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300 psf for each additional foot of embedment and 100 psf for each additional foot of <br />foundation width to a maximum value of 4,000 psf. An effective plasticity index of 25, for <br />the on -site soils, may be used in the foundation design. <br />Lateral forces on footings may be resisted by passive earth resistance and friction at the <br />bottom of the footing. Foundations may be designed for a coefficient of friction of 0.35, and <br />a passive earth pressure of 250 lb/ft2/ft. The passive earth pressure incorporates a factor of <br />safety of about 1.5. <br />All footing excavations should be cut square and level and should be free of sloughed <br />materials and trash. Subgrade soils should be pre -moistened for the assumed low expansion <br />potential (to be confirmed at the end of grading). <br />The subgrade should be moisture -conditioned and proof -rolled just prior to construction to <br />provide a firm, relatively unyielding surface, especially if the surface has been loosened by the <br />passage of construction traffic. <br />Subgrade soils should be pre -saturated to 1.3 times optimum moisture content to a depth of <br />24 inches for a high expansion potential. The minimum thickness of the floor slabs should be <br />at least 5 inches, and joints should be provided per usual practice. <br />The underslab moisture retarder (i.e. an equivalent capillary break method) should consist of <br />a 15-mil thick polyolefin (or equivalent) in conformance with ASTM E 1745 Class A <br />material underlain by a minimum 1-inch of sand, as needed. The sand layer requirements <br />above the vapor barrier are the purview of the foundation engineer/structural engineer and <br />should be provided in accordance with ACI Publication 302 "Guide for Concrete Floor and <br />Slab Construction". These recommendations must be confirmed (and/or altered) by the <br />foundation engineer, based upon the performance expectations of the foundation. Ultimately, <br />the design of the moisture retarder system and recommendations for concrete placement and <br />curing are the purview of the foundation engineer, in consideration of the project <br />requirements provided by the architect and developer. <br />4.Z4 Post -Tension Foundations <br />Based on the site geotechnical conditions and provided the remedial recommendations <br />provided herein are implemented, the site may be considered suitable for the support of the <br />anticipated structures using a post -tensioned slab -on -grade foundation system for high <br />expansion potential (91 to 130 Expansion Index). The following section summaries our <br />recommendations for the foundation system. The post -tension parameters provided in Table 3 <br />are based on the expansion potential only, and do not incorporate any increase for seismically <br />induced settlements. <br />Project i y 66n,61 18 — 834 1 3 , 2021 <br />