3815 SHORE AVE 2020-05-20 - Laserfiche WebLink

Geologically Hazardous Areas: Geotechnical Letter Kennedy Single Family Residence 3815 Shore Avenue March 23, 201 RN File No. 3279-001A Page 3 greater extent than those that were not. Part of a typical glacial sequence within the area of the site includes the following soil deposits from newest to oldest: Artificial Fill (af)—Fill material is often locally placed by human activities, consistency will depend on the source of the fill. The thickness and expanse of this material will be dependent on the extent of fill required to grade land to the desired elevations. Density of the fill will depend on earthwork activities and compaction efforts made during the placement of the material. Recessional Outwash (Ovr) —These deposits were derived from the stagnating and receding Vashon glacier and consist mostly of stratified sand and gravel, but include unstratified ablation and rnelt-out deposits. Recessional deposits were not compacted by the glacier and are typically not as dense as those that were. Vashon Till (Qvt)—The till is a non-sorted mixture of clay, sand, pebbles, cobbles and boulders, all in variable amounts. The till was deposited directly by the ice as it advanced over and eroded irregular surfaces of previously deposited formations and sediments. The till was well compacted by the advancing glacier and exhibits high strength and stability. Drainage is considered very poor in the till. Advance Outwash (Qva)—The advance outwash typically is a thick section of mostly clean, pebbly sand with increasing amounts of gravel higher in the section. The advance outwash was placed by the advancing glaciers and was overridden and well compacted by the glacier. Transitional Beds (Qtb) —The preglacial transitional beds are typically clay, silt and fine sand soils that were mostly deposited in lakes some distance from the ice front and in fluvial environments prior to the advance of the ice sheet...These beds typically grade up into the overlying advance outwash. They appear firm in outcrop and can become unstable in steep slopes because of high water content and jointing. The geologic units for this area are mapped on the Distribution and Description of Geologic Units in the Mukilteo Quadrangle. Washington, by James P. Minard (U.S. Geological Survey, 1982). The site is mapped as being underlain by deposits of transitional bed with advance outwash beds mapped further upsiope to the,southeast. Our site explorations encountered fine to medium sandy soils characteristic of transitional beds grading up into outwash deposits and laminated silts characteristic of the lower lacustrine transitional beds outcropping on the slope. Explorations We explored subsurface conditions within the site on March 19, 2018, by digging two test pits with a backhoe. The test pits were excavated to depths of 11,5 to 12.0 feet below the ground surface. Samples were collected from the test pits at observed changes in soil type. The test pits were located in the field by a geologist from this firm who also examined the soils and geologic conditions encountered, and maintained logs of the test pits.The approximate locations of the explorations are shown on the Site Plan in Figure 2. The soils were visually Robinson Noble, Inc.