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HomeMy WebLinkAboutRS_Geotechnical_Engineering_Report_260401_v1GEOTECHNICAL REPORT NE 1st Subdivision 4612 Northeast 1st Street Renton, Washington Project No. T-9277 Prepared For: Mr. Amani Imran Renton, Washington January 6, 2026 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 ....................................................................................................... 2 3.4 Geologic Hazards ................................................................................................ 3 3.4.1 Landslide Hazard Areas ............................................................................ 3 3.4.2 Erosion Hazard Areas ............................................................................... 3 3.4.3 Seismic Hazard Areas ............................................................................... 4 3.4.3 Coal Mine Hazard Areas ........................................................................... 4 3.5 Seismic Site Class ............................................................................................... 5 4.0 Discussion and Recommendations ................................................................................... 5 4.1 General ................................................................................................................ 5 4.2 Site Preparation and Grading .............................................................................. 5 4.3 Excavations ......................................................................................................... 6 4.4 Foundations ......................................................................................................... 7 4.5 Slab-on-Grade Floors .......................................................................................... 8 4.6 Infiltration Feasibility ......................................................................................... 8 4.7 Utilities ................................................................................................................ 9 4.8 Drainage .............................................................................................................. 9 4.9 Pavements ........................................................................................................... 9 5.0 Additional Services ........................................................................................................ 10 6.0 Limitations ..................................................................................................................... 10 Figures Vicinity Map ......................................................................................................................... Figure 1 Exploration Location Plan .................................................................................................... Figure 2 Appendix Field Exploration and Laboratory Testing ....................................................................... Appendix A Geotechnical Report NE 1st Subdivision 4612 Northeast 1st Street Renton, Washington 1.0 PROJECT DESCRIPTION The project consists of developing the site with six residential building lots along with associated access and utilities. Grading and development plans were not available at the time of this report. Based on the existing topography, we expect cuts and fills to achieve building lot and roadway grades will range from one to five feet. Structural loading for the residential structures is expected to be light with bearing walls carrying loads of 1 to 3 kips per foot and isolated columns carrying maximum loads of 30 to 40 kips. The recommendations in the following sections of this report are based on our understanding of the preceding design features. We should review design drawings as they become available to verify our recommendations have been properly interpreted and to supplement them, if required. 2.0 SCOPE OF WORK Our work was completed in accordance with our authorized proposal dated November 21, 2025. Accordingly, on December 2, 2025, we explored subsurface conditions at the site at three test pits excavated with a mini track- mounted excavator to maximum depths of approximately six to eight-and-one-half feet below existing site grades. Using the results of our subsurface 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.  Seismic criteria per the 2021 International Building Code (IBC).  Geologic Hazards per City of Renton Municipal Code.  Site preparation and grading.  Excavations.  Foundations.  Slab-on-grade floors.  Infiltration Feasibility.  Utilities.  Drainage.  Pavements. January 6, 2026 Project No. T-9277 Page No. 2 It should be noted, 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 contactor should be consulted to address these issues, as needed. 3.0 SITE CONDITIONS 3.1 Surface The project site consists of a single tax parcel totaling approximately 0.78 acres of land located at 4612 Northeast 1st Street in Renton, Washington. The site location is shown on Figure 1. The site is currently developed with a single-family home, detached garage, grass lawn, and ornamental trees. Site topography consists of a relatively flat area in the north that transitions to a slight slope that descends from the north to the south with an overall relief of approximately 15 feet. 3.2 Soils The soils observed in the test pits generally consisted of approximately 6 to 12 inches of organic topsoil overlying approximately 1 to 3 feet of medium dense possible fill material consisting of sand with silt and gravel over approximately 2 to 5 feet of medium dense to dense sand with gravel overlying very dense silty sand with gravel to the termination of the test pits. The Geologic Map of the Renton Quadrangle, King County, Washington: U.S. by D.R. Mullineaux (1965) shows site geology mapped as Vashon Stade ground moraine deposits (Qgt). The soils observed onsite are consistent with the glacial till mapped description. Detailed descriptions of the subsurface conditions we observed in the test pits are presented on the Test Pit Logs in Appendix A. The approximate test pit locations are shown on Figure 2. 3.3 Groundwater We observed minor groundwater seepage at approximately seven feet below current site grades in Test Pit TP-3 and mottled material in the other two test pits. We would expect that shallow groundwater seepage develops during the normally wet winter months along the contact between the upper less dense layers and underlying very dense material. The occurrence of perched groundwater is typical for sites underlain by fine-grained soils. We expect perched 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). Given the time of year our field work was completed and our experience with groundwater conditions in the area, the groundwater levels observed likely represent between the seasonal low and seasonal high levels. January 6, 2026 Project No. T-9277 Page No. 3 3.4 Geologic Hazards Section 4-3-050G.5 of the City of Renton Municipal Code (RMC) defines geologic hazards as areas susceptible to steep slopes and landslides, erosion, seismic, and coal mines. We have evaluated the site for erosion, landslide, and seismic hazards with related discussions in the sections below. 3.4.1 Landslide Hazard Areas Section 4-3-050G.5.b of the RMC defines landslide hazard areas as the following: (i)“Low Landslide Hazard (LL): Areas with slopes less than fifteen percent (15%). (ii)Medium Landslide Hazard (LM): Areas with slopes between fifteen percent (15%) and forty percent (40%) and underlain by soils that consist largely of sand, gravel or glacial till. (iii)High Landslide Hazards (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 Hazards (LV): Areas of known mapped or identified landslide deposits.” No significant sloping of over fifteen percent was observed at the site. Additionally, the site is underlain by dense to very dense glacially deposited materials that pose little to no risk of mass movement. Therefore, it is our opinion that this site would be described as a Low Landslide Hazard Area by the RMC. 3.4.2 Erosion Hazard Areas Section 4-3-050G.5.c of the RMC defines erosion hazard areas as the following: (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%). (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%).” The site soils are mapped as Alderwood gravelly sandy loam, 8 to 15 percent slopes, by the USDA NRCS Web Soil Survey. Over the site with existing slope gradients, these soils will have a moderate potential for erosion when exposed. Therefore, in our opinion, the site would be classified as a Low Erosion Hazard by the RMC. We did not observe any indications of significant active erosion at the site. However, the potential for soil erosion will increase during construction. In our opinion, proper implementation and maintenance of Best Management Practices (BMPs) for erosion prevention and sediment control, in conjunction with appropriate site drainage, will adequately mitigate the erosion potential in the planned development area. Erosion protection measures as required by the City of Renton will need to be in place prior to and during grading activities at the site. January 6, 2026 Project No. T-9277 Page No. 4 3.4.3 Seismic Hazard Areas Section 4-3-050G.5.d of the RMC defines seismic hazard areas as the following: (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.” A review of a map titled Faults and Earthquakes in Washington State, dated 2014, by Jessica L. Czajkowski and Jeffrey D. Bowman shows the closest fault to the site, the southern portion of the Seattle Fault Zone, is mapped approximately 1.5 miles north of the site. Quaternary-age activity of the fault (rupture within the last two million years) is predicted to have occurred in the Pre-Quaternary Period, or at least 11,700 years ago. Accordingly, during a seismic event, the risk of ground rupture along a fault line at the site is low. 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. The site is currently mapped on the Washington State DNR’s Natural Hazards Single-Topic Map as having very low liquefaction potential. Based on the soil and groundwater conditions we observed, it is our opinion that the risk for soil liquefaction occurring at the site is negligible due to the relative density of the soils and amount of cohesive material that would be sufficient to resist the cyclical loading of a seismic event. Therefore, in our opinion, the site would be considered a Low Seismic Hazard as defined by the RMC. 3.4.4 Coal Mine Hazard Areas Section 4-3-050G.5.c of the RMC defines coal mine hazard areas as the following: (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.” January 6, 2026 Project No. T-9277 Page No. 5 No coal mines are recorded in the area. Therefore, in our opinion, the site would be considered a Low Coal Mine Hazard as defined by the RMC. 3.5 Seismic Site Class Based on the site soil conditions and our knowledge of the area geology, per the 2021 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. Residential buildings can be supported on conventional spread footings bearing on inorganic competent native soils, competent existing fill soils, or on structural fill placed and compacted above competent inorganic soils. Pavement and floor slabs can be similarly supported. The native soils observed at the site contain a sufficient amount of soil fines that will make them difficult to compact as structural fill when too wet or too dry. The ability to use existing soils from site excavations as structural fill will depend upon its moisture content and the prevailing weather conditions at the time of construction. If grading activities take place during winter, the owner should be prepared to import clean granular material for use as structural fill and backfill. Detailed recommendations regarding these issues and other geotechnical design considerations are provided in the following sections of this report. These preliminary 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 material should be stripped and removed from the site. Surface stripping depths of approximately 6 to 12 inches should be expected to remove the organic surface soils and vegetation. Demolition of existing structures should include removal of foundations and abandonment of underground septic systems and other buried utilities. 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. Organic topsoil will not be suitable for use as structural fill but may be used for limited depths in nonstructural areas. Once clearing and 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 to verify soil conditions are as expected and suitable for support of new fill. Our representative may request a proofroll using heavy rubber-tired equipment to determine if any isolated soft and yielding areas are present. If excessively yielding areas are observed, and they cannot be stabilized in place by compaction, the affected soils should be excavated and removed to firm bearing and grade restored with new structural fill. January 6, 2026 Project No. T-9277 Page No. 6 Beneath pavement subgrades, if the depth of excavation to remove unstable soils is excessive, the use of geotextile fabrics, such as Mirafi 500X, or an equivalent fabric, can be used in conjunction with clean granular structural fill. 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 native soils encountered at the site contain a sufficient amount of soil fines that will make them difficult to compact as structural fill when too wet or too dry. The ability to use native soils from site excavations as structural fill will depend upon its moisture content, the prevailing weather conditions at the time of construction and the contractor’s ability to compact the native soils. If wet soils are encountered, the contractor will need to try the soils by aeration during dry weather conditions. Alternatively, the use of an additive, such as Portland cement, cement kiln dust (CKD), or line 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 Stormwater Pollution Prevention program (SWPPP) prepared with the Temporary Erosion and Sediment Control (TESC) plan. If grading activities are planned during the wet winter months, or if they 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 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. 4.3 Excavations All excavations at the site associated with confined spaces, such as utility trenches, must be completed in accordance with local, state, or federal requirements. Based on current Washington Industrial Safety and Health Act (WISHA) regulations, the medium dense soils would be classified as Type C soils. The very dense native soils would be classified as Type A soils. January 6, 2026 Project No. T-9277 Page No. 7 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. Based on our subsurface explorations, perched groundwater seepage should be anticipated within excavations during the wet winter months. We expect that the volume of water and rate of flow into the excavation should be relatively minor and would not be expected to impact the stability of the excavations when completed as described above. In general, 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. This 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 contractor. 4.4 Foundations The structures 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 subgrades should be prepared as recommended in Section 4.2 of this report. Perimeter foundations exposed to the weather should bear 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. The native and existing fill 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. We recommend designing foundations bearing 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. January 6, 2026 Project No. T-9277 Page No. 8 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. 4.5 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 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, 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 that if the sand or gravel layer overlying the membrane is saturated prior to pouring the slab, it will be ineffective in assisting in uniform curing of the slab and can actually serve as a water supply for moisture transmission 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. 4.6 Infiltration Feasibility The native soils observed at depth consist of very dense silty sand with gravel which characteristically exhibits low permeabilities and would not be a suitable receptor soil for discharge of development stormwater using infiltration/retention basins. Even low impact development (LID) techniques would likely fill up and overtop during rain events and causing minor local flooding. The upper shallow sand with gravel material would be suitable for some LID techniques provided the sand material remains in place and the facilities are located directly on top of the formation. The sand material is relatively shallow in thickness and therefore should only be used to support shallow LID facilities. On a preliminary basis we recommend using a long term design infiltration rate of 0.5 inches per hour to size the facilities. January 6, 2026 Project No. T-9277 Page No. 9 4.7 Utilities Utility pipes should be bedded and backfilled in accordance with American Public Works Association (APWA) or local jurisdictional requirements. As a minimum, trench backfill should be placed and compacted as structural fill, as described in Section 4.2 of this report. As noted, depending upon the soil moisture when excavated most inorganic native soils on the site should be suitable for use as backfill material during dry weather conditions. However, if utility construction takes place during the wet winter months, it will likely be necessary to import suitable wet weather fill for utility trench backfilling. 4.8 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 a positive gradient cannot be provided, provisions for collection and disposal of surface water adjacent to the structure should be provided. 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. 4.9 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 proofrolled with heavy construction equipment 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. We expect traffic for the development will consist mainly of cars and light trucks, with occasional heavy traffic in the form of moving trucks and trash/recycle vehicles. 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. January 6, 2026 Project No. T-9277 Page No. 10 The paving materials used should conform to the current Washington State Department of Transportation (WSDOT) specifications for ½-inch hot mix asphalt 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 the subgrade soils and reducing their supporting capability. For optimum 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 as they occur. 5.0 ADDITIONAL SERVICES Terra Associates, Inc., should review the final designs 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. This report is the copyrighted property of Terra Associates, Inc. and is intended for specific application to the NE 1st Subdivision project in Renton, Washington. This report is for the exclusive use of Mr. Amani Imran and their authorized representatives. No other warranty, expressed or implied, is made. The analyses and recommendations presented in this report are based on data obtained from our subsurface explorations. 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 © 2025 Zenrin 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-9277 Date: JAN 2026 RENTON, WASHINGTON NE 1ST SUBDIVISION © 2025 Microsoft Corporation © 2025 Maxar ©CNES (2025) Distribution Airbus DS © 2025 TMAP MOBILITY Earthstar Geographics SIO © 2025 TomTom © 2025 Zenrin TP-1 TP-2 TP-3 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 50 100 APPROXIMATE SCALE IN FEET SITE PLAN PROVIDED BY BING MAPS. APPROXIMATE TEST PIT LOCATION Environmental Earth Sciences Terra Associates, Inc. Consultants in Geotechnical Engineering Geology and EXPLORATION LOCATION PLAN Figure 2Proj.No. T-9277 Date: JAN 2026 RENTON, WASHINGTON NE 1ST SUBDIVISION 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-9277 Date: JAN 2026 RENTON, WASHINGTON NE 1ST SUBDIVISION Project No. T-9277 APPENDIX A FIELD EXPLORATION AND LABORATORY TESTING NE 1st Subdivision Renton, Washington On December 2, 2025, we investigated subsurface conditions at the site by excavating three test pits with a track- mounted excavator to maximum depths ranging from approximately six to eight-and-one-half feet below existing site grades. The test pit locations were approximately determined using GPS coordinates obtained from Google Earth. The approximate test pit locations are shown on Figure 2. The Test Pit Logs are presented as Figures A-2 through A-4. A geologist 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 sealed containers and taken to our laboratory for further examination and testing. Laboratory testing included determining the moisture content of all soil samples, and grain size distributions on three of the samples. The soil moisture contents are reported on the Test Pit Logs. The results of the grain size analysis are shown on Figures A-5. 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-9277 Date: JAN 2026 RENTON, WASHINGTON NE 1ST SUBDIVISION 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 A-2 T-9277 KGL Renton, Washington Grass Lawn December 2, 2025 NE 1st Subdivision LOG OF TEST PIT NO. TP-1 NA NA NA 11.2 4.2 5.5 Medium Dense to Dense Very Dense (8-inches TOPSOIL) FILL?: Brown SAND with gravel, fine to coarse sand, fine to coarse gravel, moist, trace silt, trace organic inclusions. (SP) Red to brown gravelly SAND, medium to coarse sand, fine to coarse gravel, moist. (SP) Gray silty SAND with gravel, fine to coarse sand, fine to coarse gravel, moist. (SM) Test Pit terminated at approximately 7.5 feet. No caving or water seepage was 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 A-3 T-9277 KGL Renton, Washington Grass Lawn December 2, 2025 NE 1st Subdivision LOG OF TEST PIT NO. TP-2 NA NA -2.0 feet 13.6 3.5 9.1 5.3 Medium Denseto Dense Very Dense (6-inches TOPSOIL) FILL?: Red to brown SAND with gravel, fine to coarse sand, fine to coarse gravel, moist, trace organic inclusions. (SP) Light brown to brown gravelly SAND, medium to coarse sand, fine to coarse gravel, moist. (SP) Gray silty SAND with gravel, fine to coarse sand, fine to coarse gravel, moist. (SM) Test Pit terminated at approximately 6 feet. No groundwater seepage was observed. Minor caving observed at approximately 2 feet. 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 A-4 T-9277 KGL Renton, Washington Grass Lawn December 2, 2025 NE 1st Subdivision LOG OF TEST PIT NO. TP-3 NA -7.0 feet -2.0 feet 13.2 2.6 9.2 6.8 Loose to Medium Dense Very Dense Brown to black silty SAND, fine to medium sand, moist, heavy organic inclusions. (SM) (TOPSOIL) FILL?: Red to brown silty SAND with gravel, fine to medium sand, fine to coarse gravel, moist, trace organic inclusions. (SM) Light brown to brown gravelly SAND, medium to coarse sand, fine to coarse gravel, dry to moist. (SP) Gray silty SAND with gravel, fine to coarse sand, fine to coarse gravel, moist. (SM) Test Pit terminated at approximately 8.5 feet. Minor groundwater seepage observed at 7 feet. Moderate caving observed at approximately 2 feet. Tested By: ZA Particle Size Distribution Report ASTM D422 PE R C E N T F I N E R 0 10 20 30 40 50 60 70 80 90 100 GRAIN SIZE - mm. 0.0010.010.1110100 % +3"Coarse % Gravel Fine Coarse Medium % Sand Fine Silt % Fines Clay 0.0 24.8 9.5 8.7 14.9 20.0 22.1 0.0 18.3 16.4 7.1 13.4 22.3 22.5 0.0 24.2 13.4 10.8 16.7 17.8 17.1 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 26.3038 2.7080 0.8414 0.1942 32.1210 2.5302 0.6496 0.1775 47.7364 3.8742 1.7261 0.3062 silty SAND with gravel silty SAND with gravel silty SAND with gravel T-9277 Mr. Amani Imran A-5 LL PL D85 D60 D50 D30 D15 D10 Cc Cu Material Description Test Date USCS NM Project No. Client:Remarks: Project: Location: TP-1 Depth: -6.0 feet Sample Number: 1 Location: TP-2 Depth: -6.0 feet Sample Number: 2 Location: TP-3 Depth: -8.5 feet Sample Number: 3 Terra Associates, Inc. Kirkland, WA Figure Ne 1st Subdivision Renton, Washington Tested December 2, 2025 Tested December 2, 2025 Tested December 2, 2025