9702 19TH AVE SE Geotech Report 2024-03-28

Pacific Ridge — DRH, LLC ES-7829.01 August 26, 2021 Page 3 The approximate locations of the test pits are depicted on Plate 2 (Subsurface Exploration Plan). Please refer to the test pit and boring logs provided in Appendix A for a more detailed description of subsurface conditions. Representative soil samples collected at our exploration sites were analyzed in general accordance with Unified Soil Classification System (USCS) and USDA methods and procedures. Topsoil and Fill Topsoil was generally not encountered at the test locations, except for about two inches of topsoil at one of the test pit locations. Fill was observed at four of the test pit locations and within three of the borings. The fill extended to depths of up to roughly nine feet bgs within proposed development areas; deeper fill appears to be present within the existing stockpile in the northwest corner of the site. The fill was characterized primarily as loose to medium dense silty sand with gravel (USCS: SM), and was encountered generally in a moist condition at the time of exploration. Additionally, fill may be present within proximity to existing structural improvements. Where fill is encountered during construction, ESNW should be consulted to evaluate the suitability for support of the proposed structures and/or reuse as structural fill. Native Soil Underlying topsoil, native soils consisted primarily of medium dense to very dense silty sand with gravel glacial till deposits. Overall soil relative density increased with depth. Weathered glacial till deposits were observed with an average thickness of about three feet bgs, transitioning to a dense to very dense and unweathered condition at depth. The native soil profiles were consistent across the site, and we observed unweathered glacial till deposits extending to the maximum exploration depth of about 30.5 feet bgs. Geologic Setting Geologic mapping indicates the site is underlain by glacial till deposits (Qvt). As reported on the geologic map resource, the glacial till consists of a nonsorted mixture of silt, sand, pebbles, cobbles, and boulders and commonly is referred to as "hardpan." The till was deposited directly from the glacier as it advanced over bedrock and older Quaternary sediment. Additionally, the WSS resource identifies Alderwood-urban land complex (Map Unit Symbols: 5) within the proposed development area. The Alderwood series was formed in glacial till plains. Based on our field observations, on-site native soils are generally consistent with glacial till deposits. Groundwater Groundwater seepage was not observed at the test pit or boring locations during the fieldwork (March and April 2021). However, perched groundwater seepage is common within glacially derived soil deposits and should be expected during grading activities at the site during the winter, spring, and early summer months. Groundwater seepage rates and elevations fluctuate depending on many factors, including precipitation duration and intensity, time of year, and soil conditions. Earth Solutions NW, LLC