implied by the means and methods used. A private excavation company subcontracted to SSGC dug the test holes. Soil samples were collected and stored in moisture tight for further assessment and laboratory testing. Explorations were backfilled with excavated soils and tamped when completed. Please note that backfill in the explorations will likely settle with time. Backfill material located in building areas should be re-excavated and recompacted, or replaced with structural fill. The following logs indicate the observed lithology of soils and other materials observed in the explorations at the time of excavation. Where a soil contact was observed to be gradational, our log indicates the average contact depth. Our logs also indicate the approximate depth to groundwater (where observed at the time of excavation), along with sample numbers and approximate sample depths. Soil descriptions on the logs are based on the Unified Soil Classification System. DocuSign Envelope ID: DE8E6DBB-4A7F-41D7-9A73-AA7FD64486D1 Project: Brar Short Plat SSGC Job # 20058 TEST PIT LOGS PAGE 1 OF 3 Location: S. 132nd Street, Renton, WA TEST PIT LOGS FIGURE A-1 South Sound Geotechnical Consulting PIT-1, TP-1 through TP-4 Logged by: THR Infiltration Test PIT-1 Depth (feet) Material Description 0 – 1 1 – 6 6 – 10 Topsoil Silty SAND with occasional gravel: Loose to medium dense moist, brownish gray. (SM) (Sample S-1 @ 3 feet) Silty SAND with gravel: Dense to very dense, moist, gray. (SM) (Glacial Till) Test hole completed at approximately 10 feet on 7/8/20. Infiltration test completed at 2.5 feet. Groundwater not observed at time of excavation. Approximate surface elevation: 250 feet Test Pit TP-1 Depth (feet) Material Description 0 – 0.5 0.5 – 1.5 1.5 – 3 3 – 6 Topsoil Silty SAND: Loose, moist, orangish brown. (SM) Silty SAND with occasional gravel and cobbles: Medium dense to dense, moist, mottled orange-gray. (SM) (Weathered Glacial Till) Silty SAND with gravel and cobbles: Very dense, moist, gray. (SM) (Glacial Till) Test pit completed at approximately 6 feet on 7/8/20. Groundwater not observed at time of excavation. Approximate surface elevation: 275 feet DocuSign Envelope ID: DE8E6DBB-4A7F-41D7-9A73-AA7FD64486D1 Project: Brar Short Plat SSGC Job # 20058 TEST PIT LOGS PAGE 2 OF 3 Location: S. 132nd Street, Renton, WA TEST PIT LOGS FIGURE A-1 South Sound Geotechnical Consulting PIT-1, TP-1 through TP-4 Logged by: THR Test Pit TP-2 Depth (feet) Material Description 0 – 0.5 0.5 – 2.5 2.5 – 4 4 – 6 Topsoil Silty SAND: Loose, moist, orangish brown. (SM) Silty SAND with occasional gravel and cobbles: Medium dense to dense, moist, mottled orange-gray. (SM) (Weathered Glacial Till) Silty SAND with gravel and cobbles: Very dense, moist, gray. (SM) (Glacial Till) Test pit completed at approximately 6 feet on 7/8/20. Groundwater not observed at time of excavation. Approximate surface elevation: 273 feet Test Pit TP-3 Depth (feet) Material Description 0 – 1.5 1.5 – 2 2 – 3 3 – 4 4 – 6 Fill: Silt, sand, and occasional gravel: Loose, moist, light brown. Topsoil Silty SAND: Loose, moist, orangish brown. (SM) Silty SAND with some gravel: Medium dense, moist, mottled orange-gray. (SM) (Weathered Glacial Till) SAND with silt and occasional gravel: Dense to very dense, moist, gray. (SM) (Glacial Till) Test pit completed at approximately 6 feet on 7/8/20. Groundwater not observed at time of excavation. Approximate surface elevation: 255 feet DocuSign Envelope ID: DE8E6DBB-4A7F-41D7-9A73-AA7FD64486D1 Project: Brar Short Plat SSGC Job # 20058 TEST PIT LOGS PAGE 3 OF 3 Location: S. 132nd Street, Renton, WA TEST PIT LOGS FIGURE A-1 South Sound Geotechnical Consulting PIT-1, TP-1 through TP-4 Logged by: THR Test Pit TP-4 Depth (feet) Material Description 0 – 1 1 – 1.5 1.5 – 2 2 – 4 4 – 6 Fill: Silt, sand, and occasional gravel: Loose, moist, brown. Topsoil Silty SAND: Loose, damp, orangish brown. (SM) Silty SAND with some gravel: Medium dense, moist, mottled orange-gray. (SM) (Weathered Glacial Till) Silty SAND with gravel and occasional cobble: Dense to very dense, moist, gray. (SM) (Glacial Till) Test pit completed at approximately 6 feet on 7/8/20. Groundwater not observed at time of excavation. Approximate surface elevation: 262 feet DocuSign Envelope ID: DE8E6DBB-4A7F-41D7-9A73-AA7FD64486D1 Geotechnical Engineering Report SSGC Brar Short Plat – S. 132nd St Renton, WA SSGC Project No. 20058 August 3, 2020 C-1 Appendix B Laboratory Testing and Results DocuSign Envelope ID: DE8E6DBB-4A7F-41D7-9A73-AA7FD64486D1 Geotechnical Engineering Report SSGC Brar Short Plat – S. 132nd St Renton, WA SSGC Project No. 20058 August 3, 2020 B-1 Laboratory Testing Select soil samples were tested for organic content and cation exchange capacity (CEC) by Northwest Agricultural Consultants of Kennewick, Washington. Results of the laboratory testing are included in this appendix. DocuSign Envelope ID: DE8E6DBB-4A7F-41D7-9A73-AA7FD64486D1 2545 W Falls Avenue Kennewick, WA 99336 509.783.7450 www.nwag.com lab@nwag.com Sample ID Organic Matter Cation Exchange Capacity PIT-1, S-1 2.79% 10.0 meq/100g Method ASTM D2974 EPA 9081 South Sound Geotechnical Consulting PO Box 39500 Lakewood, WA 98496 Report: 51979-1-1 Date: July 14, 2020 Project No: 20058 Project Name: Brar DocuSign Envelope ID: DE8E6DBB-4A7F-41D7-9A73-AA7FD64486D1 UNIFIED SOIL CLASSIFICATION SYSTEM Criteria for Assigning Group Symbols and Group Names Using Laboratory TestsA Soil Classification Group Symbol Group NameB Coarse Grained Soils More than 50% retained on No. 200 sieve Gravels More than 50% of coarse fraction retained on No. 4 sieve Clean Gravels Less than 5% finesC Cu  4 and 1  Cc  3E GW Well-graded gravelF Cu  4 and/or 1  Cc  3E GP Poorly graded gravelF Gravels with Fines More than 12% finesC Fines classify as ML or MH GM Silty gravelF,G, H Fines classify as CL or CH GC Clayey gravelF,G,H Sands 50% or more of coarse fraction passes No. 4 sieve Clean Sands Less than 5% finesD Cu  6 and 1  Cc  3E SW Well-graded sandI Cu  6 and/or 1  Cc  3E SP Poorly graded sandI Sands with Fines More than 12% finesD Fines classify as ML or MH SM Silty sandG,H,I Fines Classify as CL or CH SC Clayey sandG,H,I Fine-Grained Soils 50% or more passes the No. 200 sieve Silts and Clays Liquid limit less than 50 inorganic PI  7 and plots on or above “A” lineJ CL Lean clayK,L,M PI  4 or plots below “A” lineJ ML SiltK,L,M organic Liquid limit - oven dried  0.75 OL Organic clayK,L,M,N Liquid limit - not dried Organic siltK,L,M,O Silts and Clays Liquid limit 50 or more inorganic PI plots on or above “A” line CH Fat clayK,L,M PI plots below “A” line MH Elastic SiltK,L,M organic Liquid limit - oven dried  0.75 OH Organic clayK,L,M,P Liquid limit - not dried Organic siltK,L,M,Q Highly organic soils Primarily organic matter, dark in color, and organic odor PT Peat A Based on the material passing the 3-in. (75-mm) sieve B If field sample contained cobbles or boulders, or both, add “with cobbles or boulders, or both” to group name. C Gravels with 5 to 12% fines require dual symbols: GW -GM well-graded gravel with silt, GW -GC well-graded gravel with clay, GP-GM poorly graded gravel with silt, GP-GC poorly graded gravel with clay. D Sands with 5 to 12% fines require dual symbols: SW -SM well-graded sand with silt, SW -SC well-graded sand with clay, SP-SM poorly graded sand with silt, SP-SC poorly graded sand with clay E Cu = D60/D10 Cc = 6010 2 30 DxD )(D F If soil contains  15% sand, add “with sand” to group name. G If fines classify as CL-ML, use dual symbol GC-GM, or SC-SM. HIf fines are organic, add “with organic fines” to group name. I If soil contains  15% gravel, add “with gravel” to group name. J If Atterberg limits plot in shaded area, soil is a CL-ML, silty clay. K If soil contains 15 to 29% plus No. 200, add “with sand” or “with gravel,” whichever is predominant. L If soil contains  30% plus No. 200 predominantly sand, add “sandy” to group name. M If soil contains  30% plus No. 200, predominantly gravel, add “gravelly” to group name. N PI  4 and plots on or above “A” line. O PI  4 or plots below “A” line. P PI plots on or above “A” line. Q PI plots below “A” line. DocuSign Envelope ID: DE8E6DBB-4A7F-41D7-9A73-AA7FD64486D1