5107 SOUND AVE 2017-01-14 - Laserfiche WebLink

July 21, 2014 Two Single-Family residences L&A Job Ivo. 14-065 Page 4 silt, sand, gravel, and scattered cobbles and boulders. The Vashon till soil over the top tw� to thre�e feet is normally weathered to a medium-dense state, and is moderately permeable �uid compressible. The underlying fresh tili soil, commonly refezred to as "hard pan", is v�ry-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 V+Thidbey formation deposits are normally very-stiff to hard in t1,n;; ��t,vr ,��siist��rl��i s��t�. Th�y are also of e�remel� low permeability and tend to perch siormwater iniiitraiii�g inEo tlae �x�o�� pe3dyi��ka1� sa��ii�izl �oi�s. �'1�� p�r�he� groundwater may emerge out of nai,ural slopes and man-made cuts where the Whidbey formation is exposed. When the atnount of groundwater seepage increases in wet winter months, the emerging groundwater can erode and cause sloughing and siide. Two of the test piis excavated on �e project site also encountered an advance outwash deposit which usually overlies ihe Whidbey fornaation. 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 meltvvater of advancing glacial ice of the last glacier then overridden by the still advancing glacier. Due to their generally ,granular compositifln, the advance outwash deposits are of moderately high permeability and drain fairly well. The advance outwash deposits are generally dense to very dense in their naturai undisturbed state. VVhere exposed on slopes with poor vegetation cover and subjected to storm runc�ff erosion, the advance outwash soil in the top few feet can be gradually weathered and LIU & ASS�CIATES, INC.