9908 18TH AVE W 2024-12-03 - Laserfiche WebLink

Pacific Ridge — DRH, LLC ES-7852.01 March 15, 2022 Page 3 Topsoil and Fill Topsoil thickness was variable at the test pit locations, extending to depths of about 4 to 28 inches. The topsoil was characterized by dark brown color, the presence of fine organic material, and small root intrusions. Fill was encountered at test pit locations TP-1, TP-6, and TP-7, extending to depths of five and one-half feet below the existing ground surface (bgs). The fill generally consisted of silty sand soil and contained concrete, brick, and metal debris. The fill was encountered primarily in a loose condition. It is possible for deeper pockets of fill to be encountered within the western/southwestern portion of the site. Where encountered, fill soils intended for reuse as structural fill should be free of organics and other deleterious materials, and should be evaluated by ESNW for suitability at the time of construction. Native Soil Underlying the topsoil and existing fill, native soil was encountered primarily as silty sand with and without gravel (USCS: SM), consistent with the typical makeup of glacial till. The near- surface, loose to medium dense deposits may be characterized as "weathered", and the underlying, dense to very dense deposits may be characterized as "unweathered". Where fill was not present, the upper, loose to medium dense weathered glacial till deposits were observed extending to depths of about two to three and one-half feet bgs, transitioning to dense, weakly to moderately cemented, and unweathered condition at depth. Overall soil relative density generally increased with depth. Geologic Setting Geologic mapping identifies Vashon glacial till (Qvt) across the site. As reported on the geologic map resource, Vashon glacial till consists primarily of a non-sorted mixture of silt, sand, and sub- rounded to well-rounded gravels, commonly referred to as "hardpan." The till was deposited directly from the glacier as it advanced over bedrock and older Quaternary sediment. Based on our field observations, on-site native soils generally correlate with glacial till deposits. The referenced WSS identifies Alderwood-urban land complex (Map Unit Symbol: 5) as the primary soil unit underlying the subject site. The Alderwood series soils were formed in glacial till plains and are typically deposited as hills and ridges. The Alderwood-urban land complex series soils are associated with large-scale urban grading. Site soils are generally consistent with Alderwood-urban land complex. Groundwater During our subsurface exploration completed on April 1, 2021, groundwater seepage was encountered at six of the seven test pit locations between about one and one-half to five and one- half feet bgs. The groundwater seepage flow rate was variable and was observed to be heavy at TP-7. Based on the conditions observed at the test pit locations, perched groundwater seepage should be expected during grading activities at the site, particularly during the winter, spring, and early summer months. Groundwater seepage rates and elevations fluctuate depending on many factors, including precipitation duration and intensity, the time of year, and soil conditions. Earth Solutions NW, LLC