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Home My WebLink AboutRS_711 Monroe Geo Letter.pdf Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, Washington 98028 www.cobaltgeo.com (206) 331-1097 May 14, 2018 Ms. Julie Chen greenhousetex@comcast.net RE: Stormwater Feasibility Evaluation Proposed Development Expansion 711 Monroe Avenue NE Renton, Washington Dear Ms. Chen, 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. The purpose of our evaluation was to evaluate the shallow subsurface soil conditions and provide options for stormwater management. Site and Project Description The site is located at 711 Monroe Avenue NE in Renton, Washington. The site consists of one rectangular shaped parcel with a total area of about 13.552 square feet. The western portion of the site is currently developed with a single-family residence used as a business. A driveway extends through the north portion of the prope rty from the east and Monroe Avenue NE. The remainder of the property is vegetated with grasses, bushes, shrubs, and sparse deciduous and evergreen trees. The site slopes gently downward toward the southwest and west with topographic relief of less than 8 feet. There is a moderately steep slope extending downward to the west in the area just west of the property. This area is well vegetated with low- lying brush and variable diameter trees. The slope is up to 15 feet in height and has magnitudes of 30 percent. The property is bordered to the north and west by single-family residences, to the south by a commercial development, and to the east by Monroe Avenue NE. The project includes construction of a second story to the existing residence as well as expanded parking areas in the area between the building and Monroe Avenue NE. Stormwater management may include permeable pavements, infiltration systems, rain gardens, or other shallow best management practices (BMPs) from the King County Surface Water Design Manual. 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. May 14, 2018 Page 2 of 3 Stormwater Feasibility Evaluation www.cobaltgeo.com (206) 331-1097 Soil & Groundwater Conditions As part of our evaluation, we excavated two hand borings up to 6 feet below grade. Both hand borings encountered approximately 5 to 8 inches of topsoil and grass underlain by approximately 2 to 2.5 feet of loose to medium dense, silty-fine to fine grained sand with gravel (Weathered Glacial Till). This layer was underlain by medium dense to dense, silty -fine to medium grained sand with gravel (Glacial Till), which continued to the termination depths of the hand borings. Soil mottling was observed in both hand borings from about 2 to 5 feet below existing grades. Soil mottling (iron oxide staining) typically indicates the fluctuation level of seasonally perched groundwater. We also reviewed available exploration logs from nearby site locations, including across the street to the east. The soils described in these boring and test pit logs were consistent with those encountered in the hand borings. Stormwater Management Feasibility The site is underlain by glacial till which typically has a very lo w permeability. We conducted an EPA Falling Head infiltration test in HB-1 at a depth of 3 feet below grade. The factored infiltration rate was 0.28 inches/hour. The Washington State Department of Ecology (DOE) states that infiltration in soils with permeability of less than 0.3 inche s/hour is not recommended and/or potentially infeasible. It is our opinion that infiltration of stormwater runoff is not feasible due to the underlying soil and groundwater conditions. The measured infiltration rates were less than 0.3 inches/hour in the upper 4 feet. Infiltration rates in underlying unweathered glacial till are typically less than 0.1 inches/hour. Groundwater and infiltrating surface waters are likely to migrate laterally to the southwest, potentially daylighting along the moderately steep slope west of the site. Depending on the volume of runoff from the new parking areas and roof area, the following stormwater management BMPs may be feasible: • Dispersion trenches with limited flowpaths (roof area) • Sheet flow (parking lots) • Rain gardens • Detention systems (R tanks or vaults) • Permeable pavements for flow control • Direct connection to City stormwater infrastructure We recommend that all systems be located east of the existing building to reduce the potential for impacts to the slope area west of the property. Based on our observations, a combination of permeable pavements, rain gardens, and detention with overflow to City infrastructure appear feasible. It may be necessary to utilize one or more of the above BMPs for stormwater management. Once the Civil designer has calculated volumes of runoff and has developed preliminary BMPs options, we can provide additional input. We should review the final plans as they become available. May 14, 2018 Page 3 of 3 Stormwater Feasibility Evaluation www.cobaltgeo.com (206) 331-1097 Erosion and Sediment Control Erosion and sediment control (ESC) is used to reduce the transportation of eroded sediment to wetlands, streams, lakes, drainage systems, and adjacent properties. Erosion and sediment control measures should be implemented, and these measures should be in general accordance with local regulations. At a minimum, the following basic recommendations should be incorporated into the design of the erosion and sediment control features for the site: • Schedule the soil, foundation, utility, and other work requiring excavation or the disturbance of the site soils, to take place during the dry season (generally May through September). However, provided precautions are taken using Best Management Practices (BMP’s), grading activities can be completed during the wet season (generally October through April). • All site work should be completed and stabilized as quickly as possible. • Additional perimeter erosion and sediment control features may be required to reduce the possibility of sediment entering the surface water. This may include additional silt fences, silt fences with a higher Apparent Opening Size (AOS), construction of a berm, or other filtration systems. • Any runoff generated by dewatering discharge should be treated through construction of a sediment trap if there is sufficient space. If space is limited othe r filtration methods will need to be incorporated. 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, PE, LG, LEG Principal PH/sc Proposed Parking Expansion 711 Monroe Avenue NE Renton, Washington SITE PLAN FIGURE 1 Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, WA 98028 (206) 331-1097 www.cobaltgeo.com cobaltgeo@gmail.com N HB-1 HB-1 HB-2 Log of Hand Boring HB-1 Date: May 2, 2018 Contractor: Method: Hand Auger Depth: 6’ Elevation: N/A Logged By: PH Checked By: SC Initial Groundwater: None Sample Type: Grab Final Groundwater: N/A Material Description SPT N-Value Moisture Content (%)Plastic Limit Liquid Limit 10 20 30 400 50 1 2 3 4 5 6 7 8 9 10 End of Hand Boring 6’ Vegetation/Topsoil Loose to medium dense, silty-fine to medium grained sand with gravel and cobbles, dark yellowish brown to yellowish brown, moist. (Weathered Glacial Till) SM Medium dense to dense, silty-fine to medium grained sand, mottled yellowish brown to grayish brown, moist. (Glacial Till) SM Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, WA 98028 (206) 331-1097 www.cobaltgeo.com cobaltgeo@gmail.com Proposed Parking Expansion 711 Monroe Avenue NE Renton, Washington Hand Boring Log Log of Hand Boring HB-2 Date: May 2, 2018 Contractor: Method: Hand Auger Depth: 6’ Elevation: N/A Logged By: PH Checked By: SC Initial Groundwater: None Sample Type: Grab Final Groundwater: N/A Material Description SPT N-Value Moisture Content (%)Plastic Limit Liquid Limit 10 20 30 400 50 1 2 3 4 5 6 7 8 9 10 End of Hand Boring 6’ Vegetation/Topsoil Loose to medium dense, silty-fine to medium grained sand with gravel and cobbles, dark yellowish brown to yellowish brown, moist. (Weathered Glacial Till) SM Medium dense to dense, silty-fine to medium grained sand, mottled yellowish brown to grayish brown, moist. (Glacial Till) SM Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, WA 98028 (206) 331-1097 www.cobaltgeo.com cobaltgeo@gmail.com Proposed Parking Expansion 711 Monroe Avenue NE Renton, Washington Hand Boring Log