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kn KLEINFELID ER 4. Lateral Pressures Resistance to lateral forces can be provided either by friction between the base of the foundations and the underlying soil or by passive pressure of the sides of the foundations against the adjacent soil. The recommended passive soil pressure for existing clay soils is an equivalent fluid pressure of 350 psf per foot of depth. The recommended coefficient of friction is 0.40... If passive pressure and friction will be used to resist lateral forces, one of the values should be reduced by 50 percent. 5. Concrete Slab-on-Grade Floors In our opinion, the preferred method to reduce the risk of differential movement because of heaving of clay soils beneath concrete slab-on-grade floors (floor slabs) is to support the floor slabs on a minimum of 12 inches of nonexpansive Engineered Fill over 12 inches of moisture- conditioned existing soils. If additional risk of differential movement is acceptable, as an option the nonexpansive Engineered Fill can be eliminated, and the existing soils beneath floor slabs can be moisture conditioned to a minimum depth of 18 inches. The recommended degree of compaction for nonexpansive Engineered Fill beneath floor slabs, if used, is a minimum of 90 percent of the maximum dry density as determined by the ASTM D-1557 test method, at a moisture content near optimum. Moisture conditioning should consist of increasing the moisture content of the existing clay soils to at least 3 percent over optimum. A third option to reduce the risk of damage to floor slabs because of heaving of clay soils is to use post-tensioned floor slabs. Our firm can provide additional information regarding the use of post-tensioned floor slabs if this option is to be considered. Our recommendations for the preparation of the existing soils and the placement of Engineered Fill are contained in the "Site Preparation and Grading"subsection of this report. The soils beneath exterior concrete flatwork should be moisture conditioned in the same manner as for floor slabs. We suggest that the moisture-conditioned soils extend at least 2 feet beyond the flatwork outlines to help ensure the moisture conditioning extends to the recommended depth beneath the flatwork. Positive drainage away from the flatwork should also be provided to prevent ponding adjacent to the flatwork. We recommend that a minimum 4-inch thick layer of gravel or coarse-grained sand be placed beneath floor slabs in order to act as a capillary moisture break. If nonexpansive Engineered Fill is placed beneath floor slabs and the fill consists of relatively-clean sand, the gravel or sand layer can be eliminated as the Engineered Fill will act as a capillary moisture break. 20-4280-0I.G01/2019RI50 Page 5 of 12 Copyright 1999,Kleinfelder,Inc. April 14, 1999