2621 ROCKEFELLER AVE Geotech Report 2022-10-25

1 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 may occur if the subgrade is wet and cannot be dried. If the subgrade deteriorates during proofrolling or compaction, it may become necessary to modify the proofrolling or compaction criteria or methods. Structural Fill JAll fill that will support floor slabs,pavement areas or foundations, or be placed against retaining walls or in utility trenches 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. �! Materials. Materials placed within building areas are classified as structural fill for the purpose of this report. Structural fill material quality varies depending upon its use as described below: 1 1. Structural fill placed to backfill utility trenches and to support building and floor slabs may consist of on-site glacial till and advanced outwash provided that the soils are conditioned for 1 the required compaction. On-site till soils will be suitable for use as structural fill during dry weather conditions in areas needing 95 percent compaction. If structural fill is placed during wet weather, the structural fill should consist of imported gravel borrow as described in 11 Section 9-03.14(1) of the 2006 Washington State Department of Transportation (WSDOT) Standard Specifications, with the additional restriction that the fines content be limited to no more than 5 percent. If needed, building foundations should also be supported on imported gravel borrow. It is possible to use native glacial till during wet weather for areas requiring only 90 percent compaction provided the earthwork contractor implements good wet weather techniques. I 2. Structural fill placed immediately outside below-grade walls (drainage zone) should consist of washed 3/8 inch to No. 8 pea gravel or conform to Section 9-03.12(4) of the 2006 WSDOT Standard Specifications,as shown on Figure 3. 3. Structural fill placed as crushed surfacing base course below pavements should conform to Section 9-03.9(3)of the 2006 WSDOT Standard Specifications. 4. Structural fill placed as capillary break below slabs should consist of 1 Y2-inch minus clean crushed gravel with negligible sand or silt in conformance with Section 9-03.1(4)C, Grading No. 57 of the 2006 WSDOT Standard Specifications. Reuse of On-site Native Soils. Based on our explorations, most of the soils excavated for this project will be fill or glacial till. The glacial till contains a high percentage of fines and will be sensitive to changes in moisture content and difficult to handle and compact during wet weather. The existing fill is not suitable for reuse as structural fill, especially if containing debris or rubble. The glacial till soils are expected to be suitable for structural fill in areas requiring compaction to at least ' 95 percent of MDD (per ASTM D 1557), 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 sand and gravel with a low fines content to achieve adequate compaction for support of pavement areas, floor slabs and structures for wet weather construction. Imported structural fill consisting of sand and gravel (WSDOT Gravel Borrow) should be planned under all building floor slabs and foundation elements and if construction occurs during wet weather. File No.12547-001-00 Page 5 GEOENGINEERS February 3.2006 —�