8407 BROADWAY SECURE SPACE STORAGE Geotech Report 2025-04-24

ZipperGeo Broadway Everett Expansion Geoprofessional Consultants Project No. 2601.01 July 7, 2022 place during extended periods of wet weather,or if the in situ moisture conditions are elevated above the optimum moisture content, soils can become unstable or not be compactable. In the event the exposed subgrade becomes unstable, yielding, or unable to be compacted due to high moisture conditions, we recommend that the materials be removed to a sufficient depth in order to develop stable subgrade soils that can be compacted to the minimum recommended levels. The severity of construction problems will be dependent, in part, on the precautions that are taken by the contractor to protect the subgrade soils. If earthwork takes place during freezing conditions, all exposed subgrades should be allowed to thaw and then be compacted prior to placing subsequent lifts of structural fill. Alternatively, the frozen material could be stripped from the subgrade to expose unfrozen soil prior to placing subsequent lifts of fill or foundation components. The frozen soil should not be re-used as structural fill until allowed to thaw and adjusted to the proper moisture content, which may not be possible during winter months. Structural Fill Materials and Placement Structural fill includes any material placed below foundations, floor slabs and pavement sections, within utility trenches, and behind retaining walls. Prior to the placement of structural fill, all surfaces to receive fill should be prepared as previously recommended in the Site Preparation section of this report. Laboratory Testing Representative samples of on-site and imported soils to be used as structural fill should be submitted for laboratory testing at least four days in advance of the materials' intended use in order to complete the necessary Proctor tests. Re-Use of Site Soils as Structural Fill The suitability for re-use of site soils as structural fill depends on the composition and moisture content of the soil. Soils encountered in our explorations generally consisted of sand with a considerable amount of fines (21 to 25 percent) and variable gravel content. As the fines content increases, the soil becomes increasingly sensitive to small changes in moisture content. Soils containing more than about 5 percent fines cannot be consistently compacted to the appropriate levels when the moisture content is more than approximately 2 percent above or below the optimum moisture content (per ASTM D1557). Optimum moisture content is the moisture content which results in the greatest compacted dry density with a specified compactive effort. Laboratory testing of soil samples from the upper 5 feet of existing site grades indicate moisture contents ranging from about 6 to 14 percent with an average of about 9 percent. Testing of soils samples below 5 feet indicate moisture contents ranging from about 5 to 8 percent with an average of about 7 percent. We estimate optimum moisture content of site soils ranges from about 6 to 10 percent. As such, it appears in situ moisture contents are near to somewhat above optimum moisture content for compaction. Provided that earthwork occurs during the driest summer months, it appears site soils will be suitable for reuse as structural fill. However,some selective moisture conditioning such as windrowing and drying may be required. During wet weather, site soils will quickly become too wet for reuse as 7