400 RIVERSIDE RD SEPA 016 - 039 - RIVERSIDE BUSINESS PARK Geotech Report 2025-04-14

' December 8,2016 Project No. T-7388-1 I It should be noted that the recommendations outlined in this report regarding drainage are associated with soil strength, erosion, and stability. Design and performance issues with respect to moisture as it relates to the structure environment are beyond Terra Associates' purview. A building envelope specialist or contractor should be consulted to address these issues, as needed. 3.0 SITE CONDITIONS 3.1 Surface ' The undeveloped site consists of 2 tax parcels, Lots 2 and 4, having a combined area of approximately 11 acres. The site is bounded to the north and south by adjoining parcels within Riverside Business Park. Riverside Road and the Snohomish River bound the site to the west and east, respectively. The approximate location of the site is shown on Figure 1. The site's western and northern perimeters approximately match the adjacent access road and Riverside Road grades, respectively. From these areas, grades rise three to five feet to a fill pad covering the majority of Lot 4 and the western part of Lot 2. At the time of our visit, the relatively level fill pad's surface predominantly consisted of exposed soil, with vegetation consisting of sparse grasses and weeds. The eastern portion of the site is predominantly vegetated with brush. Areas of ponded water on the fill pad were observed at the time of our visit. 3.2 Soils The soil types and strengths indicated on the CPT logs generally consist of fill soils encountered to depths ranging from 3 feet at CPT-5 to 8 feet at CPT-6. The cone soundings indicate the fills consist of medium dense to dense sandy silt, silty sand, and sand. ' Native alluvial deposits underlie the fills at each of the CPT locations. The alluvial soils consist of medium dense to dense sand and silty sand found to bottom-of-layer depths ranging from 10 feet at CPT-5 to 19 feet at CPT-6. At these depths,the sands are underlain by a 4-to 11-foot thick layer of soft to medium stiff silt and clay. The CPT soundings indicate medium dense to dense alluvial sands and silty sands underlie the silts and clays to the total depths of the CPTs. The soil types and densities/consistencies at the CPT locations generally correlate with those shown on the test borings completed by Landau Associates. Their Borings B-4, B-5, B-7, and B-8 show the silty sand fills extending to depths of 6.5 to 7 feet. Loose to medium dense alluvial sand and very stiff silt underlie the fills to the total depths of B-4 and B-5. The soft silt/clay layer observed in our CPT soundings generally correlates with the very soft to soft silt encountered in the Landau Associates' Borings B-7 and B-8 at a depth of 14.5 feet. 1 ' Page No. 2