9512 EVERGREEN WAY 2022-08-29 - Laserfiche WebLink

May 4, 2017 HWA Project No. 2017-036-21 Except where apparently well compacted, the fill soil has low to moderate shear strength, moderate compressibility, and moderate permeability. Weathered Till — Immediately below fill at the locations of HH-1, HH-4, HH-5, and HH-6, and below topsoil in HH-3, weathered till was identified. The weathered till forms a poorly developed B-horizon, which was derived mostly in -situ from the native till due to chemical and physical weathering, as well as biological activity. It is typically yellowish -brown to olive brown in color and consists of silty sand with gravel and a trace of cobbles. Weathered till ranged from 1.5 to 3.2 feet thick, where it was fully penetrated in HH-5 and HH-3, respectively. This soil has moderate shear strength, low compressibility, and moderately low permeability. Vashon Till (Glacial Tilll — Dense to very dense silty sand with gravel was encountered in borings HH-2, HH-3 and HH-5 beneath the fill, or weathered till, and extended to the full depths explored. This unit has a diamicton structure (unsorted; unstratified) and is matrix -supported; i.e. the gravel particles are supported by the sand and fines. The gravel varied from fine to coarse, and may contain scattered cobbles. The till has a high fines content, although the fines within the till are non -plastic. This soil has a very high shear strength, and very low compressibility and permeability. Locally, this soil is referred to as hardpan and impedes the vertical movement to locally infiltrating water. 3.3.2 Ground Water Ground water seepage was observed in all the hand borings except HH-6. Where encountered, the seepage appeared to be perched over the dense glacial till in the overlying weathered till or fill layers. We expect the likelihood of encountering perched ground water during excavation at the site locations is high during the wet season. Therefore, we recommend that construction take place during the dry summer months. It is anticipated that the ground water conditions will change in response to rainfall, time of year, and other factors, but in general should not be of other than minor consequence to the proposed developments if managed as described in Section 4.8, below. 3.4 SURFICIAL SOIL HORIZONS According to the Soil Survey of Snohomish County Area (available online from the Natural Resources Conservation Service) the subject site is underlain by Alderwood Urban Land complex, 2 to 8 percent slopes consisting of gravelly ashy sandy loam. This soil formed on glacial till, is slowly permeable, and poses a slight erosion hazard. 3.5 GEOLOGIC HAZARDS In this section, the potential for various geologic hazards on site are discussed including erosion, landslide, and seismic (liquefaction and settlement) hazards. Final Geologic Assessment 4 HWA GeoSciences Inc.