The URL can be used to link to this page

Home My WebLink About025 Geotech Report Cobalt Geosciences P.O. Box 82243 Kenmore, Washington 98028 (206) 331-1097 March 28, 2017 Revised March 26, 2019 Mr. Bob Wenzl C/O Mr. Cliff Williams cliff@sitedme.com RE: Stormwater Feasibility Evaluation Proposed Single Family Residences 4709 NE 24th Street Renton, Washington Dear Mr. Williams, In accordance with your authorization, Cobalt Geosciences, LLC has prepared this letter to discuss the results of our stormwater feasibility evaluation at the referenced site. Site and Project Description The site is located at 4709 NE 24th Street in Renton, Washington. The site consists of one rectangular shaped parcel with a total area of about 23,770 square feet. Currently, the east portion of the site is developed with a single-family residence and driveway. The remainder of the site is landscaped with sparse trees, grasses, and shrubs/bushes. Overall the site slopes gently downward toward the west. Slopes within the site are generally less than 10 percent and the overall topographic relief is less than 10 feet. The property is bordered to the north by NE 24th Street, to the west by Duvall Avenue NE, and to the south and east by single family residences. The project consists of removing the existing structures and developing the site with two single family residences. Stormwater management may consist of infiltration, dispersion, rain gardens, or tightlines into city infrastructure. Area Geology The Geologic Map of King County, indicates that the site is underlain by Vashon Glacial Till. Vashon Glacial Till is typically characterized by an unsorted, non-stratified mixture of clay, silt, sand, gravel, cobbles and boulders in variable quantities. These materials are typically dense and relatively impermeable. The poor sorting reflects the mixing of the materials as these sediments were overridden and incorporated by the glacial ice. March 28, 2017 Revised March 26, 2019 Page 2 of 3 Stormwater Feasibility Evaluation Soil & Groundwater Conditions As part of our evaluation, we excavated and sampled two test pits up to 8 feet below grade in accessible areas of the property. Figure 1 shows the general location of the explorations. Exploration logs are attached at the end of this letter report. Test Pit TP-1 encountered approximately 6 inches of topsoil and grass underlain by approximately 2 feet of loose to medium dense, silty-fine to medium grained sand with gravel (Fill). This layer was underlain by approximately 3.5 feet of medium dense, fine to coarse grained gravel with sand and silt (Recessional Outwash). This layer was underlain by stiff silt with fine grained sand (Recessional Outwash – Lacustrine), which continued to the termination depth of the test pit. Test Pit TP-2 encountered approximately 6 inches of topsoil and grass underlain by approximately 3.5 feet of loose, silty-sand to sandy silt (Fill). This layer was underlain by medium stiff to stiff, silt with fine grained sand (Recessional Outwash – Lacustrine), which continued to the termination depth of the test pit. Groundwater was encountered in TP-1 at 5.5 feet below grade and in TP-2 at approximately 6 feet below grade. Groundwater appears to be perched on/within recessional deposits. Stormwater Management Feasibility Based on the explorations, infiltration appears to be feasible in the area of TP-1. Infiltration is not feasible in the very fine-grained silt deposits that underlie the fill and local areas of poorly graded gravel. We conducted a falling head infiltration test in TP-1 at 4 feet below grade. The short term rate was observed to be greater than 8 inches per hour. Due to the presence of groundwater and impermeable soils, we recommend limiting infiltration system depth to 2 to 4 feet below existing grade. However, the geotechnical engineer should be on site during system placement to verify that the gravelly outwash soils are present at system locations, and to verify at least 1 foot clearance above groundwater. The soils in TP-1 between 2.5 and 6 feet below grade should be considered to be “Medium Sand” per the King County Surface Water Design Manual. While the soils are very gravelly, they have areas of silt which will decrease the rate of infiltration. Therefore, each infiltration trench should have a length of 30 feet for each 1,000 square feet of impervious surface area. Infiltration does not appear feasible in the east lot. Limited dispersion or other shallow systems should be considered. Additional explorations could be performed in order to delineate areas of outwash gravels and sands that may be present within the property. Closure The information presented herein is based upon professional interpretation utilizing standard practices and a degree of conservatism deemed proper for this project. We emphasize that this report is valid for this project as outlined above and for the current site conditions and should not be used for any other site. March 28, 2017 Revised March 26, 2019 Page 3 of 3 Stormwater Feasibility Evaluation Sincerely, Cobalt Geosciences, LLC Phil Haberman, P.G., P.E.G. Principal PH/sc SITE PLAN FIGURE 1 P.O. Box 82243 Kenmore, WA 98028 (206) 331-1097 cobaltgeo@gmail.comCobalt Geosciences N Stormwater Feasibility Evaluation 4709 NE 24th Street Renton, Washington TP-1 TP-1 TP-2 TEST PIT LOGS P.O. Box 82243 Kenmore, WA 98028 (206) 331-1097 cobaltgeo@gmail.comCobalt Geosciences Test Pit TP-1 0-0.5’ Vegetation/Topsoil 0.5-2.5’ Silty Sand (SM) Loose to medium dense, silty-fine to medium grained sand trace gravel, mottled yellowish brown to grayish brown, moist. (Fill) 2.5-6’ Gravel with Sand and Silt (GP) Medium dense, fine to coarse gravel with sand and silt, yellowish brown, moist to wet (Recessional Outwash) 6-8’ Silt with Sand (ML) Stiff, silt with fine grained sand trace gravel, mottled tan to yellowish brown, moist. (Lacustrine Deposits) End of Test Pit 8’ Groundwater at 5.5 No Caving SM Recessional Outwash Topsoil/Vegetation 0.5’ Test Pit TP-2 0-0.5’ Vegetation/Topsoil 0.5-4’ Silty Sand with Gravel (SM) Loose to medium dense, silty-fine to medium grained sand with gravel, mottled yellowish brown to grayish brown, moist. (Fill) 4-8’ Silt with Sand (ML)Medium stiff to stiff, silt with fine grained sand trace gravel, mottled yellowish brown to grayish brown, moist to wet. (Lacustrine Deposits) End of Test Pit 8’ Groundwater at 6’ No Caving ML SM Fill Fill Lacustrine Deposits 0.5’ 4’ USCS Graphic USCS Graphic 4709 NE 24th Street Renton, Washington 2.5’ Topsoil/Vegetation LLC GP ML Lacustrine Deposits 6’