2712 TAYLOR DR 2023-11-01 - Laserfiche WebLink

October 6, 2019 Page 5 of 6 Limited Geotechnical Investigation Stormwater Management Feasibility The site .is'.underlain .by relatively dense, very,fine-grained glacial deposits. These soils have a very low permeability which decreases with depth. It is likely that there will be shallow perched groundwater throughout the property that further limits infiltration suitability. We, performed an in -situ small scale Pilot Infiltration Test in HB-1 at a depth of 4 feet below grade. Following testing and application of correction factors for site variability (0.33), testing (o.5), and influent control. (o.9) the infiltration rate was 0.21 inches per hour, which is lower than what the Department of Ecology considers to be, feasible. We do not recommend utilizing infiltration systems at the site. There does not appear to be adequate space for dispersion flowpaths; therefore, .we recommend direct connection `to City infrastructure with a perforated connection. 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 hrough 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. 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. www.cobaltgeo.com (2o6) 331-1097