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HomeMy WebLinkAboutRS_Geotechnical_Report_251120_v1 GEOTECHNICAL REPORT Renton Carpenter 3601 Lincoln Avenue Northeast Renton, Washington Project No. T-9165 Prepared for: JKM Holdings, LLC Puyallup, Washington July 23, 2025 7-23-2025 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 Soils .................................................................................................................... 2 3.3 Groundwater ....................................................................................................... 3 3.4 Geologic Hazards ............................................................................................... 3 3.4.1 Erosion Hazard Areas ............................................................................... 3 3.4.2 Landslide Hazard Areas ............................................................................ 5 3.4.3 Coal Mine Hazard Areas .......................................................................... 6 3.4.4 Seismic Hazard Areas ............................................................................... 6 3.5 Seismic Site Class ............................................................................................... 7 4.0 Discussion and Recommendations .................................................................................. 7 4.1 General ...................................................................................................................... 7 4.2 Site Preparation and Grading .................................................................................... 7 4.3 Relative Slope Stability ............................................................................................ 9 4.4 Excavations ............................................................................................................. 10 4.5 Foundations ............................................................................................................ 11 4.6 Slab-on-Grade Floors .............................................................................................. 11 4.7 Stormwater Facilities .............................................................................................. 12 4.8 Infiltration Feasibility ............................................................................................. 13 4.9 Drainage .................................................................................................................. 13 4.10 Utilities ................................................................................................................. 13 4.11 Pavements ............................................................................................................. 14 5.0 Additional Services ........................................................................................................ 14 6.0 Limitations ..................................................................................................................... 14 Figures Vicinity Map ......................................................................................................................... Figure 1 Exploration Location Plan .................................................................................................... Figure 2 Typical Wall Drainage Detail ............................................................................................... Figure 3 Appendices Field Exploration and Laboratory Testing ....................................................................... Appendix A Soil Testing for Post Construction Soil Quality and Depth (Ecology BMP T5.13) ........ Appendix A Slide2 Output ................................................................................................................... Appendix B Geotechnical Report Renton Carpenter 3601 Lincoln Avenue Northeast Renton, Washington 1.0 PROJECT DESCRIPTION Based on a conceptual aerial site plan, the proposed project is a residential subdivision consisting of five single- family residential building lots, a stormwater vault, and associated access and utilities. The development will be divided by existing critical areas and existing roadways. Grading plans were not available at the time of this report. Based on existing site topography in relation to the proposed building lots, we expect grading to be minimal with cuts and fills between one and five feet being necessary to achieve final lot and roadway elevations. The conceptual site plan shows a proposed stormwater vault located in the western portion of the site. We expect the single-family residences will be two- to three-story, wood-frame buildings with their main floor levels framed over a crawl space and attached garage floors constructed at grade. Foundation loads are expected to be relatively light, in the range of 1 to 2 kips per foot for bearing walls and 20 to 30 kips for isolated columns. The recommendations contained in the following sections of this report are preliminary and based on our understanding of the above design features. We should review design drawings as they become available to verify 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 Our work was completed in accordance with our authorized proposal dated March 27, 2025. Accordingly, on May 1, 2025, we explored subsurface conditions at the site in five test pits excavated with a mini-excavator to maximum depths of approximately 10 to 11 feet below existing site grades. Using the results of our field exploration 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.  Slab-on-grade floors.  Stormwater facilities. July 23, 2025 Project No. T-9165 Page No. 2  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’ purview. A building envelope specialist or contractor should be consulted to address these issues, as needed. 3.0 SITE CONDITIONS 3.1 Surface The proposed project site consists of a single tax parcel totaling approximately 1.1 acres located at 3601 Lincoln Avenue Northeast in Renton, Washington. The approximate site location is shown on Figure 1. The property is currently developed with a single-family residence and detached garage along with a septic system, associated access, and landscaping in the southern half of the site. The northern half is undeveloped and covered with shrubs and weeds. Several small- to medium-sized trees are scattered throughout the property. The very western portion of the property is forested with an associated understory. Site topography slopes gently from the southeast to the west and northwest in the approximate eastern three-quarters of the property with an overall vertical relief of approximately 15 feet. Site grades transition to a steep slope descending further to the west over vertical reliefs ranging from 10 to 35 feet with slope gradients approaching 50 to 75 percent. There is a steeper slope with an elevation relief in excess of 100 feet located off the southwest corner of the parcel. This is the northern portion of a continuous steep slope that extends farther to the south and west towards the May Creek drainage ravine, which has been mapped as an ancient landslide feature. 3.2 Soils The soils at the site generally consisted of approximately one to eight inches of organic topsoil overlying approximately two to five feet of loose to dense fill material composed of silty sand to sandy silt with variable amounts of gravel, cobbles, asphalt fragments, and concrete fragments. The native soils underlying the fill material typically consisted of medium dense sand with silt, silty sand, and sandy silt with varying gravel, cobble, and boulder content (Recessional Outwash and Weathered Till). These upper sediments overly dense to very dense silty sand to clayey sand with varying gravel and cobble content (Unweathered Till) to the termination of the test pits. There were two exceptions to this general condition. No fill material was observed overlying the upper medium dense deposits in Test Pits TP-1 and TP-5. TP-1 and TP-5 also terminated in medium dense to dense sand with silt with gravel to silty sand observed underlying the dense to very dense till-like deposits. July 23, 2025 Project No. T-9165 Page No. 3 Review of the Preliminary Geologic Map of Seattle and Vicinity, Washington by H.H. Waldron, B.A. Leisch, D.R. Mullineaux, and D.R. Crandell (1961) as well as the Geologic Map of Surficial Deposits in the Seattle 30’ by 60’ Quadrangle, Washington by J.C. Yount, J.P. Minard, and G.R. Dembroff (1993), shows the site is mapped at the contact between Vashon Till (Qt/Qvt) to the north, Younger Sand/Recessional Outwash (Qys/Qvr) to the east, and Older Clay Till and Gravel/Pre-Fraser Deposits Undifferentiated (Qc/Qpf) to the west. It is our opinion that the Qys/Qvr and Qt/Qvt mapping is generally consistent with the subsurface findings at the site, although we believe the medium dense to dense sand deposits observed below the till-like deposits are more consistent with Advance Outwash (Qva) which is mapped approximately 3,100 feet to the east. Furthermore, reconnaissance performed along the western slope indicated that the slope is underlain at depth by the Qc/Qpf mapped unit. 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. Third party laboratory test results performed on samples from the upper one foot to determine organic content, pH, and cation exchange capacity are provided in Appendix A. The approximate locations of the subsurface explorations are shown on Figure 2. 3.3 Groundwater No groundwater seepage was observed in the test pits during our subsurface explorations. However, we observed some oxidation in the upper medium dense soils and mottling in the lower dense to very dense till-like soils which suggests perched groundwater seepage has developed at times throughout most of the site. The presence of perched groundwater seepage conditions is common for sites underlain by relatively impermeable, fine-grained deposits. This occurs as a result of rainfall that infiltrates through the upper weathered soil zone and becomes perched on the underlying impermeable soils. The fine-grained and till-like soils have a relatively low permeability that impedes the continued downward migration of the infiltrated rainfall. As a result, groundwater seepage will develop and tend to flow laterally along the contact. Locally, such seepage is referred to as interflow. We expect groundwater levels and flow rates will fluctuate seasonally and will typically reach their highest levels during, and shortly following, the wet winter months (November through May). Even though our field work was completed at the end of the wet season, rainfall was well below normal and would explain the lack of observed seepage with oxidation and mottling present. This indicates that even during normal wet season rainfall, seepage flow volumes would be relatively minor. 3.4 Geologic Hazards Chapter 4-11-070 of the City of 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.” Discussions related to erosion, landslide, mine, and seismic hazards are presented as follows. 3.4.1 Erosion Hazard Areas Chapter 4-3-050G.5.c of the RMC divides Erosion Hazards into two categories: i. “Low Erosion Hazard (EL): Areas with soil 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%). July 23, 2025 Project No. T-9165 Page No. 4 ii. High Erosion Hazard (EH): Areas with soils characterized by the Natural Resource Conservation Service (formerly U.S. Soil Conservation Service) as having severe to very severe erosion potential, and a slope more than fifteen percent (15%).” The majority of the onsite soils are classified as Alderwood gravelly sandy loam, 8 to 15 percent slopes (AgC), by the United States Department of Agriculture Natural Resources Conservation Service. (NRCS). With existing gradients, these soils are rated as having a moderate susceptibility to erosion. However, Alderwood gravelly sand loam, 15 to 30 percent slopes (AgD) and Alderwood and Kitsap soils, very steep (AkF), are mapped along the northwestern and western peripheries of the site, respectively. With existing gradients, these soils are rated as having a severe susceptibility to erosion when exposed. Therefore, the majority of the site, which encompasses the proposed development area, would be classified as a Low Erosion Hazard as defined by the RMC. The northwestern corner and western edge of the property would be classified as a High Erosion Hazard as defined by the RMC. While it is not expected that exposure of the soils along the western steep slopes will occur, measures to reduce erosion and runoff onto the steep slopes will be necessary. Implementation of temporary and permanent Best Management Practices (BMPs) for preventing and controlling erosion will be required and will mitigate the erosion hazard. As a minimum, we recommend implementing the following erosion and sediment control BMPs prior to, during, and immediately following construction activities at the site: 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 periods of inactivity. Temporary cover may consist of durable plastic sheeting that is securely anchored to the ground 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 down slope 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 the steep slope.  Provide onsite sediment detention for collected runoff. The contractor should perform daily review and maintenance of all erosion and sedimentation control measures at the site. July 23, 2025 Project No. T-9165 Page No. 5 3.4.2 Landslide Hazard Areas Section 4-3-050G.5.d of the RMC divides Landslide Hazards into four categories: i. “Low Landslide Hazard (LL): Areas with slopes less than 15 percent (15%) ii. Medium Landslide Hazard (LM): Areas with slopes between fifteen percent (15%) and forty percent (40%) and underlain by soils that largely consist of sand, gravel or glacial till. iii. High Landslide Hazard (LH): Areas with slopes greater than forty percent (40%), and areas with slopes between fifteen percent (15%) and forty percent (40%) and underlain by soils consisting largely of silt and clay. iv. Very High Landslide Hazard (LV): Areas of known mapped or identified landslide deposits.” Based on topographic data available on the King County iMap, slope gradients across the eastern and central portions of the site range from approximately 10 to 15 percent and would meet the definition of a Low Landslide Hazard per the RMC. The slope north of the garage descending to the northwest occurs over a gradient of approximately 20 percent and meets the definition of a Medium Landslide Hazard per the RMC. The steep slopes along the western site margin occur over gradients ranging from approximately 50 to 75 percent with known landslide deposits in the southwestern corner of the site. Therefore, the western and southwestern margins of the site would be classified as a High Landslide Hazard and Very High Landslide Hazard per the RMC, respectively. Section 4-3-050G.5.a provides additional categorization for steep slope types as: i. Sensitive Slopes: A hillside, or portion thereof, characterized by: (a) an average slope of twenty five percent (25%) to less than forty percent (40%) as identified in the City of Renton Steep Slope Atlas or in a method approved by the City; or (b) an average slope of forty percent (40%) or greater with a vertical rise of less than fifteen feet (15') as identified in the City of Renton Steep Slope Atlas or in a method approved by the City; (c) abutting an average slope of twenty five percent (25%) to forty percent (40%) 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 forty percent (40%) or greater grade and having a minimum vertical rise of fifteen feet (15') as identified in the City of Renton Steep Slope Atlas or in a method approved by the City. The City of Renton has mapped the western slopes with slope gradients ranging from 40% to 90%. Therefore, the western slopes would be classified as Protected Slopes, per the RMC. Review of mapped landslide areas on the Washington State Department of Natural Resources (DNR) Geologic Information Portal website shows the steep, west-facing slope located within the southwestern corner of the site, is mapped with scarps, flanks, and landslide deposits. These mappings are assigned a low confidence level with an estimated relative age of greater than 150 years (Pre-historic). The mapped landslide extends southwards and then west with an approximate length of 1,500 feet. A review of LIDAR imagery confirms the presence of the mapped west-facing head scarp. Additional analysis regarding the slopes has been completed for the project and can be found in Section 4.3 below. July 23, 2025 Project No. T-9165 Page No. 6 3.4.3 Coal Mine Hazards Section 4-3-050G.5.e of the RMC divides Coal Mine Hazards into three categories: 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 two hundred feet (200’) for steeply dipping seams, or deeper than fifteen (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 two hundred feet (200’) 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.” No evidence of adits, tunnels, drifts, shafts, or other mine workings were observed while onsite. Additionally, the DNR Geologic Information Portal shows no evidence of underground mine development in the vicinity of the site. Therefore, it is our opinion that the site meets the criteria for definition as a Low Coal Mine Hazard per the RMC. 3.4.3 Seismic Hazard Areas Section 4-3-050G.5.d of the RMC divides Seismic Hazards into two categories: i. “Low Seismic Hazards (SL): Areas underlain by dense soils or bedrock. These soils generally have site classifications of A through D, as defined by the International Building Code, 2012. ii. High Seismic Hazards (SH): Areas underlain by soft or loose, saturated soils. These soils generally have site classifications of E or F, as defined in the International Building Code, 2012.” A review of a map titled Faults and Earthquakes in Washington State, dated 2014 by Jessica L. Czajkowski and Jeffrey D. Bowman shows the southeastern flank of the Seattle Fault Zone passing approximately 800 feet north of the site. Quaternary-age activity of the fault (rupture within the last two million years) is predicted to have occurred during the Holocene, or within the last 11,700 years. Accordingly, during a seismic event, the risk of ground rupture along a fault line at the site is moderate to high, in our opinion. 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 sands underlying 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. July 23, 2025 Project No. T-9165 Page No. 7 The predominant formations underlying the site consist of medium dense to very dense sand with silt, silty sand, and sandy silt formations. Given these soil conditions and lack of a groundwater table, the risk for soil liquefaction occurring at the site is negligible. Section 4.3 of this report includes a discussion relating to the susceptibility of the steep slope to seismically induced landsliding. 3.5 Seismic Site Class Based on the site soil conditions, DNR mapping, and our knowledge of the area geology, per the current 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 conditions that would preclude the planned development. The proposed structures can be supported on conventional spread footings bearing on competent native soils or competent existing fill soils underlying the upper organic topsoil layer. Floor slabs and pavements can be similarly supported. The primary geotechnical concern for the site will be the presence of steep slopes and associated landslide hazards along the west side of the site. Preliminary site plans show a proposed stormwater detention vault located approximately 25 to 50 feet from the crest of the western slopes. Most of the native soils observed in the test pits contain a sufficient amount of fines (silt- and clay-sized particles) that will make compaction as structural fill difficult when the soils are wet or dry of optimum. Accordingly, the ability to use these soils from site excavations as structural fill will depend upon their moisture content and the prevailing weather conditions at the time of construction. If grading activities take place during extended periods of wet weather, the owner should be prepared to import free-draining granular material for use as structural fill and backfill. The relatively clean outwash deposits observed below the till deposits generally have a low percentage of soil fines and should be suitable for use as structural fill in most weather conditions. Detailed recommendations regarding these issues and other geotechnical design considerations are provided in the following sections of this report. 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, organic surface soils, and other deleterious materials should be stripped from areas of new construction. We expect surface stripping depths to remove the organic surficial soils would generally be about one to eight inches. Stripped vegetation debris should be removed from the site. Organic soils will not be suitable for use as structural fill but may be used for limited depths in nonstructural areas or for landscaping purposes. Abandoned utilities should be removed from below new building areas. Abandoned utility pipes that fall outside of new building areas can be left in place provided they are sealed to prevent intrusion of groundwater seepage and soil. July 23, 2025 Project No. T-9165 Page No. 8 Some loose to medium dense fill material containing medium- to large- sized concrete and asphalt debris, as well as the underlying original topsoil horizon, were observed in the upper two and one-half to three feet in Test Pits TP- 2 and TP-4. In the vicinity of these locations and wherever excessive construction debris is exposed, the fill material and underlying topsoil should be scarified and any pieces of construction debris should be segregated from the spoils. The excavated soil may then be recompacted, meeting the requirements outlined below. The lateral extent of the removal and recompaction should be determined in the field during grading procedures. Once clearing and grubbing operations are complete, cut and fill operations to establish desired grades can be initiated. A representative of Terra Associates, Inc. should examine all bearing surfaces to verify that the conditions encountered are as anticipated and are suitable for placement of structural fill or direct support of building and pavement elements. Our representative may request proof rolling of exposed surfaces with a heavy rubber-tired vehicle to determine if any isolated soft and yielding areas are present. If unstable yielding areas are observed, they should be cut to firm bearing soil and filled to grade with structural fill. If the 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. In general, our experience has shown that a minimum of 18 inches of clean, granular structural fill over the geotextile fabric should establish a stable bearing surface. Our study indicates most of the site soils contain a significant amount of fines (silt and clay-sized particles) that will make them difficult to compact as structural fill when too wet or too dry. Provided these soils are near optimum moisture when excavated, and are placed during dry weather conditions, we anticipate they will generally be suitable for direct use as structural fill. Materials that are wet of optimum will require drying the soil by aeration during dry weather conditions or by using soil amendments such as lime or Portland cement to reduce and stabilize the soil’s moisture content. If soil amendment products are used, additional Temporary Erosion and Sedimentation Control (TESC) BMPs will need to be implemented to mitigate potential impacts to stormwater runoff associated with possible elevated pH levels. If grading activities are planned during the wet winter season, or if they are initiated during the summer and extend into fall and winter, the contractor 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 3 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 planned to be 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 may be reduced to 90 percent. July 23, 2025 Project No. T-9165 Page No. 9 4.3 Relative Slope Stability Reconnaissance of the site’s steep slopes showed occasional instances of leaning or pistoled-butted trees; however, the majority of the slope supported the growth of upright, small- to moderately sized trees. We did not observe any indications of springs or seepage along the slope face although we did observe a small number of surficial skin slides at localized, near-vertical points along the slope. However, the slopes and existing southwestern head scarp are generally well vegetated with ferns, blackberries, and other forest understory. Additionally, we did not observe any evidence of new tension-cracking or new head scarps. We evaluated the stability of the steep slopes located along the west side of the site, as well as post construction stability under detention vault loading based on the provided conceptual plans. We completed a stability analysis at locations designated as Cross Sections A-A’ through C-C’ using the computer program Slide 2. The approximate cross-section locations are shown on Figure 2. Our analysis considered both static and pseudostatic (seismic) conditions. A horizontal acceleration of 0.36g was used in the pseudostatic analysis to simulate slope performance under earthquake loading. This value is based on one-half of the maximum considered earthquake (MCE) peak ground acceleration (PGA). 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) Dense Sand 125 35 0 Till-Like Soils 125 36 50 Medium Dense Soils 125 34 0 Old Till/Qpf 125 40 500 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 Cross Section Minimum Safety Factors Existing Conditions Post Construction A-A’ 1.9 (Seismic FS = 1.3) 1.9 (Seismic FS = 1.3) B-B’ 2.1 (Seismic FS = 1.4) 2.1 (Seismic FS = 1.4) C-C’ 2.7 (Seismic FS = 1.7) 2.7 (Seismic FS = 1.8) July 23, 2025 Project No. T-9165 Page No. 10 Based on our analysis, the western slopes are stable under static and pseudo-static conditions under both current and proposed conditions. As discussed in Section 3.4.2, the west and northwest facing slopes represented by cross sections B-B’ and C-C’ would be categorized as high landslide hazard slopes per the RMC. Section 4-3-050G.2 of the RMC does not require a buffer or building setback for this classification. The results of our stability analysis confirms a buffer or building setback would not be necessary from these areas. Because of the possible presence of a previous historic landsliding the southwest facing slope represented by cross section A-A’ in our stability analysis would require a 50-foot critical area buffer along with a 15-foot building setback from the edge of buffer per the RMC. Considering that the confidence level indicated on the DNR website regarding the southwest face slope being a landside scarp is low, the results of our stability analysis which show relatively high safety factors against instability and our reconnaissance of the slope, we would categorize the landslide hazard as high due to the slope height and inclination. Mass failure involving deep seated movement is not likely. The RMC does not require a buffer or building setback for high landslide hazards. However, we would recommend establishing a minimum setback of 25 feet from the slope crest that would consist of a 10-foot buffer and 15 foot building setback. The results of our analysis are attached in Appendix B. 4.4 Excavations All excavations at the site associated with confined spaces, such as lower-building level retaining walls, must be completed in accordance with local, state, and federal requirements. Based on the Washington State Safety and Health Administration (WSHA) regulations, the loose to medium dense fill and native soils would typically be classified as Type “C” soils. The native dense to very dense unweathered till deposits would be classified as Type “A” soils. 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. Side slopes in Type A soils 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-foot 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. July 23, 2025 Project No. T-9165 Page No. 11 Groundwater seepage should be anticipated within excavations during the wet winter months near the contact between the upper loose to medium dense soils and lower dense to very dense soils. We anticipate that the volume of water and rate of flow into relatively shallow excavations will be relatively minor and are not expected to impact the stability of the excavations when completed, as described. Conventional sump pumping procedures, along with a system of collection trenches, if necessary, should be capable of maintaining a relatively dry excavation for construction purposes. 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. 4.5 Foundations The proposed buildings may be supported on conventional spread footing foundations bearing on competent native soils, competent existing fill soils, or on structural fill placed above the competent soils. Foundation subgrade should be prepared as recommended in Section 4.2 of this report. Perimeter foundations exposed to the weather should bear a minimum depth of one and one-half feet below final exterior grades for frost protection. Interior foundations can be constructed at any convenient depth below the floor slab. The native silty sand and sandy silt soils will be easily disturbed by normal construction activity particularly when wet. Care will need to be exercised during construction to avoid excessively disturbing the subgrade. If disturbed, the material should be removed, and footings lowered to undisturbed material or grade restored with structural fill. During wet-weather conditions, to avoid disturbance, consideration should be given to protecting the fill foundation subgrade with a four-inch layer of crushed rock or lean mix concrete. Foundations for single-family residences bearing on competent soils can be dimensioned 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. With structural loading as anticipated and this bearing stress applied, estimated total settlements are less than one-half inch. For designing foundations to resist lateral loads, a base friction coefficient of 0.35 can be used. Passive earth pressures acting on the side of the footing and buried portion of the foundation stem wall can also be considered. We recommend calculating this lateral resistance using an equivalent fluid weight of 300 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 foundation will be constructed neat against competent existing fill, native soil, or backfilled with structural fill as described in Section 4.2 of this report. The values recommended include a safety factor of 1.5. 4.6 Slab-on-Grade Floors Slab-on-grade floors may be supported on subgrade prepared as recommended in Section 4.2 of this report. Immediately below the floor slab, we recommend placing a four-inch-thick capillary break layer composed of clean, 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 slab. July 23, 2025 Project No. T-9165 Page No. 12 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 to 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 not be effective in assisting uniform curing of the slab and can actually serve as a water supply for moisture bleeding through the slab, potentially 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. 4.7 Stormwater Facilities Site stormwater plans were not available at the time of this report. However, the provided conceptual plans show a stormwater detention vault is proposed in the western portion of the site. Detention Vault We expect the bottom of the excavations for the detention vaults will expose native dense to very dense till and/or dense sand. Vault foundations supported by these native soils may be designed for an allowable bearing capacity of 6,000 psf. For short-term loads, such as seismic, a one-third increase in this allowable capacity can be used. Vault walls should be designed as below-grade retaining walls. The magnitude of earth pressure development on engineered retaining walls will partly depend upon the quality 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 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 pounds per square foot (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 below-grade walls under seismic loading, an additional uniform lateral pressure equivalent to 8H psf, where H is the height of the below-grade portion of the wall in feet, can be used. These values assume a horizontal backfill condition and that no other surcharge loading such as traffic, 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 given in Section 4.4 of this report. July 23, 2025 Project No. T-9165 Page No. 13 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. We should review the stormwater plans when they are completed and revise our recommendations, if required. 4.8 Infiltration Feasibility It is our opinion that stormwater management at the site should not be managed with infiltration facilities. The soils underlying the site largely consist of silty sand which exhibit low permeabilities and would impede the downward migration of stormwater. Furthermore, the proximity of the development to steep slopes in the west of the site would preclude any development of infiltration facilities or low impact development (LID) elements as the introduction of stormwater adjacent to the steep slopes has the potential to impact the stability of the slopes. 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 perimeter. If this gradient cannot be provided, surface water should be collected adjacent to the structures and directed to appropriate storm facilities. Surface water must not be allowed to flow uncontrolled over the crest of the site slopes. Surface water should be directed away from the slope crests to a point of collection and controlled discharge. If site grades do not allow for directing surface water away from slopes, then water should be collected and tightlined down the slope face in a controlled manner. 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 that is enveloped in washed ½- to ¾-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 minimum, trench backfill should be placed and compacted as structural fill, as described in Section 4.2. As noted, soils excavated onsite should be suitable for use as backfill material. However, some 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. July 23, 2025 Project No. T-9165 Page No. 14 4.11 Pavements Pavement subgrades 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 proof rolled with heavy rubber-tired construction equipment such as a loaded ten-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).  Three and one-half inches of full depth HMA. The paving materials used should conform to the Washington State Department of Transportation (WSDOT) specifications for half-inch class HMA and CRB. If the existing roadway in the southern portion of the site is intended to remain without a change in alignment, we recommend a two-inch grind and overlay to improve surface conditions along the roadway. Long-term pavement performance will depend upon 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 earthwork and foundation recommendations have been properly interpreted and implemented in project design. We should also provide geotechnical service during construction 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 Renton Carpenter project in Renton, Washington and for the exclusive use of JKM Holdings, 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. © 2025 Microsoft Corporation © 2025 TomTom SITE Environmental Earth Sciences Terra Associates, Inc. Consultants in Geotechnical Engineering Geology and Figure 1 VICINITY MAP 0 500 1000 APPROXIMATE SCALE IN FEET REFERENCE: https://www.bing.com/maps ACCESSED 2025 Proj.No. T-9165 Date: JULY 2025 RENTON, WASHINGTON RENTON CARPENTER © 2025 Microsoft Corporation © 2025 Maxar ©CNES (2025) Distribution Airbus DS © 2025 TomTom A A' B' B C C' TP-1 TP-2 TP-3 TP-4 TP-5 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 Consultants in Geotechnical Engineering Terra Associates, Inc. Geology and Environmental Earth Sciences EXPLORATION LOCATION PLAN Figure 2 LEGEND: 0 50 100 APPROXIMATE SCALE IN FEET SITE PLAN PROVIDED BY BING MAPS. APPROXIMATE TEST PIT LOCATION Proj.No. T-9165 Date: JULY 2025 RENTON, WASHINGTON RENTON CARPENTERCROSS SECTION LOCATION 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-9165 Date: JULY 2025 RENTON, WASHINGTON RENTON CARPENTER APPENDIX A FIELD EXPLORATION AND LABORATORY TESTING Renton Carpenter Renton, Washington On May 1, 2025, we explored subsurface conditions at the site at 5 test pits excavated with a mini excavator to maximum depths of approximately 10 to 11 feet below existing site grades. The test pit locations are shown on Figure 2. The test pit locations were approximately determined in the field using GPS coordinates derived from Google Earth and by pacing from existing site features. The Test Pit Logs are presented on Figures A-2 through A-6. A geotechnical engineer from our office maintained a log of each test pit as it was excavated, classified the soil conditions encountered, and obtained representative soil samples. All soil samples were visually classified in the field in accordance with the Unified Soil Classification System. A copy of this classification is presented as Figure A-1. Representative soil samples obtained from the test pits were placed in sealed plastic bags and taken to our laboratory for further examination and testing. The moisture content of each sample was measured and is reported on the Test Pit Logs. Grain size analyses were performed on selected samples. The results of the grain size analyses are shown on Figure A-7. In addition, samples of the near surface soils were submitted to AmTest Laboratories for determination of the soil cation exchange capacity (CEC), pH and organic content. The test results are attached following Figure A-7. 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. CO A R S E G R A I N E D S O I L S Mo r e t h a n 5 0 % m a t e r i a l l a r g e r th a n N o . 2 0 0 s i e v e s i z e FI N E G R A I N E D S O I L S Mo r e t h a n 5 0 % m a t e r i a l s m a l l e r th a n N o . 2 0 0 s i e v e s i z e DEFINITION OF TERMS AND SYMBOLS CO H E S I O N L E S S CO H E S I V E 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-9165 Date: JULY 2025 RENTON, WASHINGTON RENTON CARPENTER Sa m p l e N o . De p t h ( f t ) 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-2 T-9165 MJX Renton, Washington Blackberries May 1, 2025 Renton Carpenter LOG OF TEST PIT NO.TP-1 NA NA NA 14.7 21.4 9.2 9.9 Medium Dense Dense to Very Dense Dense (8-inches ORGANIC TOPSOIL) Brownish-gray silty SAND, fine to medium sand, moist, slightly oxidized, trace gravel, trace rootlets, occasional cobble, occasional boulder. (SM) (Weathered Till) Gray silty SAND to clayey SAND, fine to coarse sand, moist, mottled, trace gravel, occasional cobble, occasional rootlet, weak to moderate cementation. (SM/SC) (Unweathered Till) Gray SAND with silt to SAND with silt and gravel, fine to medium sand, fine to coarse gravel, moist, occasional boulder, occasional cemented inclusion. (SP-SM) (Advance Outwash) Test Pit terminated at approximately 10 feet. No groundwater seepage observed. No caving observed. Sa m p l e N o . De p t h ( f t ) 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-9165 MJX Renton, Washington Blackberries May 1, 2025 Renton Carpenter LOG OF TEST PIT NO.TP-2 NA NA NA 16.9 22.8 16.4 18.0 12.7 12.3 Loose to MediumDense Medium Dense Dense Dense to Very Dense (1-inch ORGANIC TOPSOIL) FILL: Brownish-gray silty SAND, fine to medium sand, moist, scattered small- to medium-sized construction debris, scattered rootlets, trace gravel. (SM) (6-inches ORGANIC TOPSOIL) Brown silty SAND, fine to medium sand, moist, occasional gravel. (SM) (Weathered Till) Brownish-gray sandy SILT, fine to medium sand, moist, occasional rootlet, occasional gravel. (ML) (Weathered Till) Gray silty SAND, fine to coarse sand, moist, occasional gravel, occasional cobble, weak to moderate cementation. (SM) (Unweathered Till) Test Pit terminated at approximately 10 feet. No groundwater seepage observed. No caving observed. Sa m p l e N o . De p t h ( f t ) 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-4 T-9165 MJX Renton, Washington Blackberries May 1, 2025 Renton Carpenter LOG OF TEST PIT NO.TP-3 NA NA NA 10.2 20.9 19.7 11.7 10.8 Dense Medium Dense Dense to Very Dense (5-inches ORGANIC TOPSOIL) FILL: Gray silty SAND with gravel, fine to coarse sand, fine to coarse gravel, trace cobbles, trace small- to medium-sized asphalt fragments, occasional charcoal debris, occasional rootlet. (SM) Brown silty SAND, fine to medium sand, moist, trace gravel. (SM) (Younger Sand/Recessional Outwash) Gray silty SAND, fine to medium sand, moist, trace gravel, occasional cobble. (SM) (Younger Sand/Recessional Outwash) Gray silty SAND to silty SAND with gravel, fine to coarse sand, fine to coarse gravel, moist, occasional sand seam, weak to moderate cementation. (SM) (Unweathered Till) Test Pit terminated at approximately 10 feet. No groundwater seepage observed. No caving observed. Sa m p l e N o . De p t h ( f t ) 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-5 T-9165 MJX Renton, Washington Blackberries May 1, 2025 Renton Carpenter LOG OF TEST PIT NO.TP-4 NA NA NA 18.0 23.5 19.8 20.9 23.2 Medium Dense Dense (4-inches ORGANIC TOPSOIL) FILL: Brown and gray silty SAND with gravel to sandy SILT with gravel, fine to coarse sand, fine to coarse gravel, moist, trace small- to medium-sized asphalt fragments, occasional large-sized concrete debris, occasional cobble, occasional rootlet. (SM/ML) (6-inches ORGANIC TOPSOIL) Brown silty SAND, fine to medium sand, moist, occasional gravel, occasional cobble. (SM) (Younger Sand/Recessional Outwash) Gray silty SAND, fine to medium sand, moist, trace gravel, occasional cobble. (SM) (Recessional Outwash) Gray silty SAND, fine to medium sand, moist, trace gravel, weak cementation. (SM) (Till- like) Test Pit terminated at approximately 11 feet. No groundwater seepage observed. No caving observed. Sa m p l e N o . De p t h ( f t ) 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-6 T-9165 MJX Renton, Washington Grass May 1, 2025 Renton Carpenter LOG OF TEST PIT NO.TP-5 NA NA NA 18.6 20.4 24.0 29.3 Medium Dense Dense Medium Dense (4-inches ORGANIC TOPSOIL) Gray silty SAND, fine to medium sand, moist, slightly oxidized, scattered sand seams, occasional rootlet. (SM) (Younger Sand/Recessional Outwash) Gray silty SAND, fine to coarse sand, moist, trace fine gravel, trace sand seams, weak cementation. (SM) (Till-Like) Gray silty SAND, fine to medium sand, moist, scattered sandy silt layers, trace heavily decomposed granite inclusions, occasional sand seam. (SM) *Increasing moisture with depth* Test Pit terminated at approximately 10 feet. No groundwater seepage observed. No caving observed. Tested By: ZA LL PL D85 D60 D50 D30 D15 D10 Cc Cu Material Description USCS AASHTO Project No.Client:Remarks: Project: Location: Test Pit TP-1 Depth: -10 feet Sample Number: 4 Location: Test Pit TP-4 Depth: -8 feet Sample Number: 4 Location: Test Pit TP-5 Depth: -10 feet Sample Number: 4 Terra Associates, Inc. Kirkland, WA Figure 8.9864 0.5058 0.3543 0.1730 0.3059 0.2261 0.2069 0.1729 0.0925 0.2092 0.1423 0.0977 SM SM silty SAND with gravel silty SAND silty SAND SM T-9165 JKM Holdings, LLC A-7 PE R C E N T F I N E R 0 10 20 30 40 50 60 70 80 90 100 GRAIN SIZE - mm. 0.0010.010.1110100 % +3"Coarse % Gravel Fine Coarse Medium % Sand Fine Silt % Fines Clay 0.0 9.0 8.6 4.0 23.5 32.9 22.0 0.0 0.0 0.0 0.3 3.2 82.5 14.0 0.0 0.0 0.0 0.2 2.1 49.2 48.5 6 i n . 3 i n . 2 i n . 1½ i n . 1 i n . ¾ i n . ½ i n . 3/ 8 i n . #4 #1 0 #2 0 #3 0 #4 0 #6 0 #1 0 0 #1 4 0 #2 0 0 Particle Size Distribution Report Renton Carpenter Tested on May 5, 2025 Tested on May 5, 2025 Tested on May 5, 2025 May 22, 2025 Am Test Inc. 13600 NE 126th Place Suite C Kirkland, WA (425) 885-1664 www.amtestlab.com Professional Analytical Services 12220 113TH AVE NE, Ste. 130 COC Number: 47739 Kirkland, WA 98034 Ted Schepper: Enclosed please find the analytical data for your Renton Carpenter project. Your sample(s) were received on Tuesday, May 6, 2025 and properly maintained prior to the subsequent analysis. The analytical procedures used at AmTest are well documented and are typically derived from the protocols of the EPA, USDA, FDA, Standard Methods or the Army Corps of Engineers. Following the analytical results you will find the Quality Control (QA/QC) results. Please note that the detection limits that are listed in the body of the report refer to the Practical Quantitation Limits (PQL's), as opposed to the Method Detection Limits (MDL's). If you should have any questions pertaining to the data package, please feel free to contact me. Attention: Ted Schepper Project Number: T-9165 Project: Renton Carpenter Terra Associates Sincerely, ElementStationManager For Aaron Young President aarony@amtestlab.com Attention: Ted Schepper Project Name: Renton Carpenter Date Reported: 05/22/25 Kirkland, WA 98034 Terra Associates 12220 113TH AVE NE, Ste. 130 Professional Analytical Services Am Test Inc. 13600 NE 126th Place Suite C Kirkland, WA (425) 885-1664 www.amtestlab.com Date Received: 05/06/25 ANALYSIS REPORT Project #: T-9165 Date ReceivedDate SampledQualifiersMatrixSample Lab ID Reported Samples A25E0082-01 TP-1 -1'Solid 05/01/2025 05/06/2025 A25E0082-02 TP-2 -1'Solid 05/01/2025 05/06/2025 A25E0082-03 TP-2 -2'Solid 05/01/2025 05/06/2025 A25E0082-04 TP-3 -1'Solid 05/01/2025 05/06/2025 A25E0082-05 TP-4 -1'Solid 05/01/2025 05/06/2025 A25E0082-06 TP-5 -1'Solid 05/01/2025 05/06/2025 [TOC_1]Samples in Report[TOC] The contents of this report apply to the sample(s) analyzed in accordance with the chain of custody document. No duplication of this report is allowed, except in its entirety.Page 2 of 8 Attention: Ted Schepper Project Name: Renton Carpenter Date Reported: 05/22/25 Kirkland, WA 98034 Terra Associates 12220 113TH AVE NE, Ste. 130 Professional Analytical Services Am Test Inc. 13600 NE 126th Place Suite C Kirkland, WA (425) 885-1664 www.amtestlab.com Date Received: 05/06/25 ANALYSIS REPORT Project #: T-9165 AMTEST Identification Number: A25E0082-01 Client Identification: TP-1 -1' Sampling Date: 05/01/25 07:00 PARAMETER METHOD DATEUNITSQ RESULT RL ANALYST Conventional Chemistry Parameters by APHA/EPA Methods CEC (Cation Exchange Capacity)05/09/2025meq/100 g 0.500 LF7.35 EPA 9081 Total Organic Matter 05/12/2025%HV3.09 SM 2540G_2011 pH 05/21/2025pH Units NR6.2 SM 4500-H+B_2011 AMTEST Identification Number: A25E0082-02 Client Identification: TP-2 -1' Sampling Date: 05/01/25 07:00 PARAMETER METHOD DATEUNITSQ RESULT RL ANALYST Conventional Chemistry Parameters by APHA/EPA Methods CEC (Cation Exchange Capacity)05/09/2025meq/100 g 0.500 LF10.0 EPA 9081 Total Organic Matter 05/12/2025%HV4.05 SM 2540G_2011 pH 05/21/2025pH Units NR5.6 SM 4500-H+B_2011 AMTEST Identification Number: A25E0082-03 Client Identification: TP-2 -2' Sampling Date: 05/01/25 07:00 PARAMETER METHOD DATEUNITSQ RESULT RL ANALYST Conventional Chemistry Parameters by APHA/EPA Methods CEC (Cation Exchange Capacity)05/09/2025meq/100 g 0.500 LF12.5 EPA 9081 Total Organic Matter 05/12/2025%HV5.79 SM 2540G_2011 pH 05/21/2025pH Units NR5.4 SM 4500-H+B_2011 [TOC_1]Sample Results[TOC] The contents of this report apply to the sample(s) analyzed in accordance with the chain of custody document. No duplication of this report is allowed, except in its entirety.Page 3 of 8 Attention: Ted Schepper Project Name: Renton Carpenter Date Reported: 05/22/25 Kirkland, WA 98034 Terra Associates 12220 113TH AVE NE, Ste. 130 Professional Analytical Services Am Test Inc. 13600 NE 126th Place Suite C Kirkland, WA (425) 885-1664 www.amtestlab.com Date Received: 05/06/25 ANALYSIS REPORT Project #: T-9165 AMTEST Identification Number: A25E0082-04 Client Identification: TP-3 -1' Sampling Date: 05/01/25 07:00 PARAMETER METHOD DATEUNITSQ RESULT RL ANALYST Conventional Chemistry Parameters by APHA/EPA Methods CEC (Cation Exchange Capacity)05/09/2025meq/100 g 0.500 LF8.00 EPA 9081 Total Organic Matter 05/12/2025%HV4.33 SM 2540G_2011 pH 05/21/2025pH Units NR5.6 SM 4500-H+B_2011 AMTEST Identification Number: A25E0082-05 Client Identification: TP-4 -1' Sampling Date: 05/01/25 07:00 PARAMETER METHOD DATEUNITSQ RESULT RL ANALYST Conventional Chemistry Parameters by APHA/EPA Methods CEC (Cation Exchange Capacity)05/09/2025meq/100 g 0.500 LF8.07 EPA 9081 Total Organic Matter 05/12/2025%HV4.12 SM 2540G_2011 pH 05/21/2025pH Units NR5.4 SM 4500-H+B_2011 AMTEST Identification Number: A25E0082-06 Client Identification: TP-5 -1' Sampling Date: 05/01/25 07:00 PARAMETER METHOD DATEUNITSQ RESULT RL ANALYST Conventional Chemistry Parameters by APHA/EPA Methods CEC (Cation Exchange Capacity)05/09/2025meq/100 g 0.500 LF7.42 EPA 9081 Total Organic Matter 05/12/2025%HV2.78 SM 2540G_2011 pH 05/21/2025pH Units NR5.3 SM 4500-H+B_2011 The contents of this report apply to the sample(s) analyzed in accordance with the chain of custody document. No duplication of this report is allowed, except in its entirety.Page 4 of 8 Attention: Ted Schepper Project Name: Renton Carpenter Date Reported: 05/22/25 Kirkland, WA 98034 Terra Associates 12220 113TH AVE NE, Ste. 130 Professional Analytical Services Am Test Inc. 13600 NE 126th Place Suite C Kirkland, WA (425) 885-1664 www.amtestlab.com Date Received: 05/06/25 ANALYSIS REPORT Project #: T-9165 Quality Control Conventional Chemistry Parameters by APHA/EPA Methods RPD LimitRPD %REC Limits%REC Source ResultUnits Spike Level Reporting LimitQualResult Analyte Batch: BCE0138 - EPA 9081 (CEC) Prepared: 05/08/25 Analyzed: 05/09/25 Calibration Blank (BCE0138-CCB1) CEC (Cation Exchange Capacity)meq/100 gUND Prepared: 05/08/25 Analyzed: 05/09/25 Calibration Check (BCE0138-CCV1) CEC (Cation Exchange Capacity)85-115%101%2.000meq/100 g0.5002.02 Source: A25E0082-03 Prepared: 05/08/25 Analyzed: 05/09/25 Duplicate (BCE0138-DUP1) CEC (Cation Exchange Capacity)20612.5meq/100 g0.50011.7 Batch: BCE0181 - No Prep - WC Soil Prepared & Analyzed: 05/12/25 Calibration Blank (BCE0181-CCB1) Organic Matter %0.200 Prepared & Analyzed: 05/12/25 Calibration Blank (BCE0181-CCB2) Organic Matter %0.190 Prepared & Analyzed: 05/12/25 Calibration Blank (BCE0181-CCB5) Organic Matter %0.150 Prepared & Analyzed: 05/12/25 Calibration Blank (BCE0181-CCB6) Organic Matter %0.150 Source: A25E0082-01 Prepared & Analyzed: 05/12/25 Duplicate (BCE0181-DUP1) Organic Matter 25.933.09%3.00 Source: A25E0082-06 Prepared & Analyzed: 05/12/25 Duplicate (BCE0181-DUP2) Organic Matter 25.932.78%2.70 Source: A25E0182-04 Prepared & Analyzed: 05/12/25 Duplicate (BCE0181-DUP5) Organic Matter 25.982.71%2.93 Source: A25E0174-02 Prepared & Analyzed: 05/12/25 Duplicate (BCE0181-DUP6) Organic Matter 25.912.65%2.62 Batch: BCE0330 - No Prep - WC Soil [TOC_1]Quality Assurance Results[TOC] The contents of this report apply to the sample(s) analyzed in accordance with the chain of custody document. No duplication of this report is allowed, except in its entirety.Page 5 of 8 Attention: Ted Schepper Project Name: Renton Carpenter Date Reported: 05/22/25 Kirkland, WA 98034 Terra Associates 12220 113TH AVE NE, Ste. 130 Professional Analytical Services Am Test Inc. 13600 NE 126th Place Suite C Kirkland, WA (425) 885-1664 www.amtestlab.com Date Received: 05/06/25 ANALYSIS REPORT Project #: T-9165 Quality Control (Continued) Conventional Chemistry Parameters by APHA/EPA Methods (Continued) RPD LimitRPD %REC Limits%REC Source ResultUnits Spike Level Reporting LimitQualResult Analyte Batch: BCE0330 - No Prep - WC Soil (Continued) Prepared: 05/20/25 Analyzed: 05/21/25 Calibration Check (BCE0330-CCV1) pH 85-115%100%6.860pH Units6.8 Prepared: 05/20/25 Analyzed: 05/21/25 Calibration Check (BCE0330-CCV2) pH 85-115%100%6.860pH Units6.9 Source: A25E0167-01 Prepared: 05/20/25 Analyzed: 05/21/25 Duplicate (BCE0330-DUP1) pH 100.56.4pH Units6.4 Source: A25E0368-02 Prepared: 05/20/25 Analyzed: 05/21/25 Duplicate (BCE0330-DUP2) pH 1036.1pH Units6.3 The contents of this report apply to the sample(s) analyzed in accordance with the chain of custody document. No duplication of this report is allowed, except in its entirety.Page 6 of 8 Attention: Ted Schepper Project Name: Renton Carpenter Date Reported: 05/22/25 Kirkland, WA 98034 Terra Associates 12220 113TH AVE NE, Ste. 130 Professional Analytical Services Am Test Inc. 13600 NE 126th Place Suite C Kirkland, WA (425) 885-1664 www.amtestlab.com Date Received: 05/06/25 ANALYSIS REPORT Project #: T-9165 Notes and Definitions DefinitionItem Dry Sample results reported on a dry weight basis. ND Analyte NOT DETECTED at or above the reporting limit. RPD Relative Percent Difference %REC Percent Recovery Source Sample that was matrix spiked or duplicated. [TOC_1]Qualifiers and Definitions[TOC] The contents of this report apply to the sample(s) analyzed in accordance with the chain of custody document. No duplication of this report is allowed, except in its entirety.Page 7 of 8 APPENDIX B SLIDE2 OUTPUT 1.91.9 400.00 lbs/ft2 1.91.9RuWater  Surface Phi  (deg) Cohesion  (psf) Strength  Type Unit  Weight  (lbs/ft3) ColorMaterial  Name 0None350Mohr‐ Coulomb125Dense  Sand 0None3650Mohr‐ Coulomb125Till‐Like  Soils 0None340Mohr‐ Coulomb125 Medium  Dense  Soils 0None40500Mohr‐ Coulomb125Old Till/ Qpf 35 0 30 0 25 0 20 0 15 0 10 0 50 -100 -50 0 50 100 150 200 250 300 350 400 Scenario Master ScenarioGroupExisting Conditions - Static Company Terra Associates, Inc.Drawn By M. Xenos File Name A-A'.slmdDateMay 8, 2025 Project T-9165 Renton Carpenter SLIDEINTERPRET 9.008 1.01.0 400.00 lbs/ft2 1.01.0RuWater  Surface Phi  (deg) Cohesion  (psf) Strength  Type Unit  Weight  (lbs/ft3) ColorMaterial  Name 0None350Mohr‐ Coulomb125Dense  Sand 0None3650Mohr‐ Coulomb125Till‐Like  Soils 0None340Mohr‐ Coulomb125 Medium  Dense  Soils 0None40500Mohr‐ Coulomb125Old Till/ Qpf 0.36 40 0 30 0 20 0 10 0 -200 -100 0 100 200 300 400 Scenario Existing Conditions - SeismicGroupExisting Conditions - Static Company Terra Associates, Inc.Drawn By M. Xenos File Name A-A'.slmdDateMay 8, 2025 Project T-9165 Renton Carpenter SLIDEINTERPRET 9.008 1.91.91.91.9RuWater  Surface Phi  (deg) Cohesion  (psf) Strength  Type Unit  Weight  (lbs/ft3) ColorMaterial  Name 0None350Mohr‐ Coulomb125Dense  Sand 0None3650Mohr‐ Coulomb125Till‐Like  Soils 0None340Mohr‐ Coulomb125 Medium  Dense  Soils 0None40500Mohr‐ Coulomb125Old Till/ Qpf 35 0 30 0 25 0 20 0 15 0 10 0 50 -150 -100 -50 0 50 100 150 200 250 300 350 400 Scenario Master ScenarioGroupPost-Construction- Static Company Terra Associates, Inc.Drawn By M. Xenos File Name A-A'.slmdDateMay 8, 2025 Project T-9165 Renton Carpenter SLIDEINTERPRET 9.008 1.01.01.01.0 RuWater  Surface Phi  (deg) Cohesion  (psf) Strength  Type Unit  Weight  (lbs/ft3) ColorMaterial  Name 0None350Mohr‐ Coulomb125Dense  Sand 0None3650Mohr‐ Coulomb125Till‐Like  Soils 0None340Mohr‐ Coulomb125 Medium  Dense  Soils 0None40500Mohr‐ Coulomb125Old Till/ Qpf 56 ft 0.36 Safety Factor 0.0 0.3 0.5 0.8 1.0 1.3 1.5 1.8 2.0 2.3 2.5 2.8 3.0 3.3 3.5 3.8 4.0 4.3 4.5 4.8 5.0 5.3 5.5 5.8 6.0+ 40 0 30 0 20 0 10 0 -200 -100 0 100 200 300 400 Scenario Post-Construction - SeismicGroupPost-Construction- Static Company Terra Associates, Inc.Drawn By M. Xenos File Name A-A'.slmdDateMay 8, 2025 Project T-9165 Renton Carpenter SLIDEINTERPRET 9.008 0.3 in0.3 in0.3 in0.3 in RuWater  Surface Phi  (deg) Cohesion  (psf) Strength  Type Unit  Weight  (lbs/ ft3) ColorMaterial  Name 0None350Mohr‐ Coulomb125Dense  Sand 0None3650Mohr‐ Coulomb125Till‐Like  Soils 0None340Mohr‐ Coulomb125 Medium  Dense  Soils 0None40500Mohr‐ Coulomb125Old Till/ Qpf 56 ft Newmark Displacement (in) 0.0 0.0 0.0 0.1 0.1 0.1 0.1 0.1 0.1 0.2 0.2 0.2 0.2 0.2 0.2 0.3 0.3 0.3 0.3 0.3 0.3 0.4 0.4 0.4 0.4+ 40 0 30 0 20 0 10 0 -200 -100 0 100 200 300 400 Scenario Master ScenarioGroupNewmark Company Terra Associates, Inc.Drawn By M. Xenos File Name A-A'.slmdDateMay 8, 2025 Project T-9165 Renton Carpenter SLIDEINTERPRET 9.008 2.12.1 400.00 lbs/ft2 2.12.1 RuWater  Surface Phi  (deg) Cohesion  (psf) Strength  Type Unit  Weight  (lbs/ ft3) ColorMaterial  Name 0None350Mohr‐ Coulomb125Dense  Sand 0None3650Mohr‐ Coulomb125Till‐Like  Soils 0None340Mohr‐ Coulomb125 Medium  Dense  Soils 0None40500Mohr‐ Coulomb125Old Till/ Qpf 35 0 30 0 25 0 20 0 15 0 10 0 50 -150 -100 -50 0 50 100 150 200 250 300 350 400 Scenario Master ScenarioGroupExisting Conidtions - Static Company Terra Associates, Inc.Drawn By M. Xenos File Name B-B'.slmdDateMay 8, 2025 Project T-9165 Renton Carpenter SLIDEINTERPRET 9.008 1.11.1 400.00 lbs/ft2 1.11.1 RuWater  Surface Phi  (deg) Cohesion  (psf) Strength  Type Unit  Weight  (lbs/ ft3) ColorMaterial  Name 0None350Mohr‐ Coulomb125Dense  Sand 0None3650Mohr‐ Coulomb125Till‐Like  Soils 0None340Mohr‐ Coulomb125 Medium  Dense  Soils 0None40500Mohr‐ Coulomb125Old Till/ Qpf 0.36 40 0 30 0 20 0 10 0 -200 -100 0 100 200 300 400 Scenario Existing Conditions - SeismicGroupExisting Conidtions - Static Company Terra Associates, Inc.Drawn By M. Xenos File Name B-B'.slmdDateMay 8, 2025 Project T-9165 Renton Carpenter SLIDEINTERPRET 9.008 2.12.12.12.1 RuWater  Surface Phi  (deg) Cohesion  (psf) Strength  Type Unit  Weight  (lbs/ ft3) ColorMaterial  Name 0None350Mohr‐ Coulomb125Dense  Sand 0None3650Mohr‐ Coulomb125Till‐Like  Soils 0None340Mohr‐ Coulomb125 Medium  Dense  Soils 0None40500Mohr‐ Coulomb125Old Till/ Qpf 35 0 30 0 25 0 20 0 15 0 10 0 50 -150 -100 -50 0 50 100 150 200 250 300 350 Scenario Master ScenarioGroupPost-Construction - Static Company Terra Associates, Inc.Drawn By M. Xenos File Name B-B'.slmdDateMay 8, 2025 Project T-9165 Renton Carpenter SLIDEINTERPRET 9.008 1.11.11.11.1 RuWater  Surface Phi  (deg) Cohesion  (psf) Strength  Type Unit  Weight  (lbs/ft3) ColorMaterial  Name 0None350Mohr‐ Coulomb125Dense  Sand 0None3650Mohr‐ Coulomb125Till‐Like  Soils 0None340Mohr‐ Coulomb125 Medium  Dense  Soils 0None40500Mohr‐ Coulomb125Old Till/ Qpf 0.36 40 0 30 0 20 0 10 0 -200 -100 0 100 200 300 400 Scenario Post-Construction - SeismicGroupPost-Construction - Static Company Terra Associates, Inc.Drawn By M. Xenos File Name B-B'.slmdDateMay 8, 2025 Project T-9165 Renton Carpenter SLIDEINTERPRET 9.008 2.72.72.72.7RuWater  Surface Phi  (deg) Cohesion  (psf) Strength  Type Unit  Weight  (lbs/ft3) ColorMaterial  Name 0None350Mohr‐ Coulomb125Dense  Sand 0None3650Mohr‐ Coulomb125Till‐Like  Soils 0None340Mohr‐ Coulomb125 Medium  Dense  Soils 0None40500Mohr‐ Coulomb125Old Till/ Qpf 22 0 20 0 18 0 16 0 14 0 0 20 40 60 80 100 120 140 160 Scenario Master ScenarioGroupExisting Conidtions - Static Company Terra Associates, Inc.Drawn By M. Xenos File Name C-C'.slmdDateMay 8, 2025 Project T-9165 Renton Carpenter SLIDEINTERPRET 9.008 1.21.21.21.2RuWater  Surface Phi  (deg) Cohesion  (psf) Strength  Type Unit  Weight  (lbs/ft3) ColorMaterial  Name 0None350Mohr‐ Coulomb125Dense  Sand 0None3650Mohr‐ Coulomb125Till‐Like  Soils 0None340Mohr‐ Coulomb125 Medium  Dense  Soils 0None40500Mohr‐ Coulomb125Old Till/ Qpf 0.36 24 0 22 0 20 0 18 0 16 0 14 0 -20 0 20 40 60 80 100 120 140 160 Scenario Existing Conditions - SeismicGroupExisting Conidtions - Static Company Terra Associates, Inc.Drawn By M. Xenos File Name C-C'.slmdDateMay 8, 2025 Project T-9165 Renton Carpenter SLIDEINTERPRET 9.008 2.72.72.72.7RuWater  Surface Phi  (deg) Cohesion  (psf) Strength  Type Unit  Weight  (lbs/ft3) ColorMaterial  Name 0None350Mohr‐ Coulomb125Dense  Sand 0None3650Mohr‐ Coulomb125Till‐Like  Soils 0None340Mohr‐ Coulomb125 Medium  Dense  Soils 0None40500Mohr‐ Coulomb125Old Till/ Qpf 24 0 22 0 20 0 18 0 16 0 14 0 -20 0 20 40 60 80 100 120 140 160 Scenario Master ScenarioGroupPost-Construction - Static Company Terra Associates, Inc.Drawn By M. Xenos File Name C-C'.slmdDateMay 8, 2025 Project T-9165 Renton Carpenter SLIDEINTERPRET 9.008 1.31.31.31.3 RuWater  Surface Phi  (deg) Cohesion  (psf) Strength  Type Unit  Weight  (lbs/ft3) ColorMaterial  Name 0None350Mohr‐ Coulomb125Dense  Sand 0None3650Mohr‐ Coulomb125Till‐Like  Soils 0None340Mohr‐ Coulomb125 Medium  Dense  Soils 0None40500Mohr‐ Coulomb125Old Till/ Qpf 0.36 22 0 20 0 18 0 16 0 14 0 -20 0 20 40 60 80 100 120 140 160 Scenario Post-Construction - SeismicGroupPost-Construction - Static Company Terra Associates, Inc.Drawn By M. Xenos File Name C-C'.slmdDateMay 8, 2025 Project T-9165 Renton Carpenter SLIDEINTERPRET 9.008