2026-0206 Cle Elum Transfer Station Project Manual

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SCJ Alliance MEMORANDUM September 26, 2025 Project No. NWG0131 Page 4 included as Appendix C should be provided to the disposal facility in order to determine if the soil can be accepted. Soil Disposal Recommendations If soil will be exported from the Site and disposed of for installation of the new UST, we recommend coordinating with disposal facilities to confirm acceptance of the soil. We can assist with soil disposal facility selection and creation of a waste profile, if necessary. New UST Design Recommendations We understand that the new UST will be a 5,000 gallon, 8-foot diameter, 16-foot long tank. We recommend that the new UST bears atop a 6-inch leveling pad (minimum thickness) of crushed surfacing base coarse, or other material directly specified by the tank manufacturer, placed and compacted directly overlying native, alluvium sand or gravel soils below any undocumented fill or coal waste observed in the test pits. We recommend an allowable soil bearing pressure of 2,000 pounds per square foot (psf) be used for design. The allowable soil bearing pressure includes a factor of safety of at least 3, and an associated total settlement estimate of less than 1 inch. The allowable soil bearing pressure is appropriate for unfactored dead loads plus live loads and can be increased by up to one-third for temporary loading condition for seismic loading. We are not aware of any buoyancy issues with the existing tank and we did not observe evidence of groundwater in our explorations that might result in buoyancy uplift. For this reason, it is our opinion that buoyancy forces will not develop, and an extended base or anchors are not needed for the UST to resist uplift. For backfill soil vertical pressure above the tank, we recommend assuming a backfill soil total unit weight of about 125 pounds per cubic foot (pcf). For soil lateral pressures from backfill acting on the tank sidewalls we recommend assuming a soil friction angle of 34° and a total unit weight of 125 pcf, resulting in an at-rest (non-yielding), triangular-distribution lateral earth pressure of 55 pcf. General Earthwork Recommendations The Site subsurface consists generally of fill underlain by alluvium. In our opinion, earthwork in these soils can be completed with conventional construction equipment. Based on our explorations and on our regional experience, the contractor should be prepared to excavate and handle large cobbles and boulders within the subsurface. All temporary cuts more than 4 feet in height that are not protected by trench boxes or otherwise shored, should be sloped in accordance with Part N of Washington Administrative Code (WAC) 296-155 for worker safety (WSL, 2022). For planning purposes, we recommend assuming site soils as Type C with a maximum allowable temporary slope inclination of 1.5H:1V – to be confirmed during construction by the contractor’s competent person(s). We recommend backfill placed directly around and in contact with the tank surface conform to the tank manufacturer’s recommendations for gradation, placement, and compaction methods. Beyond/outside this zone, the on-Site sand and gravel alluvium below the debris-laden fill and coal layer are generally suitable for reuse as structural fill provided, are relatively free from organic matter or other deleterious materials, and are screened such that they do not contain particles