3800 RIVERFRONT BLVD Geotech Report 2023-05-04

March 26, 2019 HWA Project No. 2015-061-21 Task 600-7 I I I I I I I I I I I I I I portions of the Puget Lowland filled with alluvial sediments deposited by rivers draining the western slopes of the Cascades and the eastern slopes of the Olympics. The most recent glacial advance, the Fraser Glaciation, included the Vashon Stade, during which the Puget Lobe of the Cordilleras Ice Sheet advanced and retreated through the Puget Sound Basin. Existing topography, surficial geology and hydrogeology in the project area were heavily influenced by the advance and retreat of the Vashon Ice Sheet. Surficial geological information for the site area was obtained partly from the published geological map, Geologic Map of the Everett Quadrangle (Minard, 1985). The map indicates that the site is generally underlain by younger alluvium (Holocene). Younger alluvium deposits consist of unconsolidated, stratified, clay, silt and very fine to fine sand with abundant organic material. Medium to coarse sand and gravel underlie much of the fine-grained floodplain sediment. Glacial till, advance outwash and transitional beds are mapped west of the site in the upland area. These glacial and interglacial deposits were over-consolidated by the weight of over 3,000 feet of ice during the Vashon Stade. 3.3 SUBSURFACE CONDITIONS The current and previous soil investigations at the project site have encountered six general material types: fill, refuse, alluvium, glaciolacustrine, advance outwash, and pre-Vashon alluvium, as summarized in the following sub-sections. The entire site contains surficial fill soil, placed as a cap over the refuse and regraded during preloading and recent filling activities. Dense to very dense advance outwash and/or pre-Vashon alluvium were encountered in each boring drilled for this study. The depth to the dense soils increases towards the river and generally to the northeast. Four geologic cross-sections along and perpendicular to the road alignment, presented in Figures 3A through 3D, were generated in support of the roadway design. These cross-sections were based on the current and previous exploration logs and proposed grading plans by Perteet. It is to be noted that due to the interpretive nature of cross-sections, only the exploration logs should be relied upon for subsurface detail at particular locations. Fill Soil: A fill cap was encountered in all borings for this study and previous studies within the landfill limits. This soil typically consisted of gravelly, silty sand (Unified Soil Classification SM), and ranged in thickness from approximately 5' to 20 feet. It consisted of three types as follows: 2018 Fill: Approximately 3 feet of fill material generally meeting the gradation requirements of WSDOT Common Borrow was imported and placed across the landfill site. This material was imported in the summer of 2018 from the 3-Acre Park preload and the preload at the Simpson site. HWA provided compaction testing of this material as it was placed. Compaction levels greater than 90 percent were achieved for the material as it was placed. Some degradation of the surface of this material occurred as Geotechnical Report Revision 3 26 2019.doc 5 HWA GEOSCIENCES INC.