1018 14TH ST 2023-02-09 - Laserfiche WebLink

SHANNON&WILSON,INC. 3.2 Site Geology Based on the subsurface explorations at the site, we developed a generalized subsurface profile behind the existing bulkhead (see Figure 3). This generalized subsurface profile presents the inferred subsurface conditions assumed in our analyses. General descriptions of the deposits encountered at the site are presented below: ■ Fill [Hf]. The upper 15 to 20 feet of soil behind the bulkhead consists of fill material placed by humans. Fill has widely variable properties, depending on the material used as fill and whether the fill was placed in an engineered or non-engineered fashion. Fill soils at the site are primarily material dredged from the surrounding area and are therefore similar to the underlying native deposits. Fill soils were identified from the presence of irregular clasts of one soil type within soil of another type, disturbed appearance, or from the presence of debris such as fragments of wood, sawdust, and/or cinders. ■ Estuarine Deposits [He]. In Segments C and D,the fill soils are underlain by 2 to 6 feet of Holocene estuarine silt deposits. Estuarine deposits generally consist of very soft to stiff, low-plasticity silt and clayey silt to silty clay with minor amounts of fine sand. Interbeds of organic-rich soils may be present within this unit. ■ Alluvium Deposits [Ha]. About 15 to 20 feet of Holocene alluvium was encountered below the fill and estuarine deposits in all of the borings. The alluvial deposits were likely deposited by the Snohomish River during construction of the Snohomish delta. These soils are generally granular and consist of sand, silty sand, and gravelly sand. In the borings, these deposits were very loose to loose near the top of the layer and increased to dense at the bottom of the layer. In localized areas along the project alignment,the alluvial soils are interbedded with layers of fine-grained estuarine deposits. The alluvial soils also contain zones with wood and shell fragments. ■ Beach Deposits [Hb]. Beach deposits are typically located at or near the base of Holocene units and were encountered in all of the borings except B-2. The thickest beach deposit layer was encountered in boring B-4 in Segment A (about 8 feet thick). Beach deposits typically consist of medium dense to very dense sand and gravel and may also contain scattered cobbles and locally cohesive fines. Scattered to abundant shell fragments and wood debris are commonly observed in these soils. ■ Pre-Vashon Non-glacial Fluvial Deposits [Qpnf]. Based on the borings performed for our study,the bulkhead alignment is underlain by glacially overridden soils below about elevation -20 to -25 feet. These glacially overridden deposits are primarily fluvial soils deposited of rivers and creeks in between glacial events. They are generally composed of clean to silty, fine to medium sand with lesser amounts of slightly silty, gravelly sand and sandy gravel with a trace of silt. These soils are very dense because they have been overridden by glacial ice. The organic material encountered within nonglacial fluvial deposits typically consists of small pieces of 21-1-21761-003-R I f:docx/wp/clp 21-1-21761-003 6