2320 CALIFORNIA ST Geotech Report 2025-07-25

1 Once approved, the subgrade areas should be recompacted to a firm and unyielding condition, if possible. The achievable degree of compaction will depend on when construction is performed. If the work is performed during dry weather conditions,we recommend that all subgrade areas be recompacted to at least 95 MDD in accordance with the American Society for Testing and Materials (ASTM) D 1557 test procedure (modified Proctor). If the work is performed during wet weather conditions, it may not be possible to recompact the subgrade to 95 percent of the MDD. In this case, we recommend that the subgrade be compacted to the extent possible without causing undue weaving or pumping of the subgrade soils. Subgrade disturbance or deterioration could occur if the subgrade is wet and cannot be dried. If the subgrade deteriorates during compaction, it may become necessary to modify the compaction criteria or methods. Structural Fill ' All fill, whether on-site or imported supporting the building slab, pavement areas, or in utility trenches should meet the criteria for structural fill presented below. The suitability of soil for use as structural fill depends on its gradation and moisture content. Materials. Materials used to construct the surface parking areas, used under building slabs or to backfill utility trenches or against foundations are classified as structural fill for the purpose of this report. Structural fill material quality varies depending upon its use as described below: At a minimum, structural fill placed to construct parking areas, building slab subgrades, and to backfill utility trenches and against foundations should consist of gravel borrow as described in Section 9-03.14(1) of the 2008 Washington State Department of Transportation (WSDOT) Standard Specifications,with the additional restriction that the fines content be limited to no more than 5 percent. Structural fill placed around footing drains should consist of washed 3/8 inch to No. 8 pea gravel or conform to Section 9-03.12(4)of the 2008 WSDOT Standard Specifications,as shown on Figure 3. Structural fill placed as crushed surfacing base course below pavements should conform to Section 9-03.9(3)of the 2008 WSDOT Standard Specifications. Structural fill placed below foundations should consist of control density fill (CDF),or lean concrete. CDF should have a minimum design strength of 200 psi and otherwise be in accordance with WSDOT Standard Specification 2-09.3(1) E. Reuse of On-site Soils. The near surface on-site soils consist of glacial till below the topsoil and sandy fill. On-site glacial soils are expected to be suitable for structural fill, provided the work is accomplished during the normally dry season (June through September) and that the soil can be properly moisture conditioned. It may be necessary to import Gravel Borrow to achieve adequate compaction during wet weather construction. Imported structural fill consisting of Gravel Borrow should be planned for this project. The contractor should plan to cover and maintain all fill stockpiles with plastic sheeting if it will be used as .structural fill. The reuse of on-site soils is highly dependent on the skill of the contractor and schedule, and we will work with the design team and contractor to maximize the reuse of on-site soils during the wet and dry seasons. Fill Placement and Compaction Criteria. Structural fill should be mechanically compacted to a firm, non-yielding condition. Structural fill should be placed in loose lifts not exceeding 8 to 10 inches in thickness. 111 Page 8