3301 SEAWAY BLVD Geotech Report 2022-10-25

y January 13,2016 Project No. T-7342 The site is currently undeveloped and heavily forested with both young and mature coniferous and deciduous trees and thick undergrowth consisting of blackberries, ferns, and brush. Site topography can be divided into two sections with low center of the site as the dividing point. In general, the topography on the west side of the site consists of a moderate slope that descends from the southwest to the east with an overall relief of approximately 74 feet. The center of the site is generally flat with a slight slope that ascends to the east with an overall relief of approximately six feet. The eastern portion of the site consists of a moderate to steep slope that descends from the west to the south-southeast with an overall relief of approximately 60 feet. The slopes throughout the site have gradients between 7 and 100 percent. 3.2 Soils In general, soil conditions observed below six to 18 inches of topsoil/forest duff consisted of glacially deposited sediments. In the upper west-northwest portion of the site (Test Pits TP-1, TP-2, TP-3,TP-4, TP-14, and TP-15) soils consisted of 2 to 7 feet of medium dense silty sand with gravel (weathered till) overlying dense to very dense silty sand with gravel (unweathered glacial till) to the termination of the test pits. In the lower east- southeast portion of the site (Test Pits TP-5, TP-6, TP-7, TP-8, TP-10, TP-11, and TP-12) Advance outwash composed of dense to very dense sand with varying amounts of silt and gravel was observed below the upper glacial till soils at depths of 4 to 14 '/z feet below current surface grades. The Distribution and Description of Geologic Units in the Mukilteo Quadrangle, Washington, by James P. Minard, 1982,maps the site as Vashon Till(Qvt)with Advanced Outwash(Qva) in the sites eastern extent. The soils we observed in the test pits are consistent with these mapped descriptions. The preceding discussion is intended to be a general review of the soil conditions encountered. For more detailed descriptions,please refer to the Test Pit and Boring Logs in Appendix A. 3.3 - Groundwater We observed perched groundwater seepage in Test Pits TP-4,TP-5,TP-6,TP-8,TP-10,and TP-11 between 3 and 6 feet below current site grades. The seepage observed was generally light and typically occurred within the weathered till soils or at the contact between the weathered and unweathered till soils. We also observed that the soils near the contact between the weathered and unweathered deposits were mottled in some places. This is an indication that a perched groundwater table develops at times in that zone. We would expect that shallow groundwater seepage develops during the normally wet winter months along the contact between the upper silty sand layer and underlying unweathered glacial till. This occurs as a result of rainfall that infiltrates through the upper weathered soil zone and becomes perched on the underlying dense till. The till has relatively low permeability that impedes the continued downward migration of the infiltrated rainfall. As a result, groundwater seepage will develop and tend to flow laterally along the contact. Locally,such seepage is referred to as interflow. Page No.3