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Home My WebLink AboutRS_Hardy Short Plat_Geotech Report_210106_v1Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, Washington 98028 www.cobaltgeo.com (206) 331-1097 January 6, 2021 Mr. Bob Wenzl Tuscany Homes RE: Stormwater Feasibility Evaluation Proposed Residence 857 Field Avenue NE Renton, Washington Dear Mr. Wenzl, 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 determine the feasibility of utilizing infiltration devices for stormwater runoff management. Site and Project Description The site is located at 857 Field Avenue NE in Renton, Washington. The site consists of one rectangular parcel (No. 1437650040) with a total area of about 15,130 square feet. The eastern portion of the property is developed with a residence and driveway. The remainder of the property is undeveloped and vegetated with grasses, bushes/shrubs, and variable diameter evergreen and deciduous trees. The site slope gently downward from east to west at magnitudes of 5 to 10 percent and total relief of about 12 feet. The property is bordered to the north, west, and south by residential parcels and to the east by Field Avenue NE. The project includes subdivision of the property followed by construction of a new residence. We anticipate that the existing residence will remain in place. Stormwater management may include dispersion, detention, or infiltration facilities depending on feasibility. 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. Soil & Groundwater Conditions As part of our evaluation, we excavated two test pits to determine the shallow soil and groundwater conditions, where accessible. January 6, 2021 Page 2 of 3 Stormwater Feasibility Evaluation www.cobaltgeo.com (206) 331-1097 The test pit encountered approximately 6 inches of topsoil and vegetation underlain by approximately 2.5 to 3 feet of loose to medium dense, silty-fine to medium grained sand with gravel (Weathered Glacial Till). These materials were underlain by dense, silty-fine to medium grained sand with gravel (Glacial Till), which continued to the termination depths of the test pits. Groundwater was not encountered in the test pits. Groundwater could be present seasonally perched on the underlying glacial till. Stormwater Management Feasibility The site is underlain by weathered and unweathered glacial till. Unweathered glacial till is nearly impermeable and not suitable for widespread infiltration of stormwater runoff. We performed an infiltration test in TP-1 at a depth of about 3 feet below grade. Following saturation, testing, and application of correction factors for site variability (0.8), influent control (0.9), and testing (0.5), the infiltration rate was 0.2 inches per hour. This is lower than what is considered to be feasible for widespread infiltration. We do not recommend the use of infiltration systems at this site. We recommend either dispersion trenches with vegetated flowpaths or direct connection to City infrastructure. Rain gardens and permeable pavements may be feasible depending on the location and elevation of these systems. We can provide additional input upon request. We should be provided with final plans for review to determine if the intent of our recommendations has been incorporated or if additional modifications are needed. 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 other filtration methods will need to be incorporated. January 6, 2021 Page 3 of 3 Stormwater Feasibility Evaluation www.cobaltgeo.com (206) 331-1097 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 1/6/21 Phil Haberman, PE, LG, LEG Principal PH/sc Proposed Residences 857 Field Avenue NE Renton, Washington N SITE PLAN FIGURE 1 Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, WA 98028 (206) 331-1097 www.cobaltgeo.com cobaltgeo@gmail.com Subject Property TP-1 TP-2 PT Well-graded gravels, gravels, gravel-sand mixtures, little or no fines Poorly graded gravels, gravel-sand mixtures, little or no fines Silty gravels, gravel-sand-silt mixtures Clayey gravels, gravel-sand-clay mixtures Well-graded sands, gravelly sands, little or no fines COARSE GRAINED SOILS (more than 50% retained on No. 200 sieve) Primarily organic matter, dark in color, and organic odor Peat, humus, swamp soils with high organic content (ASTM D4427)HIGHLY ORGANIC SOILS FINE GRAINED SOILS (50% or more passes the No. 200 sieve) MAJOR DIVISIONS SYMBOL TYPICAL DESCRIPTION Gravels (more than 50% of coarse fraction retained on No. 4 sieve) Sands (50% or more of coarse fraction passes the No. 4 sieve) Silts and Clays (liquid limit less than 50) Silts and Clays (liquid limit 50 or more) Organic Inorganic Organic Inorganic Sands with Fines (more than 12% fines) Clean Sands (less than 5% fines) Gravels with Fines (more than 12% fines) Clean Gravels (less than 5% fines) Unified Soil Classification System (USCS) Poorly graded sand, gravelly sands, little or no fines Silty sands, sand-silt mixtures Clayey sands, sand-clay mixtures Inorganic silts of low to medium plasticity, sandy silts, gravelly silts, or clayey silts with slight plasticity Inorganic clays of low to medium plasticity, gravelly clays, sandy clays, silty clays, lean clays Organic silts and organic silty clays of low plasticity Inorganic silts, micaceous or diatomaceous fine sands or silty soils, elastic silt Inorganic clays of medium to high plasticity, sandy fat clay, or gravelly fat clay Organic clays of medium to high plasticity, organic silts Moisture Content Definitions Grain Size Definitions Dry Absence of moisture, dusty, dry to the touch Moist Damp but no visible water Wet Visible free water, from below water table Grain Size Definitions Description Sieve Number and/or Size Fines <#200 (0.08 mm) Sand -Fine -Medium -Coarse Gravel -Fine -Coarse Cobbles Boulders #200 to #40 (0.08 to 0.4 mm) #40 to #10 (0.4 to 2 mm) #10 to #4 (2 to 5 mm) #4 to 3/4 inch (5 to 19 mm) 3/4 to 3 inches (19 to 76 mm) 3 to 12 inches (75 to 305 mm) >12 inches (305 mm) Classification of Soil Constituents MAJOR constituents compose more than 50 percent, by weight, of the soil. Major constituents are capitalized (i.e., SAND). Minor constituents compose 12 to 50 percent of the soil and precede the major constituents (i.e., silty SAND). Minor constituents preceded by “slightly” compose 5 to 12 percent of the soil (i.e., slightly silty SAND). Trace constituents compose 0 to 5 percent of the soil (i.e., slightly silty SAND, trace gravel). Relative Density Consistency (Coarse Grained Soils) (Fine Grained Soils) N, SPT, Relative Blows/FT Density 0 - 4 Very loose 4 - 10 Loose 10 - 30 Medium dense 30 - 50 Dense Over 50 Very dense N, SPT, Relative Blows/FT Consistency Under 2 Very soft 2 - 4 Soft 4 - 8 Medium stiff 8 - 15 Stiff 15 - 30 Very stiff Over 30 Hard Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, WA 98028 (206) 331-1097 www.cobaltgeo.com cobaltgeo@gmail.com Soil Classification Chart Figure C1 Test Pit Logs Cobalt Geosciences, LLC P.O. Box 82243 Kenmore, WA 98028 (206) 331-1097 www.cobaltgeo.com cobaltgeo@gmail.com Proposed Residence 857 Field Avenue NE Renton, Washington Test Pit TP-1 Date: December 4, 2020 Contractor: Client provided Depth: 5.5’ Elevation: Logged By: PH Checked By: SC Groundwater: None Material Description Moisture Content (%)Plastic Limit Liquid Limit 10 20 30 400 50 DCP Equivalent N-Value 10 Test Pit TP-2 Date: December 4, 2020 Contractor: Client provided Depth: 5.5’ Elevation: Logged By: PH Checked By: SC Groundwater: None Material Description Moisture Content (%)Plastic Limit Liquid Limit 10 20 30 400 50 1 2 3 4 5 6 DCP Equivalent N-Value 7 8 9 10 Loose to medium dense, silty-fine to medium grained sand with gravel, reddish brown to yellowish brown, moist. (Weathered Glacial Till) SM End of Test Pit 5.5’ Dense, silty-fine to medium grained sand with gravel, grayish brown, moist. (Glacial Till) -Locally cemented SM Topsoil/Grass Material Description Moisture Content (%)Plastic Limit Liquid Limit 10 20 30 400 50 1 2 3 4 5 6 DCP Equivalent N-Value 7 8 9 Loose to medium dense, silty-fine to medium grained sand with gravel, reddish brown to yellowish brown, moist. (Weathered Glacial Till) SM End of Test Pit 5.5’ Dense, silty-fine to medium grained sand with gravel, grayish brown, moist. (Glacial Till) -Locally cemented SM Topsoil/Grass