2026-0206 Cle Elum Transfer Station Project Manual

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SCJ Alliance MEMORANDUM September 26, 2025 Project No. NWG0131 Page 2 Subsurface Conditions Our interpretation of subsurface conditions was developed based on our review of geologic maps and observations made during excavation of our test pits completed in unpaved gravel or bare soil areas at the Site, laboratory testing of collected soil samples, and our experience on other projects near the Site. Geology The geologic map shows that the entirety of the Site surface is mapped as Sidestream Alluvium (Qs). Sidestream Alluvium consists of locally sourced, moderately sorted boulder to pebble gravel (Tabor et al., 1982). Fill is not mapped at the Site but was observed at the surface from previous grading and Site development. Subsurface Explorations On August 5, 2025, we completed three test pit explorations at the Site, designated TP-01 through - 03. The test pit locations are shown on Figure 2. The pits were advanced to depths of between 3.75 and 13 feet below ground surface (bgs), using a Komatsu 88 tracked excavator operated by Mass X Construction under subcontract to Geosyntec. Relative densities of the encountered materials were estimated using a ½-inch diameter steel T-probe, by observing the performance of the excavation side walls, and by observing the difficulty of excavation by the excavator. Soil samples were collected from the explorations using the excavator bucket. Upon completion of a test pit, each was backfilled with the generated soils and tamped into place with the excavator bucket. A Geosyntec geologist was present throughout the field exploration program to observe the explorations, collect soil samples, and prepare descriptive logs of the explorations. Soils were identified in general accordance with ASTM International (ASTM) D2488, Standard Practice for Description and Identification of Soils (Visual Manual Procedure). Descriptions of the soil observed in the explorations, as well as the depths where characteristics of the soil changed, are indicated on the Exploration Logs presented in Appendix A of this report. Definitions of the terminology and symbols used on the logs are included in the Exploration Log Key. The primary soil units observed at the Site, presented in stratigraphic order from top to bottom, are fill and alluvium (Qs). These units are described in more detail below. Fill We encountered fill in the upper 3 to 4 feet of each of the explorations. The fill consisted of silty sand with gravel over gravel with silt, sand, cobbles, and boulders in varying proportions (Unified Soil Classification System symbols SM, GM, and GP 1). The boulders were up to 16 inches in diameter. The fill was medium dense to very dense, slightly moist, brown, with small amounts of brick, coal fragments, plastic, rebar, asphalt, and plant debris. The fill also contained an up to 2- foot-thick section of coal waste consisting of silt to fine sand sized coal particles. The coal waste was medium dense, moist, and black. Alluvium Under the fill in TP-02 and -03, we encountered alluvium (Qs) to the bottom of each exploration at 13 feet bgs. This unit predominantly consisted of gravel with silt, sand, gravel, cobbles, and 1 Soils are classified per the United Soil Classification System (USCS) in general accordance with the ASTM International (ASTM) Method D2488 Standard Practice of Description and Identification of Soils (ASTM, 2018).