20 Civic Center Plaza - Soils Report

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Gri#in Realty SA, Inc.-Geotechnical Investigation Law/Crandall Project 70131-7-0261.0001 July 8,1997 5.0 SOIL CONDITIONS' Fill soils, 2 and 3 feet thick, were found in the two current borings. The fill soils consist of silty sand and are not uniformly well compacted. Deeper fill could occur between borings and especially adjacent to the existing basement walls. The natural soils consist of silly sand, sandy silt, clayey silt, and sand. The silty soils throughout the depth explored are variable in strength and compressibility, ranging from soft and compressible to medium stiff. The upper silty soils will become weaker and more compressible when wet. The sand and silty sand deposits are generally dense. Water was encountered at depths of 65'/1 and 67'6 feet below the existing grade. The corrosion studies indicate that the on-site soils are moderately corrosive to ferrous metals and non-deleterious to Portland cement concrete. The report of corrosion studies presented in Appendix B should be referred to for a discussion of the corrosion potential of the soils, and for potential mitigation measures. 6.0 RECOMMENDATIONS The existing fill soils are not considered suitable for support of the proposed building foundations and floor slab. The natural silty soils at the site are soft to medium stiff and compressible. Because of settlement considerations, the use of conventional spread footings is not recommended for the support of the proposed building. Accordingly, we recommend that the proposed building be supported on drilled cast-in-place friction piles. If the recommendations on grading are followed, the floor slab can be supported on grade. If a subterranean level is to be constructed, excavation of about 15 feet below the existing grade will be required. The excavation should be either sloped back and/or shored. Care should be taken in excavating adjacent to the existing buildings to the west to avoid undermining the existing footings and slabs. 4