7010 COLBY AVE 2023-06-01 - Laserfiche WebLink

Mr. Paul Korenovsky ES-6067 June 20, 2018 Page 3 Topsoil and Fill Where encountered at surface grades, topsoil extended to depths of about 2 to 12 inches, with average depths of roughly 6 inches. The topsoil was characterized by the observed dark brown hue, the presence of fine organics, and small root intrusions. Fill was encountered at several of the test pit locations to approximate depths of one-and-one- half to two-and-one-half feet below the existing ground surface (bgs). The fill was characterized primarily as a loose to medium dense, silty sand with gravel (USCS: SM) and was encountered primarily in a damp to moist condition. Fill encountered during grading should be evaluated by ESNW during subsequent construction activities. Native Soil Underlying topsoil, native soils consisted primarily of medium dense to very dense, silty sand with gravel (USCS: SM) and silty gravel with sand (UCSC: GM). Native soils were primarily encountered in a damp to moist condition. The maximum exploration depth was approximately seven feet bgs. Geologic Setting The referenced geologic map resource identifies glacial till (Qvt) deposits as the primary native soil unit underlying the subject site. As reported on the geologic map resource, glacial till typically consists of a nonsorted mixture of silt, sand, pebbles, cobbles, and boulders, which resembles concrete mix and is commonly referred to as "hardpan." The till was deposited directly from the glacier as it advanced over bedrock and older Quaternary sediment. The referenced WSS resource identifies Alderwood-Urban land complex (Map Unit Symbol: 5) as the primary soil unit underlying the subject site. The Alderwood series was formed in glacial till plains. Based on our field observations, on-site native soils are generally consistent with glacial till (Qvt) deposits. Groundwater During our subsurface exploration completed on May 15, 2018, groundwater seepage was not encountered at the test pit locations. Additionally, during SGP's subsurface exploration completed on March 20, 2018 groundwater seepage was not encountered at the test pit locations. However, it is our opinion the contractor should anticipate, and be prepared to respond to, zones of perched groundwater seepage during construction. Temporary measures to control surface water runoff and groundwater during construction would likely involve interceptor trenches and sumps. It should be noted that seepage rates and elevations fluctuate depending on many factors, including precipitation duration and intensity, the time of year, and soil conditions. In general, groundwater flow rates are higher during the wet season (October through April). Earth Solutions NW,LLC