5109 SOUND AVE 2017-08-02 - Laserfiche WebLink

July 21, 2014 Two Single-Farnily residences L&A Job No. 14-065 Page 4 silt, sand, gravel, and scattered cobbles and boulders. The Vashon till soil over the top two to three feet is normally weathered to a medium-dense state, and is moderately permeable and compressible. The underlying fresh till soil, commonly referred to as "hard pan", is very-dense and cemented. This soil unit, however, was not encountered by the test pits excavated on the project site. The Whidbey formation soil unit is consisted of pre-Fraser Glaciation sediments composed of glacial and non-glacial flood-plain deposits of light-brown to gray, fine- sandy and clayey silt. The Whidbey formation deposits are normally very-stiff to hard in their native undisturbed state. T'hey are also of extremely low permeability and tend to perch stormwater infiltrating into the more penneable surficial soils. The perched groundwater may emerge out of natural slopes and man-made cuts where the Whidbey formation is exposed. When the amount of groundwater seepage increases in wet winter months, the emerging groundwater can erode and cause sloughing and slide. Two of the test pits excavated on the project site also encountered an advance outwash tieposit which usually overlies the Whidbey formation. The deposits of advance outwash soil unit are composed of stratified sand and gravel with very minor amount of silt and clay, laid down by the meltwater of advancing glacial ice of the last glacier then overridden by the still advancing glacier. Due to their generally granular composition, the advance outwash deposits are of moderately high permeability and drain fairly well. The advance outvvash deposits are generally dense to very dense in their naturai undisturbed state. Where exposed on slopes with poor vegetation cover and subjected to storm runoff erosion, the advance outwash soil in the top few feet can be gradually weathered and LIU & ASSOCIATES, INC.