4730 COLBY AVE SEPA 016 - 027 - YMCA Geotech Report 2025-04-14

October 25, 2015 YMCA - Everett L&A Job No. 15-089 Page 4 of the Fraser Glaciation which had occurred during the later stages of the Pleistocene Epoch, and retreated from the region some 12,500 years ago. The deposits of the Vashon till soil unit were plowed directly under by glacial ice during the most recent glacial period as the glacier advanced over an eroded, irregular surface of older formations and sediments. This soil unit is composed of a mixture of unsorted clay, 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 penneable and compressible. The underlying fresh till soil, commonly referred to as "hard pan", is very dense and weakly cemented. The fresh till deposits possess a compressive strength comparable to that of low-grade concrete and can remain stable on steep natural slopes or man-make cuts for a long period. The fresh till deposits can provide excellent foundation support with little or no settlement, but are of extremely low permeability and would hardly allow stormwater to seep through. The deposits of the advance outwash soil unit, normally underlying the Vashon till soil unit, are composed of stratified sand and gravel with very minor amount of silt and clay, deposited 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 drains fairly well. The advance outwash deposits are generally dense to very dense in their natural, undisturbed state. The underlying fresh advance outwash deposits in their native, undisturbed state can provide good foundation support with little settlement expected for light to moderately heavy structures. LIU & ASSOCIATES, INC.