1228 GRAND AVE Geotech Report 2022-10-25

April 7,2008 Mr. Don Levin L&A Job No. 8A037 Page 3 The geology of the Puget Sound Lowland has been modified by the advance and retreat of several glaciers in the past and subsequent deposits and erosion. The latest glacier advanced to the Puget Sound Lowland is referred to as the Vashon Stade of the Fraser Glaciation, which has occurred during the later stages of the Pleistocene Epoch and retreated from the region some 14,500 years ago. The Vashon till soil unit is deposited directly by glacial ice during the most recent glacial period as it advanced over an eroded, irregular surface of older formations and sediments. It is a very dense mixture of unsorted clay, silt, sand, gravel, and scattered cobbles and boulders, often referred to as "hard pan". The Vashon till over the top two to four feet is normally weathered to a medium-dense state, and is moderately permeable and compressible. The underlying fresh till is very dense and is practically impervious to stormwater infiltration. The fresh till has the strength of low-grade concrete and can stand in a steep natural or cut slope for a long period. If remains undisturbed and well-drained, the fresh till can provide excellent foundation support with little settlement expected The advance outwash soil unit underlying the Vashon till is composed of stratified sand and gravel with minor amounts of silt and clay, deposited by the meltwater of advancing glacial ice. Due to their generally granular composition, the advance outwash deposits are of moderately high permeability and generally drains fairly well. The advance outwash deposits are glacially overridden and are generally dense to very dense in its natural,undisturbed state,except the top 2 to 4 feet where exposed on slopes which may be weathered to a loose to medium-dense state. The advance outwash deposits can stand in steep cuts or natural slopes for extended period when undisturbed. Where exposed on slopes with poor vegetation cover and subject to storm runoff or groundwater seepage,the advance outwash deposits can be gradually eroded and may slough and LIU & ASSOCIATES, INC.