600 RIVERSIDE RD Geotech Report 2022-10-25

1 H 11 I Landau Associates 2.0 SITE CONDITIONS This section discusses the general geologic setting of the project area and describes the surface and subsurface conditions observed at the project site at the time of our field investigation. Interpretations of the site conditions are based on the results of our review of available information, and the results of our site reconnaissance, subsurface explorations, and laboratory testing. 2.1 Geologic Setting The geologic units located near the surface of the site and within the project area include alluvium underlain by Vashon drift and the Whidbey formation. These geologic units are described in the following sections. Alluvium deposited by the Snohomish River is present across the entire site below a surficial layer of fill. The alluvium consists of layers of sand and silt with varying organic content. In general, the upper portion of this unit consists of fine to medium sand with varying silt content. This portion of the unit has been previously referred to as the upper sand unit (EMCON 1995). The upper sand unit is underlain by very soft to stiff silt with trace sand, lenses of sand, and organics. This portion of the unit has been previously referred to as the silt unit (EMCON 1995). Soil encountered at depths below the silt unit primarily consist of silty, clayey sand, or interbedded clayey silt and sand grading to gray, medium sand with trace fines and pieces of wood. Others have referred to this portion of the unit as the lower sand unit (EMCON 1995). The silt unit forms an aquitard between an upper sand aquifer and an underlying lower sand aquifer. Vashon drift includes, from youngest to oldest, Vashon recessional outwash, Vashon till, and Vashon advance outwash. Glacial rivers or streams deposited the units of the Vashon drift sequentially during the advance and later recession of the Vashon glaciation. No units of the Vashon drift were found in excavations at the site; only younger alluvium overlying the Vashon drift was observed within the depths of the excavations and borings at the site. However, Vashon drift is present in the hillsides west and south of the site and may underlie the alluvium at depth beneath the site. Regional geologic data indicate that the Vashon drift is underlain by the Whidbey formation. This unit consists mostly of medium- to coarse -grained, cross -stratified sand, and interbeds of fine-grained sand and silty sand. This unit was not observed in any of the explorations completed at the site. 2.2 Hydrologic Setting Three hydrologic units have been identified by others beneath the project site (EMCON 1995). These units include a shallow unconfined aquifer in the fill and upper sand unit, an underlying aquitard unit, and a semiconfined aquifer in the lower sand unit. Geotechnical Engineering Report 0121030.010.011 Riverside Business Park March 22, 2018 Public Access Trails and Roadway Improvements 2-1