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HomeMy WebLinkAboutRS_Geotechnical_Report_220303_V1.pdfGEOTECHNICAL REPORT Towns on 12th 3916 Northeast 12th Street Renton, Washington Project No. T-8566 Prepared for: Madrona Place, LLC Redmond, Washington August 5, 2021 Revised February 15, 2022 TERRA ASSOCIATES, Inc. Mr. Ben Paulus Madrona Place, LLC 18300 Redmond Way, Suite 140 Redmond, Washington 98052 Subject: Geotechnical Report Towns on 12th Consultants in Geotechnical Engineering, Geology and Environmental Earth Sciences August 5, 2021 Revised February 15, 2022 Project No. T-8566 39 I 6 Northeast 12th Street Renton, Washington Dear Mr. Paulus: As requested, we have conducted a geotechnical engineering study for the subject project. The attached report presents our findings and recommendations for the geotechnical aspects of project design and construction. In general, the soil conditions at the site consisted of approximately 6 to 12 inches of topsoil overlying approximately three to eight feet of medium dense silty sand over medium dense to very dense sand with silt and gravel and silty sand with gravel to the termination of the test pits. The exception to this was in Test Pits TP-6 and TP-7, where we observed approximately five and two feet, respectively, of loose, fill material overlying the native soils. The fill material consisted of sand and silty sand with varying amounts of debris and organic material. Groundwater seepage was observed in Test Pit TP-2 at approximately eight feet below current site grades. In our opinion, the soil conditions we observed at the site will be suitable for support of the proposed development, provided the recommendations presented in this report are incorporated into project design and construction. We trust the information presented in this report is sufficient for your current needs. If you have any questions or require additional information, please call. 12220 113th Avenue NE, Ste. 130, Kirkland, Washington 98034 Phone (425) 821-7777 • Fax (425) 821-4334 2-15-2022 TABLE OF CONTENTS Page No. 1.0 Project Description ........................................................................................................... 1 2.0 Scope of Work ................................................................................................................. 1 3.0 Site Conditions ................................................................................................................. 2 3.1 Surface ................................................................................................................ 2 3.2 Subsurface ........................................................................................................... 2 3.3 Groundwater ....................................................................................................... 3 3.4 Geologic Hazards ................................................................................................ 3 3.4.1 Erosion Hazard Areas ................................................................................ 3 3.4.2 Flood Hazard Areas .................................................................................... 4 3.4.3 Steep Slope and Landslide Hazard Areas .................................................. 4 3.4.4 Seismic Hazard Areas ................................................................................ 5 3.4.5 Coal Mine Hazard Areas ............................................................................ 6 3.5 Seismic Design Parameters ................................................................................. 6 4.0 Discussion and Recommendations ................................................................................... 6 4.1 General ................................................................................................................ 6 4.2 Site Preparation and Grading .............................................................................. 7 4.3 Relative Slope Stability ....................................................................................... 8 4.4 Excavations ......................................................................................................... 9 4.5 Foundations ....................................................................................................... 10 4.6 Slab-on-Grade Floors ........................................................................................ 10 4.7 Stormwater Facilities ........................................................................................ 11 4.8 Infiltration Feasibility ....................................................................................... 12 4.9 Drainage ............................................................................................................ 12 4.10 Utilities .............................................................................................................. 12 4.11 Pavement ........................................................................................................... 13 5.0 Additional Services ........................................................................................................ 13 6.0 Limitations ..................................................................................................................... 13 Figures Vicinity Map ......................................................................................................................... Figure 1 Exploration Location Plan..................................................................................................... Figure 2 Typical Wall Drainage Detail ............................................................................................... Figure 3 Appendix Field Exploration and Laboratory Testing ....................................................................... Appendix A Slope Stability Results ..................................................................................................... Appendix B Geotechnical Report Towns on 12th 3916 Northeast 12th Street Renton, Washington 1.0 PROJECT DESCRIPTION The project consists of developing the approximately 6.5-acre site with residential building lots, stormwater facilities, associated access, and utilities. Development plans and grading plans were not available at the time of this report. Based on current site grades, grading to achieve building lot and roadway elevations is expected to be moderate with cuts and fills from one to ten feet. We expect that the residential structures constructed on the lots will be two- to three-story, wood-frame buildings with their main floor levels framed over a crawl space and attached garages constructed at grade. Foundation loads should be relatively light, in the range of 2 to 3 kips per foot for bearing walls and 25 to 50 kips for isolated columns. The recommendations contained in the following sections of this report are based on our understanding of the conceptual design features discussed above. We should review design drawings as they become available to verify that our recommendations have been properly interpreted and incorporated into project design, and to amend or supplement our recommendations, if required. 2.0 SCOPE OF WORK On June 10, 2021, we investigated subsurface conditions at the site by observing soil conditions in nine test pits excavated to maximum depths of 6.5 to 11.5 feet below existing surface grades. Using the results of our field study and laboratory testing, analyses were undertaken to develop geotechnical recommendations for project design and construction. Specifically, this report addresses the following: Soil and groundwater conditions. Geologic Hazards per the City of Renton Municipal Code (RMC). Seismic design parameters per the current International Building Code (IBC). Site preparation and grading. Relative slope stability. Excavations. Foundations. August 5, 2021 Revised February 15, 2022 Project No. T-8566  Page No. 2 Slab-on-Grade floors. Stormwater facilities. Infiltration feasibility. Drainage. Utilities. Pavements. It should be noted that recommendations outlined in this report regarding drainage are associated with soil strength, design earth pressures, erosion, and stability. Design and performance issues with respect to moisture as it relates to the structure environment are beyond Terra Associates, Inc’s purview. A building envelope specialist or contractor should be consulted to address these issues, as needed. 3.0 SITE CONDITIONS 3.1 Surface The project site consists of seven tax parcels totaling approximately 6.5 acres located at 3916 Northeast 12th Street in Renton, Washington. The approximate site location is shown on Figure 1. The site is currently occupied by a couple of residential buildings and associated storage structures in the southern portion of the site. The remaining site and the areas surrounding the buildings are undeveloped and covered with trees and forest duff. Site topography consists of a slight slope that descends from the south to the north with an overall relief of approximately 25 feet. Near the north-northeast properly line the slope transitions from moderate to steep and descends to Northeast Sunset Boulevard with an overall relief of approximately 30 feet. 3.2 Subsurface In general, the soil conditions at the site consisted of approximately 6 to 12 inches of topsoil overlying approximately three to eight feet of medium dense silty sand over medium dense to very dense sand with silt and gravel and silty sand with gravel to the termination of the test pits. The exception to this was in Test Pits TP-6 and TP-7, where we observed approximately five and two feet, respectively, of loose, fill material overlying the native soils. The fill material consisted of sand and silty sand with varying amounts of debris and organic material. The Geologic Map of Surficial Deposits in the Seattle 30’ x 60’ Quadrangle, Washington by Yount, J.C., Minard, J.P., and Dembroff, G.R. (1993), shows the site soils mapped as Till (Qvt). The native soils observed in the test pits are generally consistent with this mapped description. August 5, 2021 Revised February 15, 2022 Project No. T-8566  Page No. 3 The preceding discussion is intended to be a general review of the soil conditions encountered. For more detailed descriptions, please refer to the Test Pit Logs in Appendix A. The approximate location of the test pits is shown on attached Figure 2. 3.3 Groundwater Minor groundwater seepage was observed in Test Pit TP-2 at a depth of approximately eight feet below existing grade. The groundwater seepage was observed near the contact between the upper weathered till material and lower unweathered till material. Groundwater seepage was not observed in any other subsurface exploration. 3.4 Geologic Hazards Section 4-3-050G of the Renton Municipal Code (RMC) defines geologically hazardous areas as “areas which may be prone to one or more of the following conditions: erosion, flooding, landslides, coal mine hazards, or seismic activity.” 3.4.1 Erosion Hazard Areas Section 4-3-050G 5c of the RMC defines erosion hazard areas as: “i. Low Erosion Hazard (EL): Areas with soils characterized by the Natural Resource Conservation Service (formerly U.S. Soil Conservation Service) as having slight or moderate erosion potential, and a slope less than fifteen percent (15 percent). ii. High Erosion Hazard (EH): Areas with soils characterized by the Natural Resource Conservation Service (formerly U.S. Soil Conservation Service) as having severe or very severe erosion potential, and a slope more than fifteen percent (15 percent).” The soils observed on-site are predominately classified as Alderwood gravelly sandy loam, 8 to 15 percent slopes (AgC) and Arents, Alderwood material, 6 to 15 percent slopes (AmC) by the United States Department of Agriculture Natural Resources Conservation Service (NRCS). With the existing slope gradients onsite, these soils will have a slight to moderate potential for erosion when exposed. Therefore, in our opinion, the proposed development area is not an erosion hazard as defined by the RMC. The soils on the steep slope in the north and northeastern portion of the site near Northeast Sunset Boulevard will have a severe to very severe potential for erosion when exposed. Therefore, the steep slope would be considered an erosion hazard area as defined by the RMC. Implementation of temporary and permanent Best Management Practices (BMPs) for preventing and controlling erosion will be required and will mitigate the erosion hazard during construction on the eastern slopes. As a minimum, we recommend implementing the following erosion and sediment control BMPs prior to, during, and immediately following construction activities at the site. August 5, 2021 Revised February 15, 2022 Project No. T-8566  Page No. 4 Prevention Limit site clearing and grading activities to the relatively dry months (typically May through September). Limit disturbance to areas where construction is imminent. Locate temporary stockpiles of excavated soils no closer than ten feet from the crest of the slope. Provide temporary cover for cut slopes and soil stockpiles during period of inactivity. Temporary cover may consist of durable plastic sheeting that is securely anchored to the ground surface or straw mulch. Establish permanent cover over exposed areas that will not be disturbed for a period of 30 days or more by seeding, in conjunction with a mulch cover or appropriate hydroseeding. Containment Install a silt fence along site margins and downslope of areas that will be disturbed. The silt fence should be in place before clearing and grading is initiated. Intercept surface water flow and route the flow away from the slope to a stabilized discharge point. Surface water must not discharge at the top or onto the face of steep slope. Provide onsite sediment retention for collected runoff. The contractor should perform daily review and maintenance of all erosion and sedimentation control measures at the site. 3.4.2 Flood Hazard Areas Section 4-3-050G 4a of the RMC defines flood hazard areas as “the land in the floodplain subject to one percent or greater chance of flooding in any given year. Designation on flood maps always includes the letters A or V.” Based on a review of the project site and topography, the area is not within a mapped flood hazard area. Therefore, in our opinion the site would not be considered a flood hazard area as definitely the RMC. 3.4.3 Steep Slope and Landslide Hazard Areas Section 4-3-050G 5a of the RMC defines steep slope hazard areas as: “i. Sensitive Slopes: A hillside, or portion thereof, characterized by: (a) an average slope of 25 percent to less than 40 percent as identified in the City of Renton Steep Slope Atlas or in a method approved by the City; or (b) an average slope of 40 percent or greater with a vertical rise of less than 15 feet as identified in the City of Renton Steep Slope Atlas or in a method approved by the City; August 5, 2021 Revised February 15, 2022 Project No. T-8566  Page No. 5   (c) abutting an average slope of 25 percent to 40 percent as identified in the City of Renton Steep Slope Atlas or in a method approved by the City. This definition excludes engineered retaining walls. ii. Protected Slopes: A hillside, or portion thereof, characterized by an average slope of 40 percent or greater grade and having a minimum vertical rise of 15 feet as identified in the City of Renton Steep Slope Atlas or in a method approved by the City.” Section 4-3-050G 5b defines landslide hazard areas as: “i. Low Landslide Hazard (LL): Areas with slopes less than 15 percent. ii. Medium Landslide Hazard (LM): Areas with slopes between 15 percent and 40 percent and underlain by soils that consist largely of sand, gravel, or glacial till. iii. High Landslide Hazards (LH): Areas with slopes greater than 40 percent, and areas with slopes between 15 percent and 40 percent and underlain by soils consisting largely of silt and clay. iv. Very High Landslide Hazards (LV): Areas of known mapped or identified landslide deposits.” The existing topography within the proposed development area is relatively flat with an overall relief of no more than 15 feet and it is not mapped as a steep slope by the City of Renton. Therefore, in our opinion, the proposed development area would be classified as a Low Hazard Area as defined by the RMC. However, the slope that borders the north and northeast of the site near Northeast Sunset Boulevard is mapped as a steep slope and landslide hazard area by the City. Therefore, a buffer and building setback are required from all steep slope areas. This buffer is typically 50 feet with a 15-foot building setback. However, we are able to reduce the buffer, provided additional analysis is completed to determine if the proposed development will impact the steep slope areas. The required analysis is below in Section 4.3 of this report. 3.4.4 Seismic Hazard Areas Section 4-3-050G 5d of the RMC defines seismic hazard areas as: “i. Low Seismic Hazard (SL): Areas underlain by dense soils or bedrock. These soils generally have site classifications of A through D, as defined in the International Building Code, 2012. ii. High Seismic Hazard (SH): Areas underlain by soft or loose, saturated soils. These soils generally have site classifications E or F, as defined in the International Building Code, 2012.” Liquefaction is a phenomenon where there is a reduction or complete loss of soil strength due to an increase in water pressure induced by vibrations. Liquefaction mainly affects geologically recent deposits of fine grained sand that is below the groundwater table. Soils of this nature derive their strength from intergranular friction. The generated water pressure or pore pressure essentially separates the soil grains and eliminates this intergranular friction; thus, eliminating the soil’s strength. August 5, 2021 Revised February 15, 2022 Project No. T-8566  Page No. 6   Based on the soil and groundwater conditions we observed at the site, it is our opinion that the risk for damage resulting from soil liquefication or subsidence during a severe seismic event is negligible. Therefore, in our opinion, unusual seismic hazard areas do not exist at the site and design in accordance with local building codes for determining seismic forces would adequately mitigate impacts associated with ground shaking and the site would be classified as a Low Seismic Hazard per the City of Renton. 3.4.5 Coal Mine Hazard Areas Section 4-3-050G 5e of the RMC defines coal mine hazard areas as: “i. Low Coal Mine Hazards (CL): Areas with no known mine workings and no predicted subsidence. While no mines are known in these areas, undocumented mining is known to have occurred. ii. Medium Coal Mine Hazards (CM): Areas where mine workings are deeper than 200 feet for steeply dipping seams, or deeper than 15 times the thickness of the seam or workings for gently dipping seams. These areas may be affected by subsidence. iii. High Coal Mine Hazard (CH): Areas with abandoned and improperly sealed mine openings and areas underlain by mine workings shallower than 200 feet in depth for steeply dipping seams, or shallower than fifteen (15) times the thickness of the seam or workings for gently dipping seams. These areas may be affected by collapse or other subsidence.” Based on a review of documented mining operations in the area, there are no noted coal mines nearby. Therefore, the site would be considered a Low Coal Mine Hazard per the RMC. 3.5 Seismic Design Parameters Based on soil conditions noted in the test pits and our knowledge of the area geology, per Chapter 16 of the 2018 International Building Code (IBC), site class “C” should be used in structural design. 4.0 DISCUSSION AND RECOMMENDATIONS 4.1 General Based on our study, there are no geotechnical considerations that would preclude development of the site, as currently planned. The structures can be supported on conventional spread footings bearing on competent inorganic native soils or on structural fill placed and compacted above the native soils. Pavement and floor slabs can be similarly supported. The loose organic fill material observed in Test Pits TP-6 and TP-7 would not be suitable for support of building elements and will need to be removed from below all building elements with grade restored using new structural fill. The lateral extent of the overexcavation and restoration will need to be determined in the field during grading. August 5, 2021 Revised February 15, 2022 Project No. T-8566  Page No. 7   The majority of the native soils encountered at the site contain a sufficient percentage of fines that will make them difficult to compact as structural fill when too wet or dry. The ability to use soil from site excavations as structural fill or backfill will depend on its moisture content and the prevailing weather conditions at the time of construction. If grading activities will take place during the winter season, the contractor should be prepared to import free-draining granular material for use as structural fill and backfill. The following sections provide detailed recommendations regarding the preceding issues and other geotechnical design and construction considerations. These recommendations should be incorporated into the final design drawings and construction specifications. 4.2 Site Preparation and Grading To prepare the site for construction, all vegetation and organic surface soils should be stripped and removed from below the building lots and roadway areas. Surface stripping depths of approximately 6 to 12 inches should be expected to remove the organic surficial soils. Soil containing organic material will not be suitable for use as structural fill, but may be used for limited depths in nonstructural areas. In the developed portions of the site, demolition of existing structures should include removal of existing foundations and abandonment of underground septic systems and other buried utilities. Abandoned utility pipes that fall outside of the new building area can be left in place, provided they are sealed to prevent intrusion of groundwater seepage and soil. In the vicinity of Test Pits TP-6 and TP-7, overexcavation of the organic fill material should be expected. The overexcavation will extend to depths of up to five feet below existing grade and the grade should be restored using new structural fill placed and compacted as outlined below. The lateral extent of the overexcavation will need to be determined in the field during grading. Once stripping operations are complete, cut and fill operations can be initiated to establish desired grades. Prior to placing fill, all exposed bearing surfaces should be observed by a representative of Terra Associates, Inc. to verify soil conditions are as expected and suitable for support of building foundations and pavement elements or placement of structural fill. Our representative may request proofrolling the exposed surface with a heavy rubber- tired vehicle to determine if any isolated soft and yielding areas are present. If unsuitable yielding areas are observed, they should be cut to firm bearing soil and filled to grade with structural fill. If depth of excavation to remove unstable soils is excessive, use of geotextile fabric, such as Mirafi 500X or equivalent in conjunction with structural fill can be considered in order to limit the depth of removal. Our experience has shown that, in general, a minimum of 18 inches of a clean, granular structural fill placed and compacted over the geotextile fabric should establish a stable bearing surface. Our study indicates that the site soils contain a sufficient percentage of fine, silt size particles that will make them difficult to compact as structural fill if they are too wet or too dry. The ability to use the existing fill and native soils as structural fill will depend on their moisture content and the prevailing weather conditions when site grading activities take place. If wet soils are encountered, the contractor will need to dry the soils by aeration during dry weather conditions. Alternatively, the use of an additive such as Portland cement or lime to stabilize the soil moisture can be considered. If the soil is amended, additional Best Management Practices (BMPs) addressing the potential for elevated pH levels will need to be included in the Storm Water Pollution Prevention Program (SWPPP) prepared with the Temporary Erosion and Sedimentation Control (TESC) plan. August 5, 2021 Revised February 15, 2022 Project No. T-8566  Page No. 8   If grading activities are planned during the wet winter months, or if they are initiated during the summer and extend into fall and winter, the owner should be prepared to import wet-weather structural fill. For this purpose, we recommend importing a granular soil that meets the following grading requirements: U.S. Sieve Size Percent Passing 6 inches 100 No. 4 75 maximum No. 200 5 maximum* * Based on the 3/4-inch fraction. Prior to use, Terra Associates, Inc. should examine and test all materials imported to the site for use as structural fill. Structural fill should be placed in uniform loose layers not exceeding 12 inches and compacted to a minimum of 95 percent of the soil’s maximum dry density, as determined by American Society for Testing and Materials (ASTM) Test Designation D-1557 (Modified Proctor). The moisture content of the soil at the time of compaction should be within two percent of its optimum, as determined by this ASTM standard. In nonstructural areas, the degree of compaction can be reduced to 90 percent. 4.3 Relative Slope Stability The project site is bordered by a mapped potential landslide hazard area and has steep slopes along the north- northeast property line. In order to determine the buffer for the steep slopes and the overall stability of the project site, we completed slope stability analyses. Our analyses were completed at locations designated as Cross-Section A-A’ and Cross Section B-B’ using the computer program Slide 2. The approximate cross-section location is shown on Figure 2. Our analysis considered both static and the pseudostatic (seismic) conditions. A horizontal acceleration of 0.30g was used in the pseudostatic analysis to simulate slope performance under earthquake loading. This acceleration is equal to one-half of the peak horizontal ground acceleration, with a 2 percent in 50-year probability of exceedance as defined by the 2018 International Building Code. Based on our field exploration, laboratory testing, and previous experience with similar soil types, we chose the following parameters for our analysis: Table 1 – Slope Stability Analysis Soil Parameters Soil Type Unit Weight (pcf) Friction Angle (Degrees) Cohesion (psf) Silty SAND 120 34 0 Weathered Glacial Till 125 34 100 Glacial Till 130 38 250 August 5, 2021 Revised February 15, 2022 Project No. T-8566  Page No. 9   The results of our slope stability analysis, as shown by the lowest safety factors for each condition, are presented in the following table: Table 2 – Slope Stability Analysis Results Based on our analysis, the slope is stable in its current condition. We would expect the buffer for the steep slopes can be reduced from 50 feet to 15 feet, while maintaining overall site stability and meeting the standard factors of safety of 1.5 for static and 1.1 for seismic. However, additional analysis will be required once the grading plans have been completed. The results of our analysis are attached in Appendix B. 4.4 Excavations All excavations at the site associated with confined spaces, such as utility trenches, must be completed in accordance with local, state, and federal requirements. Based on regulations outlined in the Washington Industrial Safety and Health Act (WISHA), the weathered glacial till soils would be classified as Type C soil. The deeper dense to very dense unweathered till soils would be classified as Type A soil. Accordingly, temporary excavations in Type C soils should have their slopes laid back at an inclination of 1.5:1 (Horizontal:Vertical) or flatter, from the toe to the crest of the slope. For Type A soils, side slopes can be laid back at a slope inclination of 0.75:1 or flatter. For temporary excavation slopes less than 8 feet in height in Type A soils, the lower 3.5 feet can be cut to a vertical condition, with a 0.75:1 slope graded above. For temporary excavation slopes greater than 8 feet in height up to a maximum height of 12 feet, the slope above the 3.5 feet vertical portion will need to be laid back at a minimum slope inclination of 1:1. No vertical cut with a backslope immediately above is allowed for excavation depths that exceed 12 feet. In this case, a four-foot vertical cut with an equivalent horizontal bench to the cut slope toe is required. All exposed temporary slope faces that will remain open for an extended period of time should be covered with a durable reinforced plastic membrane during construction to prevent slope raveling and rutting during periods of precipitation. The above information is provided solely for the benefit of the owner and other design consultants and should not be construed to imply that Terra Associates, Inc. assumes responsibility for job site safety. It is understood that job site safety is the sole responsibility of the project general contractor. Cross Section Existing Conditions A-A’ 2.57 (Seismic FS = 1.25) B-B’ 2.85 (Seismic FS = 1.46) August 5, 2021 Revised February 15, 2022 Project No. T-8566  Page No. 10   4.5 Foundations The buildings may be supported on conventional isolated or continuous footing foundations bearing on competent native soils or on structural fills placed above competent soils. Foundation subgrades should be prepared as recommended in Section 4.2 of this report. Perimeter foundations exposed to the weather should be at a minimum depth of 1.5 feet below final exterior grades for frost protection. Interior foundations can be constructed at any convenient depth below the floor slab. As noted above, the existing fill material observed in Test Pits TP-6 and TP-7 will need to be removed from below building foundations. Overexcavation of up to five feet should be expected in the vicinity of these test pits. The lateral extent of the overexcavation should be determined in the field during grading. We recommend designing foundations supported on competent material for a net allowable bearing capacity of 2,500 pounds per square foot (psf). For short-term loads, such as wind and seismic, a one-third increase in this allowable capacity can be used in design. With the anticipated loads and this bearing stress applied, building settlements should be less than one-inch total and one-half inch differential. For designing foundations to resist lateral loads, a base friction coefficient of 0.35 can be used. Passive earth pressure acting on the sides of the footings may also be considered. We recommend calculating this lateral resistance using an equivalent fluid weight of 350 pounds per cubic foot (pcf). We recommend not including the upper 12 inches of soil in this computation because they can be affected by weather or disturbed by future grading activity. This value assumes the foundations will be constructed neat against competent native soil or the excavations are backfilled with structural fill, as described in Section 4.2 of this report. The recommended passive and friction values include a safety factor of 1.5. 4.6 Slab-on-Grade Floors Slab-on-grade floors may be supported on subgrades prepared as recommended in Section 4.2 of this report. Immediately below the floor slabs, we recommend placing a four-inch-thick capillary break layer of clean, free- draining, coarse sand or fine gravel that has less than five percent passing the No. 200 sieve. This material will reduce the potential for upward capillary movement of water through the underlying soil and subsequent wetting of the floor slabs. The capillary break layer will not prevent moisture intrusion through the slab caused by water vapor transmission. Where moisture by vapor transmission is undesirable, such as covered floor areas, a common practice is to place a durable plastic membrane on the capillary break layer and then cover the membrane with a layer of clean sand or fine gravel to protect it from damage during construction, and aid in uniform curing of the concrete slab. It should be noted, if the sand or gravel layer overlying the membrane is saturated prior to pouring the slab, it will be ineffective in assisting uniform curing of the slab and can actually serve as a water supply for moisture seeping through the slab and affecting floor coverings. Therefore, in our opinion, covering the membrane with a layer of sand or gravel should be avoided if floor slab construction occurs during the wet winter months and the layer cannot be effectively drained. We recommend floor designers and contractors refer to the current American Concrete Institute (ACI) Manual of Concrete Practice for further information regarding vapor barrier installation below slab-on-grade floors. August 5, 2021 Revised February 15, 2022 Project No. T-8566  Page No. 11   4.7 Stormwater Facilities Drainage plans were not available at the time of this report; however, we anticipate that stormwater will be directed to either a detention vault or detention pond before being released in a controlled manner to the drainage basin. Detention Vault If development stormwater will be collected and routed to a detention vault, we expect that the bottom of the excavation for the vault would expose native, dense soils observed three to four feet below current site grades. Vault foundations supported by these native soils may be designed for an allowable bearing capacity of 5,000 psf, provided that the foundation subgrade is at least five feet below current site grades. It is possible that the excavation for construction of the detention vault may intercept seasonal perched groundwater. However, we expect that the volume of groundwater that might find its way into the excavation as seepage will likely be minor. We expect that conventional dewatering procedures, consisting of routing seepage along trenches to a sump, will provide for relatively dry working conditions. The magnitude of earth pressures developing on below-grade walls will depend on the quality and compaction of the wall backfill. We recommend placing and compacting wall backfill as structural fill, as described in Section 4.2 of this report. To prevent overstressing the walls during backfilling, heavy construction machinery should not be operated within five feet of the back of the wall. Wall backfill in this zone should be compacted with hand- operated equipment. To prevent hydrostatic pressure development, wall drainage must also be installed. A typical wall drainage detail is shown on Figure 3. With wall backfill placed and compacted as recommended and drainage properly installed, we recommend designing unrestrained walls for an active earth pressure equivalent to a fluid weighing 35 pounds per cubic foot (pcf). For restrained walls, an additional uniform load of 100 psf should be added to the 35 pcf. To account for typical traffic surcharge loading, the walls can be designed for an additional imaginary height of two feet (two- foot soil surcharge). For evaluation of wall performance under seismic loading, a uniform pressure equivalent to 8H psf, where H is the height of the below-grade portion of the wall, should be applied in addition to the static lateral earth pressure. These values assume a horizontal backfill condition and that no other surcharge loading, sloping embankments, or adjacent buildings will act on the wall. If such conditions exist, then the imposed loading must be included in the wall design. Friction at the base of foundations and passive earth pressure will provide resistance to these lateral loads. Values for these parameters are provided in Section 4.5 of this report. If it is not possible to discharge collected water at the footing invert elevation, the invert elevation of the wall drainpipe could be set equivalent to the outfall invert. For any portion of the wall that falls below the invert elevation of the wall drain, an earth pressure equivalent to a fluid weighing 85 pcf should be used. Detention Pond If fill berms would be constructed, the berm locations should be stripped of topsoil, duff, and soils containing organic material prior to the placement of fill. The fill berms should be constructed by placing structural fill in accordance with recommendations outlined in Section 4.2 of this report. Material used to construct pond berms should consist of predominately granular soils with a maximum size of three inches and a minimum of 20 percent fines. Terra Associates, Inc. should examine and test all onsite or imported materials proposed for use as berm fill prior to their use. August 5, 2021 Revised February 15, 2022 Project No. T-8566  Page No. 12   Due to exposure to fluctuating stored water levels and wave action, soils exposed on the interior side slopes of the ponds may be subject to some risk of periodic shallow instability or sloughing. Establishing interior slopes at a 3:1 gradient will significantly reduce or eliminate this potential. Exterior berm slopes and interior slopes above the maximum water surface should be graded to a finished inclination no steeper than 2:1. Finished slope faces should be thoroughly compacted and vegetated to guard against erosion. We should review the stormwater plans when they are completed and revise our recommendations, if required. 4.8 Infiltration Feasibility In our opinion, the soils observed in the test pits are not suitable for stormwater management using onsite infiltration or low impact development (LID) natural drainage practices (NDPs) due to the presence of the weathered glacial till at shallow depths. Conventional water quality/detention facilities with controlled release to the drainage basin should be used to manage development stormwater. 4.9 Drainage Surface Final exterior grades should promote free and positive drainage away from the site at all times. Water must not be allowed to pond or collect adjacent to foundations or within the immediate building areas. We recommend providing a positive drainage gradient away from the building perimeters. If this gradient cannot be provided, surface water should be collected adjacent to the structures and directed to appropriate storm facilities. Subsurface We recommend installing a continuous drain along the outside lower edge of the perimeter building foundations. The drains can be laid to grade at an invert elevation equivalent to the bottom of footing grade. The drains can consist of four-inch diameter perforated PVC pipe enveloped in washed half- to three-quarter inch gravel-sized drainage aggregate. The aggregate should extend six inches above and to the sides of the pipe. The foundation drains and roof downspouts should be tightlined separately to an approved point of controlled discharge. All drains should be provided with cleanouts at easily accessible locations. These cleanouts should be serviced at least once each year. 4.10 Utilities Utility pipes should be bedded and backfilled in accordance with American Public Works Association (APWA) or local jurisdictional requirements. At a minimum, trench backfill should be placed and compacted as structural fill, as described in Section 4.2 of this report. As noted, soils excavated onsite should be suitable for use as backfill material. However, the vast majority of the site soils are fine-grained and moisture sensitive; therefore, moisture conditioning may be necessary to facilitate proper compaction. If utility construction takes place during the winter, it may be necessary to import suitable wet weather fill for utility trench backfilling. August 5, 2021 Revised February 15, 2022 Project No. T-8566  Page No. 13   4.11 Pavement Pavement subgrade should be prepared as described in Section 4.2 of this report. Regardless of the degree of relative compaction achieved, the subgrade must be firm and relatively unyielding before paving. The subgrade should be proofrolled with heavy rubber-tire construction equipment such as a loaded 10-yard dump truck to verify this condition. The pavement design section is dependent upon the supporting capability of the subgrade soils and the traffic conditions to which it will be subjected. For residential access, with traffic consisting mainly of light passenger vehicles with only occasional heavy traffic, and with a stable subgrade prepared as recommended, we recommend the following pavement sections:  Two inches of hot mix asphalt (HMA) over four inches of crushed rock base (CRB).  3.5 inches full depth HMA over prepared subgrade. The paving materials used should conform to the Washington State Department of Transportation (WSDOT) specifications for half-inch class HMA and CRB. Long-term pavement performance will depend on surface drainage. A poorly drained pavement section will be subject to premature failure as a result of surface water infiltrating into the subgrade soils and reducing their supporting capability. For optimum pavement performance, we recommend surface drainage gradients of at least two percent. Some degree of longitudinal and transverse cracking of the pavement surface should be expected over time. Regular maintenance should be planned to seal cracks when they occur. 5.0 ADDITIONAL SERVICES Terra Associates, Inc. should review the final design drawings and specifications in order to verify that earthwork and foundation recommendations have been properly interpreted and implemented in project design. We should also provide geotechnical services during construction in order to observe compliance with our design concepts, specifications, and recommendations. This will allow for design changes if subsurface conditions differ from those anticipated prior to the start of construction. 6.0 LIMITATIONS We prepared this report in accordance with generally accepted geotechnical engineering practices. No other warranty, expressed or implied, is made. This report is the copyrighted property of Terra Associates, Inc. and is intended for specific application to the Towns on 12th project in Renton, Washington. This report is for the exclusive use of Madrona Place, LLC, and their authorized representatives. The analyses and recommendations presented in this report are based on data obtained from the subsurface explorations completed onsite. Variations in soil conditions can occur, the nature and extent of which may not become evident until construction. If variations appear evident, Terra Associates, Inc. should be requested to reevaluate the recommendations in this report prior to proceeding with construction. © 2022 Microsoft Corporation © 2022 TomTom SITE Environmental Earth Sciences Terra Associates, Inc. Consultants in Geotechnical Engineering Geology and Figure 1 VICINITY MAP 0 1000 2000 APPROXIMATE SCALE IN FEET REFERENCE: https://www.bing.com/maps ACCESSED 2/16/2022 Proj.No. T-8566 Date: FEB 2022 RENTON, WASHINGTON TOWNS ON 12TH TP-1 TP-2 TP-3 TP-4 TP-5 TP-6 TP-8 TP-7 TP-9 A B' B A' REFERENCE: REFERENCE ONLY AND SHOULD NOT BE USED FOR DESIGN OR CONSTRUCTION PURPOSES. DIMENSIONS ARE APPROXIMATE. IT IS INTENDED FOR NOTE: THIS SITE PLAN IS SCHEMATIC. ALL LOCATIONS AND LEGEND: 0 120 240 APPROXIMATE SCALE IN FEETSITE PLAN PROVIDED BY KING COUNTY iMAP. APPROXIMATE TEST PIT LOCATION Environmental Earth Sciences Terra Associates, Inc. Consultants in Geotechnical Engineering Geology and EXPLORATION LOCATION PLAN Figure 2Proj.No. T-8566 Date: FEB 2022 RENTON, WASHINGTON TOWNS ON 12TH 12" COMPACTED STRUCTURAL FILL EXCAVATED SLOPE (SEE REPORT TEXT FOR APPROPRIATE INCLINATIONS) SLOPE TO DRAIN 12" MINIMUM 3/4" MINUS WASHED GRAVEL 3" BELOW PIPE 12" OVER PIPE 4" DIAMETER PERFORATED PVC PIPE SEE NOTE 6"(MIN.) NOT TO SCALE NOTE: MIRADRAIN G100N PREFABRICATED DRAINAGE PANELS OR SIMILAR PRODUCT CAN BE SUBSTITUTED FOR THE 12-INCH WIDE GRAVEL DRAIN BEHIND WALL. DRAINAGE PANELS SHOULD EXTEND A MINIMUM OF SIX INCHES INTO 12-INCH THICK DRAINAGE GRAVEL LAYER OVER PERFORATED DRAIN PIPE. Environmental Earth Sciences Terra Associates, Inc. Consultants in Geotechnical Engineering Geology and TYPICAL WALL DRAINAGE DETAIL Figure 3Proj.No. T-8566 Date: FEB 2022 RENTON, WASHINGTON TOWNS ON 12TH   APPENDIX A FIELD EXPLORATION AND LABORATORY TESTING Towns on 12th Renton, Washington On June 10, 2021, we completed our site exploration by observing soil and groundwater conditions at nine test pits. The test pits were excavated using a trackhoe to a maximum depth of about 11.5 feet below existing site grades. Test pit locations were determined in the field by measurements from existing site features. The approximate location of the test pits is shown on the attached Exploration Location Plan, Figure 2. The Test Pit Logs are attached as Figures A-2 through A-10. A geotechnical engineer from our office conducted the field exploration. Our representative classified the soil conditions encountered, maintained a log of each test pit, obtained representative soil samples, and recorded water levels observed during excavation. All soil samples were visually classified in accordance with the Unified Soil Classification System (USCS) described on Figure A-1. Representative soil samples obtained from the test pits were placed in closed containers and taken to our laboratory for further examination and testing. The moisture content of each sample was measured and is reported on the individual Test Pit Logs. Grain size analyses were performed on selected samples. The results of the analysis are shown on Figures A-11 and A-12. Environmental Earth Sciences Terra Associates, Inc. Consultants in Geotechnical Engineering Geology and MAJOR DIVISIONS LETTER SYMBOL TYPICAL DESCRIPTION GRAVELS More than 50% of coarse fraction is larger than No. 4 sieve Clean Gravels (less than 5% fines) GW Well-graded gravels, gravel-sand mixtures, little or no fines. GP Poorly-graded gravels, gravel-sand mixtures, little or no fines. Gravels with fines GM Silty gravels, gravel-sand-silt mixtures, non-plastic fines. GC Clayey gravels, gravel-sand-clay mixtures, plastic fines. SANDS More than 50% of coarse fraction is smaller than No. 4 sieve Clean Sands (less than 5% fines) SW Well-graded sands, sands with gravel, little or no fines. SP Poorly-graded sands, sands with gravel, little or no fines. Sands with fines SM Silty sands, sand-silt mixtures, non-plastic fines. SC Clayey sands, sand-clay mixtures, plastic fines. SILTS AND CLAYS Liquid Limit is less than 50% ML Inorganic silts, rock flour, clayey silts with slight plasticity. CL Inorganic clays of low to medium plasticity. (Lean clay) OL Organic silts and organic clays of low plasticity. SILTS AND CLAYS Liquid Limit is greater than 50% MH Inorganic silts, elastic. CH Inorganic clays of high plasticity. (Fat clay) OH Organic clays of high plasticity. HIGHLY ORGANIC SOILS PT Peat.COARSE GRAINED SOILSMore than 50% material largerthan No. 200 sieve sizeFINE GRAINED SOILSMore than 50% material smallerthan No. 200 sieve sizeDEFINITION OF TERMS AND SYMBOLS COHESIONLESSCOHESIVE Standard Penetration Density Resistance in Blows/Foot Very Loose 0-4 Loose 4-10 Medium Dense 10-30 Dense 30-50 Very Dense >50 Standard Penetration Consistancy Resistance in Blows/Foot Very Soft 0-2 Soft 2-4 Medium Stiff 4-8 Stiff 8-16 Very Stiff 16-32 Hard >32 2" OUTSIDE DIAMETER SPILT SPOON SAMPLER 2.4" INSIDE DIAMETER RING SAMPLER OR SHELBY TUBE SAMPLER WATER LEVEL (Date) Tr TORVANE READINGS, tsf Pp PENETROMETER READING, tsf DD DRY DENSITY, pounds per cubic foot LL LIQUID LIMIT, percent PI PLASTIC INDEX N STANDARD PENETRATION, blows per foot UNIFIED SOIL CLASSIFICATION SYSTEM Figure A-1Proj.No. T-8566 Date: FEB 2022 RENTON, WASHINGTON TOWNS ON 12TH Sample No.Depth (ft)PROJECT NAME: PROJ. NO: LOGGED BY: LOCATION: DATE LOGGED: APPROX. ELEV: DEPTH TO CAVING: FIGURE DEPTH TO GROUNDWATER: SURFACE CONDITIONS: Description Consistency/ Relative Density W (%)interpreted as being indicative of other locations at the site. NOTE: This subsurface information pertains only to this test pit location and should not be 0 1 2 3 4 5 6 7 8 9 10 A-2 T-8566 ZN Renton, Washington Trees and shrubs June 10, 2021 Towns on 12th LOG OF TEST PIT NO. TP-1 NA NA NA 1 2 3 8.2 8.9 7.5 Medium Dense Dense (6 inches TOPSOIL) Yellow-brown to olive-brown silty SAND, fine to medium sand, moist, abundant tree roots. (SM) Olive-gray SAND with silt and gravel, fine to coarse sand, fine gravel, moist, mottled. (SP-SM) Test Pit terminated at approximately 8 feet. No groundwater seepage observed. Sample No.Depth (ft)PROJECT NAME: PROJ. NO: LOGGED BY: LOCATION: DATE LOGGED: APPROX. ELEV: DEPTH TO CAVING: FIGURE DEPTH TO GROUNDWATER: SURFACE CONDITIONS: Description Consistency/ Relative Density W (%)interpreted as being indicative of other locations at the site. NOTE: This subsurface information pertains only to this test pit location and should not be 0 1 2 3 4 5 6 7 8 9 10 11 12 A-3 T-8566 ZN Renton, Washington Trees and shrubs June 10, 2021 Towns on 12th LOG OF TEST PIT NO. TP-2 NA 8 feet NA 1 2 3 8.9 10.3 14.4 Medium Dense Dense to Very Dense (6 inches TOPSOIL) Yellow-brown to olive-gray silty SAND with gravel, fine to medium sand, fine to coarse gravel, moist. (SM) (Weathered Glacial Till) *Becomes olive-gray, mottled. Olive-gray silty SAND with gravel, fine to coarse sand, fine gravel, moist to wet. (Glacial Till) Test Pit terminated at approximately 10 feet. Minor groundwater seepage observed at approximately 8 feet. Sample No.Depth (ft)PROJECT NAME: PROJ. NO: LOGGED BY: LOCATION: DATE LOGGED: APPROX. ELEV: DEPTH TO CAVING: FIGURE DEPTH TO GROUNDWATER: SURFACE CONDITIONS: Description Consistency/ Relative Density W (%)interpreted as being indicative of other locations at the site. NOTE: This subsurface information pertains only to this test pit location and should not be 0 1 2 3 4 5 6 7 8 9 10 11 12 13 A-4 T-8566 ZN Renton, Washington Trees and shrubs June 10, 2021 Towns on 12th LOG OF TEST PIT NO. TP-3 NA NA NA 1 2 3 6.4 8.1 9.7 Medium Dense Medium Dense to Dense Very Dense (6 inches TOPSOIL) Yellow-brown to olive-gray silty SAND, fine to medium sand, moist. (SM) (Weathered Glacial Till) Olive-gray silty SAND with gravel, fine to medium sand, fine to coarse gravel, moist, mottled. (SM) (Weathered Glacial Till) Olive-gray silty SAND with gravel, fine to medium sand, fine to coarse gravel, moist. (SM) (Glacial Till) Test Pit terminated at approximately 11.5 feet. No groundwater seepage observed. Sample No.Depth (ft)PROJECT NAME: PROJ. NO: LOGGED BY: LOCATION: DATE LOGGED: APPROX. ELEV: DEPTH TO CAVING: FIGURE DEPTH TO GROUNDWATER: SURFACE CONDITIONS: Description Consistency/ Relative Density W (%)interpreted as being indicative of other locations at the site. NOTE: This subsurface information pertains only to this test pit location and should not be 0 1 2 3 4 5 6 7 8 9 A-5 T-8566 ZN Renton, Washington Trees and shrubs June 10, 2021 Towns on 12th LOG OF TEST PIT NO. TP-4 NA NA NA 1 2 3 4.6 7.3 6.5 Medium Dense Medium Dense to Dense Very Dense (6 inches TOPSOIL) Yellow-brown to olive-gray silty SAND with gravel, fine to medium sand, fine to coarse gravel, moist. (SM) (Weathered Glacial Till) *Becomes olive-gray, mottled. Olive-gray silty SAND with gravel, fine to medium sand, fine to coarse gravel, moist. (SM) (Glacial Till) Test Pit terminated at approximately 7.5 feet. No groundwater seepage observed. Sample No.Depth (ft)PROJECT NAME: PROJ. NO: LOGGED BY: LOCATION: DATE LOGGED: APPROX. ELEV: DEPTH TO CAVING: FIGURE DEPTH TO GROUNDWATER: SURFACE CONDITIONS: Description Consistency/ Relative Density W (%)interpreted as being indicative of other locations at the site. NOTE: This subsurface information pertains only to this test pit location and should not be 0 1 2 3 4 5 6 7 8 9 10 11 12 13 A-6 T-8566 ZN Renton, Washington Trees and shrubs June 10, 2021 Towns on 12th LOG OF TEST PIT NO. TP-5 NA NA NA 1 2 3 10.3 8.4 9.6 Medium Dense Dense to Very Dense (6 inches TOPSOIL) Yellow-brown to olive-gray silty SAND, fine to medium sand, moist, abundant tree roots. (SM) (Weathered Glacial Till) Olive-gray silty SAND with gravel, fine to medium sand, fine to coarse gravel, moist, abundant tree roots. (SM) (Glacial Till) *Boulder at approximately 6 feet. Test Pit terminated at approximately 11.5 feet. No groundwater seepage observed. Sample No.Depth (ft)PROJECT NAME: PROJ. NO: LOGGED BY: LOCATION: DATE LOGGED: APPROX. ELEV: DEPTH TO CAVING: FIGURE DEPTH TO GROUNDWATER: SURFACE CONDITIONS: Description Consistency/ Relative Density W (%)interpreted as being indicative of other locations at the site. NOTE: This subsurface information pertains only to this test pit location and should not be 0 1 2 3 4 5 6 7 8 9 10 11 A-7 T-8566 ZN Renton, Washington Trees and shrubs June 10, 2021 Towns on 12th LOG OF TEST PIT NO. TP-6 NA NA NA 1 2 3 4 16.0 8.7 12.0 8.5 Loose Dense Very Dense Dark brown silty SAND, fine sand, moist, tree roots. (SM) (TOPSOIL) FILL: Olive-brown SAND, fine to medium sand, moist, piece of concrete foundation, piece of fabric, piece of steel pipe, piece of PVC pipe, tree roots. (SP) Olive-gray silty SAND, fine to medium sand, moist. (SM) (Weathered Glacial Till) Olive-gray silty SAND with gravel, fine to medium sand, fine to coarse gravel, moist. (SM) (Glacial Till) Test Pit terminated at approximately 9.5 feet. No groundwater seepage observed. Sample No.Depth (ft)PROJECT NAME: PROJ. NO: LOGGED BY: LOCATION: DATE LOGGED: APPROX. ELEV: DEPTH TO CAVING: FIGURE DEPTH TO GROUNDWATER: SURFACE CONDITIONS: Description Consistency/ Relative Density W (%)interpreted as being indicative of other locations at the site. NOTE: This subsurface information pertains only to this test pit location and should not be 0 1 2 3 4 5 6 7 8 9 A-8 T-8566 ZN Renton, Washington Trees and shrubs June 10, 2021 Towns on 12th LOG OF TEST PIT NO. TP-7 NA NA NA 1 2 3 8.2 9.2 6.7 Loose Medium Dense Dense to Very Dense (6 inches TOPSOIL) FILL: Yellow-brown silty SAND, fine to medium sand, moist, piece of steel pipe, piece of PVC pipe, tree roots. Yellow-brown to olive-gray silty SAND, fine to medium sand, moist. (SM) (Weathered Glacial Till) Olive-gray silty SAND with gravel, fine to medium sand, fine to coarse gravel, moist. (SM) (Glacial Till) Test Pit terminated at approximately 7.5 feet. No groundwater seepage observed. Sample No.Depth (ft)PROJECT NAME: PROJ. NO: LOGGED BY: LOCATION: DATE LOGGED: APPROX. ELEV: DEPTH TO CAVING: FIGURE DEPTH TO GROUNDWATER: SURFACE CONDITIONS: Description Consistency/ Relative Density W (%)interpreted as being indicative of other locations at the site. NOTE: This subsurface information pertains only to this test pit location and should not be 0 1 2 3 4 5 6 7 8 A-9 T-8566 ZN Renton, Washington Trees and shrubs June 10, 2021 Towns on 12th LOG OF TEST PIT NO. TP-8 NA NA NA 1 2 7.7 6.6 Medium Dense Dense to Very Dense (6 inches TOPSOIL) Yellow-brown to olive-gray silty SAND, fine to medium sand, moist. (SM) (Weathered Glacial Till) Olive-gray silty SAND with gravel, fine to medium sand, fine to coarse gravel, moist. (SM) (Glacial Till) Test Pit terminated at approximately 6.5 feet. No groundwater seepage observed. Sample No.Depth (ft)PROJECT NAME: PROJ. NO: LOGGED BY: LOCATION: DATE LOGGED: APPROX. ELEV: DEPTH TO CAVING: FIGURE DEPTH TO GROUNDWATER: SURFACE CONDITIONS: Description Consistency/ Relative Density W (%)interpreted as being indicative of other locations at the site. NOTE: This subsurface information pertains only to this test pit location and should not be 0 1 2 3 4 5 6 7 8 9 10 11 12 A-10 T-8566 ZN Renton, Washington Trees and shrubs June 10, 2021 Towns on 12th LOG OF TEST PIT NO. TP-9 NA NA NA 1 2 3 4 5.5 76.9 28.4 7.4 Medium Dense Stiff Dense (6 inches TOPSOIL) Olive-brown silty SAND, fine to medium sand, moist. (SM) (Weathered Glacial Till) *Becomes dark brown. Olive-gray sandy SILT, moist, mottled. (ML) Olive-gray silty SAND with gravel, fine to medium sand, fine to coarse gravel, moist. (SM) (Glacial Till) Test Pit terminated at approximately 10 feet. No groundwater seepage observed. Tested By: FQ Checked By: ZN LL PL D85 D60 D50 D30 D15 D10 Cc Cu Material Description USCS AASHTO Project No.Client:Remarks: Project: Location: TP-1 Depth: 4 feet Sample Number: 2 Location: TP-2 Depth: 9 feet Sample Number: 3 Terra Associates, Inc. Kirkland, WA Figure 6.6902 2.4075 1.6266 0.4278 0.1266 7.2818 0.7730 0.4344 0.1939 SAND with silt and gravel SP-SM Silty SAND with gravel SM T-8566 Madrona Place, LLC A-11PERCENT FINER0 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 0.0 20.6 24.6 24.9 18.2 11.7 0.0 0.0 21.0 9.3 20.2 31.0 18.56 in.3 in.2 in.1½ in.1 in.¾ in.½ in.3/8 in.#4#10#20#30#40#60#100#140#200Particle Size Distribution Report Towns on 12th Tested By: FQ Checked By: ZN LL PL D85 D60 D50 D30 D15 D10 Cc Cu Material Description USCS AASHTO Project No.Client:Remarks: Project: Location: TP-6 Depth: 2.5 feet Sample Number: 2 Location: TP-9 Depth: 8 feet Sample Number: 3 Terra Associates, Inc. Kirkland, WA Figure 3.7815 0.5619 0.4171 0.2556 0.1689 0.1367 0.85 4.11 0.1791 SAND SP Sandy SILT ML T-8566 Madrona Place, LLC A-12PERCENT FINER0 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 0.0 12.6 8.1 28.6 45.7 5.0 0.0 0.0 1.1 2.3 5.1 22.4 69.16 in.3 in.2 in.1½ in.1 in.¾ in.½ in.3/8 in.#4#10#20#30#40#60#100#140#200Particle Size Distribution Report Towns on 12th   APPENDIX B SLOPE STABILITY RESULTS                                                 2.5722.5722.5722.572RuWater SurfacePhi (deg)Cohesion (psf)Strength TypeUnit Weight (lbs/ft3)ColorMaterial Name0None34100Mohr‐Coulomb125Weathered Glacial Till0None38250Mohr‐Coulomb130Glacial Till650600550500450400350-50050100150200250300350400450500550600ScenarioStaticGroupGroup 1CompanyTerra Associates, Inc.Drawn ByZakeyo Ngoma, P.E.File NameX-Section A-A'.slmdDate7/16/2021, 10:49:14 AMProjectTowns on 12thSLIDEINTERPRET 9.008 1.2461.2461.2461.246RuWater SurfacePhi (deg)Cohesion (psf)Strength TypeUnit Weight (lbs/ft3)ColorMaterial Name0None34100Mohr‐Coulomb125Weathered Glacial Till0None38250Mohr‐Coulomb130Glacial Till 0.3650600550500450400350-50050100150200250300350400450500550600ScenarioSeismicGroupGroup 1CompanyTerra Associates, Inc.Drawn ByZakeyo Ngoma, P.E.File NameX-Section A-A'.slmdDate7/16/2021, 10:49:14 AMProjectTowns on 12thSLIDEINTERPRET 9.008 2.8542.8542.8542.854RuWater SurfacePhi (deg)Cohesion (psf)Strength TypeUnit Weight (lbs/ft3)ColorMaterial Name0None340Mohr‐Coulomb120Silty SAND0None34100Mohr‐Coulomb125Weathered Glacial Till0None38250Mohr‐Coulomb130Glacial Till6005755505255004754504254003750255075100125150175200225250275300325350375400425450475ScenarioStaticGroupGroup 1CompanyTerra Associates, Inc.Drawn ByZakeyo Ngoma, P.E.File NameX-Section B-B'.slmdDate7/16/2021, 9:17:07 AMProjectTowns on 12thSLIDEINTERPRET 9.008 1.4561.4561.4561.456RuWater SurfacePhi (deg)Cohesion (psf)Strength TypeUnit Weight (lbs/ft3)ColorMaterial Name0None340Mohr‐Coulomb120Silty SAND0None34100Mohr‐Coulomb125Weathered Glacial Till0None38250Mohr‐Coulomb130Glacial Till 0.36005755505255004754504254003753500255075100125150175200225250275300325350375400425450475ScenarioSeismicGroupGroup 1CompanyTerra Associates, Inc.Drawn ByZakeyo Ngoma, P.E.File NameX-Section B-B'.slmdDate7/16/2021, 9:17:07 AMProjectTowns on 12thSLIDEINTERPRET 9.008