2220 GIBSON PL Geotech Report 2022-10-25

August 18, 2014 Gibson Bluff L&A Job No. 14-056 Page 4 The deposits of the Vashon till soil unit were plowed directly under 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 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. This soil unit, however, was not encountered by the test borings drilled on the 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. They are also of extremely low permeability and would perch stormwater infiltrating into the more permeable surficial soils. The perched groundwater may emerge out of natural slopes and man-made cuts where the Whidbey formation is exposed. This soil unit was encountered by one of the three test borings at 3 L0 feet deep. The Whidbey formation soil unit is normally overlain by an Esperance Sand soil unit. This soil unit was encountered by each of the three test borings. The Esperance Sand soil unit is composed of light-brown to light-gray, fine to medium sand with various amount of gravel, deposited by melt water of advancing glacier. The.Esperance sand deposits are dense to very-dense in its undisturbed native state. Due to its generally granular composition, the Esperance sand deposits are of moderately high permeability and would LIU & ASSOCIATES, INC.