2710 14TH ST BLDG A Geotech Report 2023-12-29

4.5.4.1. Subgrade Protection Site soils contain significant fines content (silt/clay) and will be highly sensitive and susceptible to moisture and equipment loads. Once the existing pavement is removed, the exposed subgrade soils can deteriorate rapidly in wet weather and under equipment loads. The contractor should take necessary measures to prevent site subgrade soils from becoming disturbed or unstable. Construction traffic during the wet season should be restricted to specific areas of the site, preferably areas that are surfaced with the existing asphalt pavement or working pad materials not susceptible to wet weather disturbance. Existing asphalt should be left in place through the winter months, where practical, to limit subgrade disturbance. It may be possible to initially remove only pavement areas within building footprints, while leaving existing areas around the future buildings for equipment lay down purposes and for routing equipment. Consideration should be given to removingthe remaining asphalt pavement (outside of building footprints) in dry weather just before final site grading and final paving activities for the project. Protectingthe existing soils with a thin layer of crushed rock will not be adequate during the wet season and the subgrade will still deteriorate under equipment loads. If the contractor removes the existing pavement prior to the wet season, consideration should be given to protecting the exposed subgrade areas with asphalt -treated base (ATB), or a thicker section of crushed rock or recycled asphalt grindings overlying a geotextile separator. 4.5.5. Structural Fill All fill, whether existing on -site soils or imported soil, that will support floor slabs, pavement areas or foundations, or be placed against retaining walls or in utility trenches are classified as structural fill and should generally meet the criteria for structural fill presented below. The suitability of soil for use as structural fill depends on its gradation and moisture content. 4.5.5.1. Materials Structural fill material quality varies depending upon its use as described below: ■ Structural fill placed below foundations (designed for 3,000 psf or lower), floor slabs, or as subbase material below pavement areas should meet the criteria for gravel borrow as described in Section 9-03.14(1) of the 2018 WSDOT Standard Specifications. ■ CDF used to support building foundations designed for bearing pressures exceeding 4,000 psf should be in accordance with 2018 WSDOT Standard Specification Section 2-09.3(1)E and should have a minimum compressive strength of 200 psi. The mix design should be adjusted to obtain this minimum compressive strength. Lean concrete may also be used under building foundations. ■ Structural fill placed to raise site grades or to backfill utility trenches should meet the criteria for common borrow as described in Section 9-03.14(3) of the 2018 WSDOT Standard Specifications during dry weather conditions (typically June through September). Common borrow materials are highly moisture sensitive. For wet weather construction (October through May), structural fill placed to raise site grades or in utility trenches should meet the criteria for gravel borrow as described in Section 9-03.14(1) of the 2018 WSDOTStandard Specifications, exceptthatthe fines content (material passing the US No. 200 sieve) should not exceed 5 percent. GEOENGINEEHS� October30, 2019 Page 13 ' File No 21288-002 00