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RS_Geotechnical_Engineering_Report_240903_v1.pdf
GEOTECHNICAL REPORT Imran Townhomes 2811 Northeast 13th Street Renton, Washington Project No. T-8918 Prepared for: Mr. Amani Imran Renton, Washington October 16, 2023 10-16-2023 TABLE OF CONTENTS Page No. 1.0 Project Description .......................................................................................................... 1 2.0 Scope of Work ................................................................................................................. 1 3.0 Site Conditions ................................................................................................................ 2 3.1 Surface ................................................................................................................ 2 3.2 Subsurface .......................................................................................................... 2 3.3 Groundwater ....................................................................................................... 2 3.4 Geologic Hazards ............................................................................................... 3 3.4.1 Erosion Hazards ....................................................................................... 3 3.4.2 Landslide Hazards .................................................................................... 3 3.4.3 Coal Mine Hazards ................................................................................... 4 3.4.4 Seismic 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 ........................................................................................................ 7 4.4 Foundation Support ............................................................................................ 7 4.5 Slab-on-Grade Floors ......................................................................................... 8 4.6 Stormwater Facilities .......................................................................................... 8 4.7 Infiltration Feasibility ......................................................................................... 9 4.8 Drainage ........................................................................................................... 10 4.9 Utilities ............................................................................................................. 10 4.10 Pavements ......................................................................................................... 10 5.0 Additional Services ....................................................................................................... 11 6.0 Limitations ..................................................................................................................... 11 Figures Vicinity Map ......................................................................................................................... Figure 1 Exploration Location Plan .................................................................................................... Figure 2 Typical Wall Drainage Detail ............................................................................................... Figure 3 Appendix Field Exploration and Laboratory Testing ........................................................................ Appendix A Geotechnical Report Imran Townhomes 2811 Northeast 13th Street Renton, Washington 1.0 PROJECT DESCRIPTION The project consists of redeveloping the site with five townhome units, along with associated access and utilities. Grading and development plans were unavailable at the time of this report. Based on existing topography, we would expect grading to be minor, with cuts and fills between one and five feet. We would expect that the structures will be a three-story, wood-frame buildings with the basement and main floors constructed out of reinforced concrete. Foundation loads are expected to be relatively light, in the range of 4 to 8 kips per foot for bearing walls and 50 to 75 kips for isolated columns. 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 We completed our work in accordance with our proposal, dated June 12, 2023. Accordingly, on July 19, 2023, we observed soil and groundwater conditions at 3 soil test pits excavated with a mini excavator to maximum depths of approximately 8 to 10 feet below existing grades. Using the information obtained from the subsurface exploration and laboratory testing, we performed analyses 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. Seismic Site Class per the current International Building Code (IBC). Site preparation and grading. Excavations. Foundation support. Slab-on-grade floors. Stormwater Facilities. Infiltration Feasibility. Drainage. Utilities. Pavements. October 16, 2023 Project No. T-8918 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 site consists of a single tax parcel totaling about 0.36 acres located at 2811 Northeast 13th Street in Renton, Washington. The approximate site location is shown on Figure 1. The site is currently developed with a duplex building, an outbuilding, and associated landscaping. A few small- to medium-sized trees occupy the southern portion of the property. Site topography is relatively flat with no obvious signs of sloping. 3.2 Subsurface The soils observed in the test pits generally consisted of approximately two inches of organic topsoil over approximately five to six and-one-half feet of medium dense to very dense silty sand with varying gravel and cobble content (weathered and unweathered till) over medium dense to dense sand with silt to silty sand with varying gravel and cobble content to the termination of the test pits. The exception to this general condition was observed in Test Pit TP-1 where no weathered silty sand material was observed. The Preliminary Geologic Map of Seattle and Vicinity, Washington by H.H. Waldron, B.A. Leisch, D.R. Mullineaux, D.R. Crandell (1961) maps the site as Vashon Till (Qt). The medium dense to very dense silty sand with gravel material observed in the test pits are consistent with the Vashon Till mapped description. The United States Department of Agriculture Natural Resources Conservation Service (NRCS) classifies the onsite soils as Arents, Alderwood material. A soil horizon, consisting of this material, is typically by glacial processes in the form of till plains and is derived from Basal till which is consistent with our exploratory findings and knowledge of the area’s geologic setting. The preceding discussion is intended to be a general review of the soil conditions encountered. For more detailed descriptions, please refer to the Test Pit Logs in Appendix A. The approximate locations of the test pits are shown on the Exploration Location Plan, Figure 2. 3.3 Groundwater We did not observe any indication of groundwater seepage during our explorations. However, the occurrence of shallow 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). October 16, 2023 Project No. T-8918 Page No. 3 3.4 Geologic Hazards We evaluated site conditions for the presence of Geologic Hazards as defined in Section 4-11-070 of the City of Renton Municipal Code (RMC). Discussions related to erosion, landslide, coal mine, and seismic hazards are given below. 3.4.1 Erosion Hazards Section 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%). 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 onsite soils are classified as Arents, Alderwood material, 6 to 15 percent slopes, by the United States Department of Agriculture Natural Resources Conservation Service. (NRCS). With existing gradients, these soils are rated as slight to moderate susceptibility to erosion. Therefore, the site would be classified as a Low Erosion Hazard as defined by the RMC, in our opinion. 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. BMPs for erosion prevention and sediment control must be in place prior to and throughout grading activity at the site. 3.4.2 Landslide Hazards 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.” October 16, 2023 Project No. T-8918 Page No. 4 Existing site topography throughout the site is relatively flat with no obvious signs of sloping. Therefore, the site meets the criteria for definition of a Low Landslide Hazard per the RMC, in our opinion. 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 Washington State Department of Natural Resources map titled Inactive and Abandoned Mines, dated September 2004, shows no evidence of underground mine development in the vicinity of the site. Therefore, it is our opinion the site meets the criteria for definition as a Low Coal Mine Hazard per the RMC. 3.4.4 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 closest fault to the site, the southeastern portion of the Seattle Fault Zone, is located approximately one and one-half miles 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 low. October 16, 2023 Project No. T-8918 Page No. 5 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 Liquefaction Susceptibility Map of King County, Washington, dated September 2004, 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 meets the above criteria for definition as a Low Seismic Hazard per the RMC. 3.5 Seismic Site Class Based on soil conditions observed in the test pits, and our knowledge of the area geology, per Chapter 16 of the 2018 International Building Code (IBC), Site Class “D” should be used in structural design. 4.0 DISCUSSION AND RECOMMENDATIONS 4.1 General Based on our study, there are no geotechnical considerations that would preclude development of the site as currently planned. The residential buildings can be supported on conventional spread footings bearing on competent native soils, or on structural fill placed on the competent soils observed below the organic surface horizon. Pavement and floor slabs can be similarly supported. The majority of 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. The ability to use the native soils from site excavations as structural fill will depend on its moisture content and the prevailing weather conditions at the time of construction. If grading activities will take place during winter, the owner should be prepared to import clean granular material for use as structural fill and backfill. The following sections provide detailed recommendations regarding the preceding issues and other geotechnical design considerations. These recommendations should be incorporated into the final design drawings and construction specifications. 4.2 Site Preparation and Grading To prepare the site for construction, all vegetation, organic surface soils, and other deleterious material should be stripped and removed from the site. Surface stripping depths of two inches should be expected to remove the organic surface soils and vegetation. In the developed portions of the site, demolition of existing structures should include removal of existing foundations and buried asphalt 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. October 16, 2023 Project No. T-8918 Page No. 6 Once clearing and stripping operations are complete, cut and fill operations can be initiated to establish desired building grades. Prior to placing fill, all exposed bearing surfaces should be observed by a representative of Terra Associates, Inc. to verify soil conditions are as expected and suitable for support of new fill or building elements. 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. 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, 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 majority of 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 on 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 dry the soils by aeration during dry weather conditions. Alternatively, the use of an additive, such as Portland cement, cement kiln dust (CKD), or lime to stabilize the soil moisture can be considered. If the soil is amended, additional Best Management Practices (BMPs) addressing the potential for elevated pH levels will need to be included in the Stormwater Pollution Prevention Program (SWPPP) prepared with the Temporary Erosion and Sedimentation Control (TESC) plan. Cobbles and boulders in excess of six inches should be removed from the structural fill. If grading activities are planned during the wet winter months, or if they are initiated during the summer and extend into fall and winter, the owner should be prepared to import wet-weather structural fill. For this purpose, we recommend importing a granular soil that meets the following grading requirements: U.S. Sieve Size Percent Passing 6 inches 100 No. 4 75 maximum No. 200 5 maximum* * Based on the 3/4-inch fraction. Prior to use, Terra Associates, Inc. should examine and test all materials imported to the site for use as structural fill. Structural fill should be placed in uniform loose layers not exceeding 12 inches and compacted to a minimum of 95 percent of the soil’s maximum dry density, as determined by American Society for Testing and Materials (ASTM) Test Designation D-1557 (Modified Proctor). The moisture content of the soil at the time of compaction should be within two percent of its optimum, as determined by this ASTM standard. In nonstructural areas, the degree of compaction can be reduced to 90 percent. October 16, 2023 Project No. T-8918 Page No. 7 4.3 Excavations All excavations at the site associated with confined spaces, such as utility trenches, must be completed in accordance with local, state, and federal requirements. Based on regulations outlined in the Washington Industrial Safety and Health Act (WISHA), the medium dense sand with silt and silty sand soils would be classified as Type C soils. The dense to very dense soils would be classified as Type B 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 B soils can be laid back at a slope inclination of 1:1 or flatter. 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. Groundwater seepage should be anticipated within excavations during the wet winter season. We anticipate that the volume of water and rate of flow into the excavation 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 relative 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. 4.4 Foundation Support The residential buildings may be supported on conventional spread footing foundations bearing on competent native 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. A majority of the native 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 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 inch. October 16, 2023 Project No. T-8918 Page No. 8 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 350 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 reworked 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. 4.5 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 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. 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. 4.6 Stormwater Facilities Site stormwater plans were not available at the time of this report. Detention Vault We expect the bottom of the excavations for a detention vault would expose medium dense to dense sand with silt and gravel. Vault foundations supported by these dense native soils may be designed for an allowable bearing capacity of 4,000 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 on 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. October 16, 2023 Project No. T-8918 Page No. 9 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 will 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. 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. Stormwater Ponds If fill berms will be constructed, the berm locations should be stripped of topsoil, duff, and soils containing organic material prior to the placement of fill. The fill berms should be constructed by placing structural fill in accordance with recommendations outlined in Section 4.2 of this report. Material used to construct pond berms should consist of predominately granular soils with a maximum size of three inches and a minimum of 20 percent fines. Terra Associates, Inc. should examine and test all onsite or imported materials proposed for use as berm fill prior to their use. It is likely that sandy soils may be exposed within the pond area. Therefore, it may be necessary to line the dead storage portion of the pond for water quality purposes depending on the final grades and exposed soils. Due to the exposure to fluctuating stored water levels and wave action, soils exposed on the interior side slopes of the ponds may be subject to some risk of periodic shallow instability or sloughing. Establishing interior slopes at a 3:1 gradient will significantly reduce or eliminate this potential. Exterior berm slopes and interior slopes above the maximum water surface should be graded to a finished inclination no steeper than 2:1. Finished slope faces should be thoroughly compacted and vegetated to guard against erosion. We should review the stormwater plans when they are completed and revise our recommendations, if required. 4.7 Infiltration Feasibility The native glacially consolidated soils composed of silty sand with gravel characteristically exhibit low permeabilities and would not be a suitable receptor soil for discharge of development stormwater using infiltration/retention basins. The upper weathered silty sand deposits such as those observed in the upper three to four feet of Test Pits TP-2 and TP-3 would support the use of low impact development (LID) techniques such as shallow gravel infiltration trenches provided the trenches are installed with an overflow system tied into an approved point of discharge. October 16, 2023 Project No. T-8918 Page No. 10 The trenches should be installed such that a minimum of two feet of separation is provided between the bottom of the trench and the contact between the weathered and unweathered soils. Based on the results of the laboratory testing, we recommend using a preliminary infiltration rate of 0.1 inches per hour for the design of the system. The permeability of the native weathered till soils will be significantly impacted by the intrusion of soil fines (silt- and clay-sized particles). A relatively minor amount of soil fines can reduce the permeability of the formation by a factor of ten. The greatest exposure to soil fines contamination will occur during mass grading and construction. Therefore, we recommend that the Temporary Erosion and Sedimentation Control (TESC) plans route construction stormwater to a location other than the permanent infiltration facility. 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 this gradient cannot be provided, surface water should be collected adjacent to the structures and directed to appropriate storm facilities. Subsurface We recommend installing perimeter foundation drains adjacent to shallow foundations. The drains can be laid to grade at an invert elevation equivalent to the bottom of footing grade. The drains can consist of four-inch diameter perforated PVC pipe enveloped in washed pea gravel-sized drainage aggregate. The aggregate should extend six inches above and to the sides of the pipe. Roof and foundation drains should be tightlined separately to the storm drains. All drains should be provided with cleanouts at easily accessible locations. 4.9 Utilities Utility pipes should be bedded and backfilled in accordance with American Public Works Association (APWA) or the local jurisdictional specifications. At a minimum, trench backfill should be placed and compacted as structural fill as described in Section 4.2 of this report. As noted, most native soils excavated 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. 4.10 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 rubber-tired construction equipment such as a loaded ten-yard dump truck to verify this condition. October 16, 2023 Project No. T-8918 Page No. 11 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. Long-term pavement performance will depend on surface drainage. A poorly drained pavement section will be subject to premature failure as a result of surface water infiltrating into the subgrade soils and reducing their supporting capability. For optimum pavement performance, we recommend surface drainage gradients of at least two percent. Some degree of longitudinal and transverse cracking of the pavement surface should be expected over time. Regular maintenance should be planned to seal cracks when they occur. 5.0 ADDITIONAL SERVICES Terra Associates, Inc. should review the final design drawings and specifications in order to verify earthwork and foundation recommendations have been properly interpreted and implemented in project design. We should also provide geotechnical services 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 Imran Townhomes project in Renton, Washington. This report is for the exclusive use of Mr. Amani Imran, and his 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. © 2023 Microsoft Corporation © 2023 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 2023 Proj.No. T-8918 Date: OCT 2023 RENTON, WASHINGTON IMRAN TOWNHOMES © 2023 Microsoft Corporation © 2023 Maxar ©CNES (2023) Distribution Airbus DS © 2023 TomTom TP-1TP-2TP-3REFERENCE:REFERENCE ONLY AND SHOULD NOT BE USED FORDESIGN OR CONSTRUCTION PURPOSES.DIMENSIONS ARE APPROXIMATE. IT IS INTENDED FORNOTE:THIS SITE PLAN IS SCHEMATIC. ALL LOCATIONS ANDConsultants in Geotechnical EngineeringTerraAssociates, Inc.Geology andEnvironmental Earth SciencesEXPLORATION LOCATION PLANFigure 2LEGEND:02040APPROXIMATE SCALE IN FEETSITE PLAN PROVIDED BY BING MAPS.APPROXIMATE TEST PIT LOCATIONProj.No. T-8918Date: OCT 2023RENTON, WASHINGTONIMRAN TOWNHOMES 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-8918 Date: OCT 2023 RENTON, WASHINGTON IMRAN TOWNHOMES Project No. T-8918 APPENDIX A FIELD EXPLORATION AND LABORATORY TESTING Imran Townhomes 2811 Northeast 13th Street Renton, Washington On July 19, 2023, we investigated subsurface conditions at the site by excavating 3 test pits with a mini- excavator to depths of about 8 to 10 feet below existing grades. The test pit locations were approximately determined in the field by sighting and pacing from existing surface features. The approximate test pit locations are shown on Figure 2. The Test Pit Logs are presented as Figures A-2 through A-4. A geotechnical engineer from our office conducted the field exploration. Our representative classified the soil conditions encountered, maintained a log of each test pit, obtained representative soil samples, and recorded water levels observed during excavation. All soil samples were visually classified in accordance with the Unified Soil Classification System (USCS) described on Figure A-1. Representative soil samples obtained from the test pits were placed in 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 select soil samples. The results are shown on Figures A-5 and A-6. Environmental Earth Sciences Terra Associates, Inc. Consultants in Geotechnical Engineering Geology and MAJOR DIVISIONS LETTER SYMBOL TYPICAL DESCRIPTION GRAVELS More than 50% of coarse fraction is larger than No. 4 sieve Clean Gravels (less than 5% fines) GW Well-graded gravels, gravel-sand mixtures, little or no fines. GP Poorly-graded gravels, gravel-sand mixtures, little or no fines. Gravels with fines GM Silty gravels, gravel-sand-silt mixtures, non-plastic fines. GC Clayey gravels, gravel-sand-clay mixtures, plastic fines. SANDS More than 50% of coarse fraction is smaller than No. 4 sieve Clean Sands (less than 5% fines) SW Well-graded sands, sands with gravel, little or no fines. SP Poorly-graded sands, sands with gravel, little or no fines. Sands with fines SM Silty sands, sand-silt mixtures, non-plastic fines. SC Clayey sands, sand-clay mixtures, plastic fines. SILTS AND CLAYS Liquid Limit is less than 50% ML Inorganic silts, rock flour, clayey silts with slight plasticity. CL Inorganic clays of low to medium plasticity. (Lean clay) OL Organic silts and organic clays of low plasticity. SILTS AND CLAYS Liquid Limit is greater than 50% MH Inorganic silts, elastic. CH Inorganic clays of high plasticity. (Fat clay) OH Organic clays of high plasticity. HIGHLY ORGANIC SOILS PT Peat.COARSE GRAINED SOILSMore than 50% material largerthan No. 200 sieve sizeFINE GRAINED SOILSMore than 50% material smallerthan No. 200 sieve sizeDEFINITION OF TERMS AND SYMBOLS COHESIONLESSCOHESIVE Standard Penetration Density Resistance in Blows/Foot Very Loose 0-4 Loose 4-10 Medium Dense 10-30 Dense 30-50 Very Dense >50 Standard Penetration Consistancy Resistance in Blows/Foot Very Soft 0-2 Soft 2-4 Medium Stiff 4-8 Stiff 8-16 Very Stiff 16-32 Hard >32 2" OUTSIDE DIAMETER SPILT SPOON SAMPLER 2.4" INSIDE DIAMETER RING SAMPLER OR SHELBY TUBE SAMPLER WATER LEVEL (Date) Tr TORVANE READINGS, tsf Pp PENETROMETER READING, tsf DD DRY DENSITY, pounds per cubic foot LL LIQUID LIMIT, percent PI PLASTIC INDEX N STANDARD PENETRATION, blows per foot UNIFIED SOIL CLASSIFICATION SYSTEM Figure A-1Proj.No. T-8918 Date: OCT 2023 RENTON, WASHINGTON IMRAN TOWNHOMES Sample No.Depth (ft)PROJECT NAME: PROJ. NO: LOGGED BY: LOCATION: DATE LOGGED: APPROX. ELEV: DEPTH TO CAVING: FIGURE DEPTH TO GROUNDWATER: SURFACE CONDITIONS: Description Consistency/ Relative Density W (%)interpreted as being indicative of other locations at the site. NOTE: This subsurface information pertains only to this test pit location and should not be 0 1 2 3 4 5 6 7 8 9 10 A-2 T-8918 MJX Renton, Washington Grass Lawn July 19, 2023 Imran Townhomes LOG OF TEST PIT NO.TP-1 NA NA NA 5.0 10.0 9.0 Dense to Very Dense Medium Dense to Dense (2-inches organic TOPSOIL) Gray silty SAND with gravel, fine to coarse sand, fine to coarse gravel, dry to moist, trace cobbles, moderate cementation. (SM) Gray SAND with silt to silty SAND, fine to coarse sand, moist, trace gravel, trace cobbles, weak cementation, interbedded, cemented silty sand layers. (SP-SM) Test Pit terminated at approximately 8 feet. No groundwater seepage observed. No caving observed. Sample No.Depth (ft)PROJECT NAME: PROJ. NO: LOGGED BY: LOCATION: DATE LOGGED: APPROX. ELEV: DEPTH TO CAVING: FIGURE DEPTH TO GROUNDWATER: SURFACE CONDITIONS: Description Consistency/ Relative Density W (%)interpreted as being indicative of other locations at the site. NOTE: This subsurface information pertains only to this test pit location and should not be 0 1 2 3 4 5 6 7 8 9 10 11 12 A-3 T-8918 MJX Renton, Washington Grass Lawn & Exposed Soil July 19, 2023 Imran Townhomes LOG OF TEST PIT NO.TP-2 NA NA NA 7.0 5.5 7.5 7.3 Medium Dense Dense Medium Dense to Dense (2-inches organic TOPSOIL) Brownish-gray silty SAND with gravel, fine to coarse sand, fine to coarse gravel, dry, scattered rootlets, trace cobbles. (SM) Gray silty SAND with gravel, fine to coarse sand, fine to coarse gravel, dry to moist, trace cobbles, moderate cementation. (SM) Gray silty SAND, fine to coarse sand, moist, trace gravel, occasional cobble, weak cementation. (SM) Test Pit terminated at approximately 10 feet. No groundwater seepage observed. No caving observed. Sample No.Depth (ft)PROJECT NAME: PROJ. NO: LOGGED BY: LOCATION: DATE LOGGED: APPROX. ELEV: DEPTH TO CAVING: FIGURE DEPTH TO GROUNDWATER: SURFACE CONDITIONS: Description Consistency/ Relative Density W (%)interpreted as being indicative of other locations at the site. NOTE: This subsurface information pertains only to this test pit location and should not be 0 1 2 3 4 5 6 7 8 9 10 A-4 T-8918 MJX Renton, Washington Grass Lawn July 19, 2023 Imran Townhomes LOG OF TEST PIT NO.TP-3 NA NA NA 4.9 7.6 5.7 7.3 Medium Dense Dense Medium Dense to Dense (2-inches organic TOPSOIL) Brownish-gray silty SAND with gravel, fine to coarse sand, fine to coarse gravel, dry, trace rootlets, trace cobbles, occasional boulder. (SM) Gray silty SAND with gravel, fine to coarse sand, fine to coarse gravel, dry to moist, trace cobbles, moderate cementation. (SM) Gray SAND with silt to silty SAND, fine to coarse sand, moist, trace gravel, occasional cobble, interbedded and cemented silty sand layers. (SP-SM) Test Pit terminated at approximately 8 feet. No groundwater seepage observed. No caving observed. Tested By: KJ LL PL D85 D60 D50 D30 D15 D10 Cc Cu Material Description USCS AASHTO Project No. Client:Remarks: Project: Location: Test Pit TP-2 Depth: -4 feet Sample Number: 2 Location: Test Pit TP-2 Depth: -10 feet Sample Number: 4 Terra Associates, Inc. Kirkland, WA Figure 20.4541 0.5464 0.3629 0.1651 3.4633 0.5466 0.4271 0.2435 0.0936 silty SAND with gravel SM silty SAND SM T-8918 Mr. Amani Imran A-5PERCENT FINER0 10 20 30 40 50 60 70 80 90 100 GRAIN SIZE - mm. 0.0010.010.1110100 % +3"Coarse % Gravel Fine Coarse Medium % Sand Fine Silt % Fines Clay 0.0 16.1 4.8 5.5 19.4 30.9 23.3 0.0 2.3 10.7 4.6 32.6 35.2 14.66 in.3 in.2 in.1½ in.1 in.¾ in.½ in.3/8 in.#4#10#20#30#40#60#100#140#200Particle Size Distribution Report Imran Townhomes Tested on July 25, 2023 Tested on July 25, 2023 Tested By: KJ LL PL D85 D60 D50 D30 D15 D10 Cc Cu Material Description USCS AASHTO Project No. Client:Remarks: Project: Location: Test Pit TP-3 Depth: -1.5 feet Sample Number: 1 Location: Test Pit TP-3 Depth: -8 feet Sample Number: 4 Terra Associates, Inc. Kirkland, WA Figure 16.7975 0.7302 0.3967 0.1302 3.3214 0.4772 0.3510 0.1951 silty SAND with gravel SM silty SAND SM T-8918 Mr. Amani Imran A-6PERCENT FINER0 10 20 30 40 50 60 70 80 90 100 GRAIN SIZE - mm. 0.0010.010.1110100 % +3"Coarse % Gravel Fine Coarse Medium % Sand Fine Silt % Fines Clay 0.0 13.1 14.8 5.9 14.9 24.3 27.0 0.0 0.0 12.3 5.3 26.2 38.1 18.16 in.3 in.2 in.1½ in.1 in.¾ in.½ in.3/8 in.#4#10#20#30#40#60#100#140#200Particle Size Distribution Report Imran Townhomes Tested on July 25, 2023 Tested on July 25, 2023