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HomeMy WebLinkAboutRS_Geotechnical_Report_170503_v1 Cobalt Geosciences P.O. Box 82243 Kenmore, Washington 98028 (206) 331-1097 August 22, 2016 Updated April 24, 2017 pmvrsingh@gmail.com Mr. Satwant Singh RE: Limited Stormwater & Geologic Hazard Evaluation Proposed Six Lot Short Plat 13609 156th Avenue SE Renton, Washington Dear Mr. Singh, In accordance with your authorization, Cobalt Geosciences, LLC has prepared this letter to discuss the results of our limited stormwater and geologic hazard evaluation at the referenced site. The proposed development includes six single-family residences, access driveways, and landscaped areas. Site and Project Description The site is located at 13609 – 156th Avenue SE in Renton, Washington (Figure 1). The site consists of one rectangular parcel (No. 146340-0045) with a total area of approximately 1.3 acres. The site is developed with a single-family residence and access driveway extending onto the property from the north and near the east end of the property. There are several outbuildings within the remainder of the property. The site slopes gently downward toward the southwest with total topographic relief of about 25 feet. The site is vegetated with grasses, blackberry vines, and local deciduous and evergreen trees. The site is bordered to the south and west by single-family residences, to the north by SE 2nd Place and to the east by 156th Avenue SE. The project consists of subdividing the existing parcel into six building lots. The existing residence and outbuildings would be removed as part of site development. Stormwater management will include a stormwater detention vault located in the west portion of the site. 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 unsor ted, 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. August 22, 2016 Updated April 12, 2017 Page 2 of 3 Limited Stormwater & Geologic Hazard Evaluation Limited Soils Evaluation We investigated the shallow soil conditions at three locations to confirm the mapped geologic unit and to determine if infiltration is feasible. Figure 2 shows the test pit locations. The test pit logs can be found in Figure 3. All of the test pits encountered about 6 to 18 inches of vegetation and topsoil underlain by approximately 1.5 to 2.5 feet of loose to medium dense, silty-fine to medium grained sand with gravel (Weathered Glacial Till). This material was underlain by dense, silty-fine to medium grained sand with gravel (Unweathered Glacial Till). The unweathered till was cemented in many places. We did not observe groundwater in the test pits during the excavation work in August, 2016. We anticipate that perched groundwater may develop at the contact between the weathered and unweathered glacial till during the wet season. We observed localized soil mottling from 3 to 4 feet in the test pits and the soils below these layers were partially cemented . Limited Infiltration Evaluation On April 21, 2017, we performed two small-scale Pilot Infiltration Tests (PIT) within the property to evaluate the infiltration potential of the shallow soils. Two areas were excavated to a depth of approximately 3 feet below grade and at least 40 square feet in area. Groundwater seepage was observed entering the excavations at a depth of 2.5 feet below grade. These excavation locations are indicated on the site plan in Figure 2. The shallow soil and groundwater conditions indicate that infiltration of stormwater runoff is not feasible. If groundwater was not present, the dense glacial till would likely have an infiltration rate of less than 0.1 inches per hour, which is lower than the Washington State Department of Ecology recommended minimum rate of 0.3 inches per hour. Geologic Hazards There are localized slope areas up to 25 percent in magnitude are located within and adjacent to the western portion of the site. We conducted a reconnaissance of the area and did not observe evidence of earth movements, erosion, or indicators of future instability. Based on the site geology and field explorations, the site is underlain by relatively dense soil deposits, which are not typically prone to landslide activity under the current conditions. The Natural Resources Conservation Services (NRCS) maps for King County indicate that the site is underlain by Alderwood soils. We anticipate the soils to consist of very gravelly sandy loams with a “Slight” erosion potential in a disturbed state. It is our opinion that soil erosion potential at this project site can be reduced through landscaping and surface water runoff control. Typically erosion of exposed soils will be most noticeable during periods of rainfall and may be controlled by the use of normal temporary erosion control measures, such as silt fences, hay bales, mulching, control ditches and diversion trenches. The typical wet weather season, with regard to site grading, is from October 31 st to April 1st. Erosion control measures should be in place before the onset of wet weather. August 22, 2016 Updated April 12, 2017 Page 3 of 3 Limited Stormwater & Geologic Hazard Evaluation Conclusions & Recommendations The site is underlain by weathered and unweathered glacial till. The moderately steep slope in the western portion of the site appears stable at this time. The proposed development will not increase the likelihood of landslide activity or erosion on the property and adjacent areas provided that the geotechnical firm reviews the final plans and periodically monitors grading activities in the area during construction. Recent subsurface explorations indicated dense glacial till with areas of perched groundwater at shallow depths. Due to the presence of shallow groundwater along with very dense, fine -grained soil conditions, infiltration of stormwater runoff is not feasible at this site. We should review the final plans to determine if additional geotechnical provisions are warranted as part of the proposed site development and stormwater system construction. 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. Sincerely, Cobalt Geosciences, LLC Phil Haberman, P.G., P.E.G. Principal Engineering Geologist PH/sc N VICINITY MAP FIGURE 1 P.O. Box 82243 Kenmore, WA 98028 (206) 331-1097 cobaltgeo@gmail.com Cobalt Geosciences Proposed 6-Lot Short Plat 13609 - 156th Ave SE Renton, Washington Project Location Kent WASHINGTON SITE SITE PLAN FIGURE 2 P.O. Box 82243 Kenmore, WA 98028 (206) 331-1097 cobaltgeo@gmail.com Cobalt Geosciences N TP-1 Proposed 6-Lot Short Plat 13609 - 156th Ave SE Renton, Washington Test Pit TP-1 0-0.5’ Grass/Topsoil 0.5-2.5’ Silty-SAND with Gravel (SM) Loose to medium dense, silty-fine to medium grained sand with gravel, mottled yellowish brown to grayish brown, dry to moist. 2.5-6.0’ Silty-SAND with Gravel (SM) Dense to very dense, silty-fine to medium grained sand with gravel, yellowish brown to grayish brown, moist. End of Test Pit 6’ No Groundwater No Caving SM SM Test Pit TP-3 Test Pit TP-2 TEST PIT LOGS FIGURE 3 P.O. Box 82243 Kenmore, WA 98028 (206) 331-1097 cobaltgeo@gmail.com Cobalt Geosciences Paul Bunyan Training & Education Center Puyallup, Washington 0-0.25’ Grass/Topsoil 0.25-3.0’ Silty Sand with Gravel (SM) Loose, silty-sand with gravel, yellowish brown, dry to moist. 3.0’-5.0’ Silty Sand with Gravel, Silty-Gravel with Sand (SM-GM) Loose to medium dense, coarse gravel with sand and silt, trace cobbles, yellowish brown, moist. 5.0’-8.0’ Gravel with Sand to Silty-Gravel with Sand (GP-GM) Medium dense, gravel with sand and silt, yellowish to grayish brown, moist. End of Test Pit 8’ No Groundwater SM SM 0-0.5’ Grass/Topsoil 0.5-3.5’ Silty GRAVEL with sand (GM) Loose to medium dense, fine to coarse gravel with cobbles, sand, and silt, yellowish brown, dry to moist (Drift). 3.5-4.0’ Dense to very dense, fine to coarse gravel with cobbles, sand, and silt, yellowish brown, moist (Drift/Till). End of Test Pit 4’ No Groundwater GM GM Test Pit TP-3 0-0.5’ Grass/Topsoil 0.5’-2.0’ Silty Sand with Gravel (SM) Loose, silty-sand with gravel, yellowish brown, dry to moist (Fill). 2.0’-7.0’ Silty Sand with Gravel (SM) Loose to medium dense, silty-sand with gravel, trace cobbles, yellowish brown, moist. End of Test Pit 7’ No Groundwater SM SM Test Pit TP-2 0-0.25’ Grass/Topsoil 0.25-3.0’ Silty Sand with Gravel (SM) Loose, silty-sand with gravel, yellowish brown, dry to moist. 3.0’-5.0’ Silty Sand with Gravel, Silty-Gravel with Sand (SM-GM) Loose to medium dense, coarse gravel with sand and silt, trace cobbles, yellowish brown, moist. 5.0’-8.0’ Gravel with Sand to Silty-Gravel with Sand (GP-GM) Medium dense, gravel with sand and silt, yellowish to grayish brown, moist. End of Test Pit 8’ No Groundwater SM SM-GM GP-GM Test Pit TP-2 0-0.25’ Grass/Topsoil 0.25-3.0’ Silty Sand with Gravel (SM) Loose, silty-sand with gravel, yellowish brown, dry to moist. 3.0’-5.0’ Silty Sand with Gravel, Silty-Gravel with Sand (SM-GM) Loose to medium dense, coarse gravel with sand and silt, trace cobbles, yellowish brown, moist. 5.0’-8.0’ Gravel with Sand to Silty-Gravel with Sand (GP-GM) Medium dense, gravel with sand and silt, yellowish to grayish brown, moist. End of Test Pit 8’ No Groundwater SM SM-GM GP-GM Test Pit TP-2 0-0.25’ Grass/Topsoil 0.25-3.0’ Silty Sand with Gravel (SM) Loose, silty-sand with gravel, yellowish brown, dry to moist. 3.0’-5.0’ Silty Sand with Gravel, Silty-Gravel with Sand (SM-GM) Loose to medium dense, coarse gravel with sand and silt, trace cobbles, yellowish brown, moist. 5.0’-8.0’ Gravel with Sand to Silty-Gravel with Sand (GP-GM) Medium dense, gravel with sand and silt, yellowish to grayish brown, moist. End of Test Pit 8’ No Groundwater SM SM-GM GP-GM Test Pit TP-2 0-0.25’ Grass/Topsoil 0.25-3.0’ Silty Sand with Gravel (SM) Loose, silty-sand with gravel, yellowish brown, dry to moist. 3.0’-5.0’ Silty Sand with Gravel, Silty-Gravel with Sand (SM-GM) Loose to medium dense, coarse gravel with sand and silt, trace cobbles, yellowish brown, moist. 5.0’-8.0’ Gravel with Sand to Silty-Gravel with Sand (GP-GM) Medium dense, gravel with sand and silt, yellowish to grayish brown, moist. End of Test Pit 8’ No Groundwater SM SM-GM GP-GM Limited Stormwater Evaluation Renton, Washington 0-1.5’ Grass/Topsoil 1.5-3.0’ Silty-SAND with Gravel (SM) Loose to medium dense, silty-fine to medium grained sand with gravel, mottled yellowish brown to grayish brown, dry to moist. 3.0-5.0’ Silty-SAND with Gravel (SM) Dense to very dense, silty-fine to medium grained sand with gravel, yellowish brown to grayish brown, moist. End of Test Pit 5’ No Groundwater No Caving Test Pit TP-3 0-1.5’ Grass/Topsoil 1.5-4.0’ Silty-SAND with Gravel (SM) Loose to medium dense, silty-fine to medium grained sand with gravel, reddish brown to dark yellowish brown, moist. 4.0-6.0’ Silty-SAND with Gravel (SM) Medium dense to dense, silty-fine to medium grained sand with gravel, yellowish brown to grayish brown, moist. End of Test Pit 6’ No Groundwater No Caving SM SM