3410 TERMINAL AVE 2018-03-09 - Laserfiche WebLink

SI IAN NON t_.SON,INC. ice for these glaciations originated in the Coast Range and Rocky Mountains of British ' Columbia, Canada, and generally advanced(flowed) southward into the Puget Lowland. Each glaciation deposited new sediment and partially eroded previous sediments. During the intervening periods when glacial ice was not present,normal stream processes,wave action, weathering, and landsliding eroded and reworked some of the glacially derived sediment, further complicating the geologic setting. , During the most recent glaciation that covered the central Puget Lowland, approximately 18,000 to 16,000 years before present(termed Vashon) (Porter and Swanson, 1998),the glacial ' ice is estimated to have been about 3,000 feet thick in the project area(Thorson, 1989). The weight of the glacial ice resulted in compaction(overconsolidation)of the glacial and nonglacial soils beneath the ice. As the last ice to reach the Puget Lowland(Vashon Stade)retreated to the north, deposits of sand, gravel, silt, and clay were laid down by meltwater streams issuing from the glacial ice front. These deposits are termed glacial recessional soils and are not glacially 1 consolidated. The proposed bulkhead replacement site is located along the east shore of Possession Sound. ' Prior to construction of the original bulkhead and other Port facilities,the site was likely situated at the southern edge of the Snohomish River delta. The glacial and interglacial deposits are overlain by younger(Holocene Epoch),relatively loose and soft,post-glacial soils that include peat, alluvial, lacustrine, and fill deposits. 5.3 Site Geology ' Based on our subsurface exploration and our review of the historical subsurface explorations at the site,the following soil deposits underlie the site. General descriptions of the deposits encountered at the site in our exploration are presented below: ■ Fill [Hf]. The upper 45 feet of soil behind(east of)the wharf consists of fill material placed by humans likely during the construction of the wharf and apron. The depth of the fill is under the wharf is unknown, since the historical borings were completed prior to construction of the wharf. Fill has widely variable properties, depending on the material used as fill and whether the fill was placed in an engineered or non- engineered condition. Fill soils at the site are likely 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. 21-1-21962-003-R2.docx/wp/cp 21-1-21962-003 6 1