3333 NASSAU ST 2024-08-07 - Laserfiche WebLink

October 3, 2016 Single-Family Residence L&A Job No. 16-124 Page 6 feet,respectively. This advance autwash deposit is consisted of light-brown to light-gray, medium-dense to dense, clean to slightly-silty, fine sand. Also encountered in Test Pit 4 (the upper-most test pit) between the weathered soil and the advance outwash layers is a layer of glacial till. This glacial till layer is Aout 3.5 feet thick and is composed of gray, very-dense, cemented, silty fine sand with trace gravel. The deposits of the Vashon till soil unit were plowed directly under glacial ice during the most recent glaciation period as the glacier advanced over an eroded, irregular surface of older formations and sediments. They are composed of a mixture of very-dense, cemented unsorted clay, silt, sand, gravel, and scattered cobbles and boulders. Glacial 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. GROUNDWATER CONDITION Groundwater was not encountered by any of the four test pits excavated on the project site. The very-dense cemented glacial till and the very-stiff to hard transitional beds soil layers underlying the site at shallow depth are of extremely low permeability and would perch stomrwater infiltrating into the more permeable surficial soils. The amount of and the depth to the near-surface perched groundwater would fluctuate seasonally, depending on precipitation, surface runoff, ground vegetation cover, site utilization, and other factors. The perched groundwater would accumulate and rise in the wet winter months and may dry up completely during the dryer summer months. This perched groundwater may build up and emerge out of gully banks in the winter months and cause erosion and landslides. LIU & ASSOCIATES, INC. I S3