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Home My WebLink AboutRS_Palermo SP_ Geotech Report_220511_v1 2105 South C Street 17625 130th Avenue NE, Suite 102 Tacoma, Washington 98402 www.robinson-noble.com Woodinville, Washington 98072 P: 253.475.7711 | F: 253.472.5846 P: 425.488.0599 | F: 425.488.2330 October 21, 2021 Mr. and Mrs. Nelson and Rhoda Palermo 9414 South 202nd Street Kent, Washington 98031 Infiltration Evaluation Southeast 192nd Street and 120th Avenue Southeast Renton, Washington RN File No. 3546-001A Dear Mr. and Mrs. Palermo: Introduction This letter presents our evaluation of infiltration potential at King County Parcel 6198400340 located at the northwest corner of Southeast 192nd Street and 120th Avenue Southeast, Renton, Washington. The approximate location of the site is shown on the Vicinity Map, presented as Figure 1. The site is approximately 1.0 acres in size and currently undeveloped. Site grades are generally flat within the project and surrounding area. We understand that you plan to short-plat the property. You have requested that site subsurface soils be evaluated for infiltration capabilities of collected stormwater from the planned new structures. The City of Renton uses the 2016 King County Stormwater Design Manual (KCSM) with supplemental City of Renton Surface Water Design Manual (RSWM) for determination of infiltration feasibility. We evaluated infiltration feasibility of the site for shallow infiltration systems including drywells, infiltration trenches and permeable pavement. Subsurface Soil and Ground Water Conditions The geology of the area is mapped on the Geologic Map of the Renton Quadrangle, King County, Washington by D.R. Mullineaux (U.S. Geological Survey, 1965). The site is mapped as being underlain by glacial till. We explored subsurface conditions at the site on September 20 and 21, 2021, by performing four pilot infiltration tests (PITs). PITs were excavated to depths of approximately 2.0 to 2.8 feet to test infiltration capabilities. After testing for infiltration, explorations extended down beneath the base of the PIT to evaluate soil and potential groundwater mounding conditions beneath the test. The PIT explorations were continued to depths of approximately 3.3 to 4.4 feet below the ground surface. The explorations were located in the field by a representative from this firm who also examined the soils and geologic conditions encountered, and maintained logs of the explorations. The approximate locations of the explorations are shown on the Site Plan in Figure 2. The soils were Infiltration Evaluation 120th Ave SE and SE 192nd St October 21, 2021 RN File No. 3546-001A Page 2 Robinson Noble, Inc. visually classified in general accordance with the Unified Soil Classification System, a copy of which is presented as Figure 3. The logs of the explorations are presented in Figures 4 and 5. The explorations generally encountered a dark brown topsoil layer that was approximately 1.0 to 1.4 feet in thickness. Underlying the topsoil, we encountered weathered till from depths of approximately 1.0 to 3.5 feet. The weathered till consisted of loose to medium dense, brown silty sand with gravel and roots. Below the weathered till, we encountered dense to very dense, gray to grayish-brown mottled brown silty sand with gravel. We interpret this material as partially weathered glacial till, though less deeply weathered than the overlying soils. The partially weathered till was observed from depths of approximately 2.0 to 4.4 feet. These soils are consistent with the mapped geology and suggest the entire site is underlain by glacial till. We did not encounter groundwater seepage during our site explorations. We did observe indications of groundwater based on the mottled layering observed in the partially weathered till at a depth of approximately 2.0 feet. The silty, glacially consolidated soils interpreted to underlie the site are considered poorly draining. During the wetter times of the year, we expect perched water conditions to occur as pockets of water on top of these materials. Perched water does not represent a regional groundwater “table” within the upper soil horizons. Volumes of perched groundwater vary depending upon the time of year and the upslope recharge conditions. Conclusions and Recommendations It is our opinion that infiltration is feasible at the site at shallow depths. We have not been provided with a proposed site plan for the project. Our testing was performed to evaluate shallow infiltration systems such as permeable pavements, drywells and infiltration trenches at existing site grades. Evidence of high groundwater elevations were encountered as shallow as 2 feet below existing grades. This distinction would eliminate the feasibility of using infiltration systems such as a trench or drywell. Permeable pavements appear to be the only feasible approach for infiltration at the site. Site grading could affect this infiltration feasibility. We do not expect significant grading at the site due to the relatively flat topography within the project area. We understand that the City of Renton uses the 2017 RSWM as derived from the 2016 KCSM. Hydrologic Analysis and Design is presented in Chapter 5.2 of the RSWM. Due to the indications of high groundwater elevations, we considered the use of permeable pavements at the site. Chapter 5.2.1 of the RWSM provides design considerations and requirements for permeable pavements. Infiltration Testing The infiltration rates were measured for the site by conducting four Small Pilot Infiltration Tests (PITs) in the weathered till soils while trying to maintain water levels during the testing under the topsoil layer. The approximate locations of these tests are shown on the attached Site Plan as Figure 2. The tests were completed in general accordance with Reference 6-A of the RSWM. Infiltration Evaluation 120th Ave SE and SE 192nd St October 21, 2021 RN File No. 3546-001A Page 3 Robinson Noble, Inc. Table 1 below presents the measured unfactored infiltration rates. Table 1. Measured Unfactored Infiltration Rates PIT-1 PIT-2 PIT-3 PIT-4 PIT area 15.8 ft2 14.8 ft2 14.0 ft2 14.9 ft2 Test Depth (feet) 2.8 2.0 2.3 2.2 Unfactored Infiltration Rate (inches/hour) 2.4 0.7 0.6 3.0 In general, the measured infiltration rates for PITs 1 through 4 are consistent and in agreement with the anticipated rates for weathered glacial till soils. Chapter 5.2.1 of the RSWM provides correction factors, or ranges of factors, for infiltration design. These factors are dictated by type of test performed, geometry of the facility, and subgrade soils below the infiltration gallery. Permeable Pavements: The manual recommends that a correction factor of 0.33 to 1 be applied to the measured infiltration rates for permeable pavements and is based on site variability and size of the infiltration facility. In general, slight variation was observed in the measured infiltration rates and observed geology on the subject property. We therefore recommend that a correction factor of 0.7 be used for this variability. An additional correction factor of 0.9 to 1 should be placed on the measured infiltration rate based on quality of aggregate base material that will be used under the pervious pavement. We are unaware of the aggregate to be placed at this time, so we recommend that this correction factor be applied as 0.9. Table 2 provides correction factors based on the RSWM correction factors for permeable pavements. Table 2. Design Infiltration Rates PIT-1 PIT-2 PIT-3 PIT-4 Unfactored Infiltration Rate 2.4 0.7 0.6 3.0 Total Correction Factor 0.63 0.63 0.63 0.63 Design Infiltration Rate (in/hr) 1.5 0.4 0.4 1.9 It is our opinion that the infiltration rates determined at the site can be used for permeable pavement design. Based on the results of our analysis, we recommend an average infiltration rate of 1.0 inches per hour be used for permeable pavement design. Infiltration Evaluation 120th Ave SE and SE 192nd St October 21, 2021 RN File No. 3546-001A Page 4 Robinson Noble, Inc. Groundwater Protection Water being infiltrated from a pollution generating surface should meet the requirements of Groundwater Protection as set forth in Section 5.2.1 of the RSWM. We have reviewed the King County iMap GIS database and the site appears to be located within a groundwater protection area related to Wellhead Protection Areas – five year time of travel. For sites located within a groundwater protection area, the soil is considered acceptable for protection of groundwater if the soil has a cation exchange capacity of greater than 5 and an organic content of greater than 1.0 percent and meets one of the following criteria: 1. The soil has a measured infiltration rate of less than 2.4 inches per hour or logged as one of the classes from the USDA Textural Triangle (Figure 5.2.1.A of the RSWM) with the exception of sand, loamy sand, and sandy loam, OR 2. The soil has a measured infiltration rate less than or equal to 9 inches per hour, and is composed of less than 25% gravel by weight with at least 75% of the soil passing the #4 sieve. The portion passing the #4 sieve must meet one of the following gradations: a. At least 50% must pass the #40 sieve and at least 2% must pass the #100 sieve. b. At least 25% must pass the #40 sieve and at least 5% must pass the #200 sieve. Table 3 presents the results from water quality standards testing from the samples collected in the infiltration test areas. The detailed results are provided in Appendix A. The grain-size analyses results are shown in Appendix B. Table 3. Water Quality Standards PIT-1 PIT-2 PIT-3 PIT-4 Cation Exchange (Na, mEq/100 g) 9.1 7.71 15.8 6.2 Organic Content (%) 4.94 3.08 4.66 3.88 Gravel Content (%) 13.9 25.1 13.3 23.3 Sand or Finer Content (%) 86.1 74.9 86.7 76.7 Percent Passing #40 Sieve (%)* 81.2 79.1 78.4 69.2 Percent Passing #100 Sieve (%)* 50.4 54.3 57.4 46.0 Percent Passing #200 Sieve (%)* 37.1 43.0 47.2 35.3 USDA Textural Triangle Class* Sandy Loam Sandy Loam Sandy Loam Sandy Loam *Based on total weight of soil smaller than gravel (#4 sieve) As shown in Table 3, all four samples have cation exchange capacity of greater than 5 and organic content of greater than 1.0 percent. Although measured infiltration rates were generally less than 2.4 inches per hour the soils were classified as Sandy Loam, not meeting the requirements of Item 1 of the water quality standards above. Infiltration Evaluation 120th Ave SE and SE 192nd St October 21, 2021 RN File No. 3546-001A Page 5 Robinson Noble, Inc. We then evaluated Item 2 of the water quality standards above. Measured infiltration rates were evaluated as less than 9 inches per hour. We took an average value of the gravel and sand content percentages in Table 3. This calculated value was determined to be 18.9 percent gravel and 76.9 percent sand meeting the requirements of Item 2 above. Item 2a was further evaluated and samples appear to contain more than 50 percent passing the #40 sieve and 2 percent passing the #100 sieve. It is our opinion, based on the results of water quality and infiltration testing, that the underlying soils at the site will provide adequate water quality treatment for the planned permeable pavement. Use of This Letter We have prepared this letter for Mr. and Mrs. Palermo and their agents, for use in design of this project. This letter is not a complete geotechnical report. The data and letter should be provided to prospective contractors for their bidding and estimating purposes, but our letter, conclusions, and interpretations should not be construed as a guarantee of subsurface conditions. Our recommendations are based on the soil conditions encountered during this study. Within the limitations of scope, schedule and budget for our services, we have strived to take care that our services have been completed in accordance with generally accepted practices followed in this area at the time this letter was prepared. No other conditions, expressed or implied, should be understood. We appreciate the opportunity to be of service to you. If there are any questions concerning this letter or if we can provide additional services, please call. Sincerely, Robinson Noble, Inc. Barbara A. Gallagher, PE Associate Engineer MGL:BAG:am Five Figures Appendix A – Water Quality Test Results Appendix B – Grain Size Results Note: Basemap taken from Renton 7.5-minute series. USGS 2020 Nelson and Rhoda Palermo: Infiltration Evaluation Vicinity Map Figure 1 Project Site PM: JRW October 2021 3546-001A Scale 1" = 50'0 50 100TP-4TP-3TP-2TP-1AA’Approximate Location ofCross Section A-A’TP-1 PIT-1 Number and Approximate Location of Pilot Infiltration Test LEGEND 40’0’ Approximate Scale Figure 2 Site Plan Nelson and Rhoda Palermo: Infiltration Evaluation PM: JRW October 2021 3546-001A Note: Basemap taken from King County iMap. https://gismaps.kingcounty.gov/iMap/ PIT-1PIT-2 PIT-3 P I T-4 Unified Soil Classification System MAJOR DIVISIONS GROUP SYMBOL GROUP NAME WELL-GRADED GRAVEL, FINE TO COARSE GRAVEL POORLY-GRADED GRAVEL SILTY GRAVEL CLAYEY GRAVEL WELL-GRADED SAND, FINE TO COARSE SAND GW GP GM GC SW SP POORLY-GRADED SAND SILTY SAND CLAYEY SAND SILT CLAY ORGANIC SILT, ORGANIC CLAY SILT OF HIGH PLASTICITY, ELASTIC SILT CLAY OF HIGH PLASTICITY, FAT CLAY ORGANIC CLAY, ORGANIC SILT PEATPTHIGHLY ORGANIC SOILS GRAVEL CLEAN GRAVEL GRAVEL WITH FINES SAND CLEAN SAND SAND WITH FINES INORGANIC INORGANIC ORGANIC ORGANIC COARSE - GRAINED SOILS MORE THAN 50% RETAINED ON number 200 SIEVE MORE THAN 50% OF COARSE FRACTION RETAINED ON NO. 4 SIEVE MORE THAN 50% OF COARSE FRACTION PASSES NO. 4 SIEVE SM SC ML CL OL MH CH OH FINE - GRAINED SOILS MORE THAN 50% PASSES NO. 200 SIEVE SILT AND CLAY SILT AND CLAY LIQUID LIMIT LESS THAN 50% LIQUID LIMIT 50% OR MORE NOTES: 1) Field classification is based on visual examination of soil in general accordance with ASTM D 2488-83. 2) Soil classification using laboratory tests is based on ASTM D 2487-83. 3) Descriptions of soil density or consistency are based on interpretation of blowcount data, visual appearance of soils, and/or test data. SOIL MOISTURE MODIFIERS Dry- Absence of moisture, dusty, dry to the touch Moist- Damp, but no visible water Wet- Visible free water or saturated, usually soil is obtained from below water table PM: JRW October 2021 3546-001A Unified Soil Classification System Figure 3 Nelson and Rhoda Palermo: Infiltration Evaluation LOG OF EXPLORATION DEPTH (ft) USC SOIL DESCRIPTION ROBINSON NOBLE, INC. FILE NO 3546-001A FIGURE 4 PIT ONE 0.0 – 1.3 SM Dark brown to black silty fine to medium sand with gravel and organics (loose, moist) (Topsoil) 1.3 – 2.3 SM Light brown silty fine to medium sand with gravel and roots (loose to medium dense, moist) (Weathered Till) 2.3 – 3.5 SM Grayish brown mottled brown silty fine to medium sand with gravel and roots (medium dense to dense, moist) (Weathered Till) 3.5 – 4.4 SM Gray mottled brown silty fine to medium sand with gravel (very dense, moist) (Partially Weathered Till) Samples were collected at 2.2, 3.0, and 4.1 feet Groundwater seepage was not encountered Test pit caving was not encountered Test pit was completed at 4.4 feet on 9/20/2021 PIT TWO 0.0 – 1.0 SM Dark brown to black silty fine to medium sand with gravel and organics (loose, moist) (Topsoil) 1.0 – 1.7 SM Brown silty fine to medium sand with gravel and roots (loose to medium dense, moist) (Weathered Till) 1.7 – 2.1 SM Grayish brown silty fine to coarse sand with gravel and trace roots (dense, moist) (Weathered Till) 2.1 – 3.5 SM Gray mottled brown silty fine to coarse sand with gravel (very dense, moist) (Partially Weathered Till) Samples were collected at 1.6 and 2.8 feet Groundwater seepage was not encountered Test pit caving was not encountered Test pit was completed at 3.5 feet on 9/20/2021 PIT THREE 0.0 – 1.2 SM Dark brown to black silty fine to medium sand with gravel and organics (loose, moist) (Topsoil) 1.2 – 2.0 SM Grayish brown silty fine to medium sand with gravel and roots (loose to medium dense, moist) (Weathered Till) 2.0 – 3.3 SM Light gray mottled brown silty fine to coarse sand with gravel and trace roots (very dense, moist) (Partially Weathered Till) Samples were collected at 1.6 and 2.8 feet Groundwater seepage was not encountered Test pit caving was not encountered Test pit was completed at 3.3 feet on 9/21/2021 LOG OF EXPLORATION DEPTH (ft) USC SOIL DESCRIPTION ROBINSON NOBLE, INC. FILE NO 3546-001A FIGURE 5 PIT FOUR 0.0 – 1.4 SM Dark brown to black silty fine to medium sand with gravel and organics (loose, moist) (Topsoil) 1.4 – 2.1 SM Brown silty fine to medium sand with gravel and roots (loose to medium dense, moist) (Weathered Till) 2.1 – 3.7 SM Grayish brown mottled brown silty fine to coarse sand with gravel and trace roots (very dense, moist) (Partially Weathered Till) Samples were collected at 1.8 and 3.0 feet Groundwater seepage was not encountered Test pit caving was not encountered Test pit was completed at 3.7 feet on 9/21/2021 RN File No. 3546-001A October 2021 Appendix A  Water Quality Standards RN File No. 3546-001A October 2021 Appendix B  Grain Size Analysis Sample Description USCS Classification Source:PIT 1 SM, Silty Sand Depth (ft):2.8 Moisture (%):35.9% Atterberg Limits Coefficients Specification Liquid Limit n/a D10 0.020 No Specification Given Plastic Limit n/a D30 0.061 Plasticity Index n/a D60 0.205 D90 1.541 CC 0.89 CU 10.14 The results of this analysis were obtained in general accordance with ASTM D6913. Grain Size Analysis Report 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0.0010.010.1110100Percent Finer (Passing) by WeightGrain Size in Millimeters Sieve Opening in Inches Mesh Openings per Inch Hydrometer Analysis #41½#10¾⅜#60#100#2003#40#20½Sieve Analysis, U.S. Standard Sieve Sizes Fine Gravels Sands Fines (Silt or Clay) Coarse Fine Coarse Medium 0.0 %62.9 %37.1% Figure A1 Grain Size Analysis Report Palermo Short Plat JRW Oct. 2021 3546-001A Sample Description USCS Classification Source:PIT 2 SM, Silty Sand Depth (ft):2 Moisture (%):26.0% Atterberg Limits Coefficients Specification Liquid Limit n/a D10 0.017 No Specification Given Plastic Limit n/a D30 0.052 Plasticity Index n/a D60 0.196 D90 1.482 CC 0.80 CU 11.22 The results of this analysis were obtained in general accordance with ASTM D6913. Grain Size Analysis Report 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0.0010.010.1110100Percent Finer (Passing) by WeightGrain Size in Millimeters Sieve Opening in Inches Mesh Openings per Inch Hydrometer Analysis #41½#10¾⅜#60#100#2003#40#20½Sieve Analysis, U.S. Standard Sieve Sizes Fine Gravels Sands Fines (Silt or Clay) Coarse Fine Coarse Medium 0.0 %57.0 %43.0% Figure A2 Grain Size Analysis Report Palermo Short Plat JRW Oct. 2021 3546-001A Sample Description USCS Classification Source:PIT 3 SM, Silty Sand Depth (ft):2.3 Moisture (%):51.1% Atterberg Limits Coefficients Specification Liquid Limit n/a D10 0.016 No Specification Given Plastic Limit n/a D30 0.048 Plasticity Index n/a D60 0.174 D90 1.889 CC 0.82 CU 10.98 The results of this analysis were obtained in general accordance with ASTM D6913. Grain Size Analysis Report 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0.0010.010.1110100Percent Finer (Passing) by WeightGrain Size in Millimeters Sieve Opening in Inches Mesh Openings per Inch Hydrometer Analysis #41½#10¾⅜#60#100#2003#40#20½Sieve Analysis, U.S. Standard Sieve Sizes Fine Gravels Sands Fines (Silt or Clay) Coarse Fine Coarse Medium 0.0 %52.8 %47.2% Figure A3 Grain Size Analysis Report Palermo Short Plat JRW Oct. 2021 3546-001A Sample Description USCS Classification Source:PIT 4 SM, Silty Sand Depth (ft):2.2 Moisture (%):50.2% Atterberg Limits Coefficients Specification Liquid Limit n/a D10 0.021 No Specification Given Plastic Limit n/a D30 0.064 Plasticity Index n/a D60 0.289 D90 2.091 CC 0.66 CU 13.61 The results of this analysis were obtained in general accordance with ASTM D6913. Grain Size Analysis Report 0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% 0.0010.010.1110100Percent Finer (Passing) by WeightGrain Size in Millimeters Sieve Opening in Inches Mesh Openings per Inch Hydrometer Analysis #41½#10¾⅜#60#100#2003#40#20½Sieve Analysis, U.S. Standard Sieve Sizes Fine Gravels Sands Fines (Silt or Clay) Coarse Fine Coarse Medium 0.0 %64.7 %35.3% Figure A4 Grain Size Analysis Report Palermo Short Plat JRW Oct. 2021 3546-001A