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• (4) If moisture sensitive floor coverings are used, a minitnum 4-inch layer of free-draining <br />gravel should be placed beneath the slabs. This material should consist of minus 2-inch <br />aggregate with less than 50 % passing the No. 4 sieve and less than 3 % passing the No. <br />200 sieve. As an alternative, the slab could be constructed directly on a 3-inch thick <br />layer of clean sand overlying a vapor barrier such as PVC or polyethylene. <br />(5) To reduce the risk of differential floor slab movement, a minimum 3-foot thick layer of <br />select, non-expansive compacted fill should be placed beneath floor slabs in structures <br />on moderate to high swelling sites. This may require over-excavation of the on-site <br />clayey materials to accommodate the layer. The geotechnical engineer should evaluate <br />the suitability of all proposed fill materials to be used at the site prior to placement. Fill <br />should be placed and compacted according to the criteria presented in the "Site Grading" <br />• section of this report. <br />FOUNDATION AND 12ETAIAIING WALLS <br />Based on our current understanding of the proposed construction, the need for deep <br />below-grade basement walls is not anticipated, but some retaining walls will be needed for grade <br />changes and culverts, wing walls and tunnels will be a part of this project. The design and <br />construction criteria presented below should be observed for foundation walls subject to ]ateral <br />earth pressure. The constrnction details should be considered when preparing constrnction <br />documents. <br />(1) The following tabulates design equivalent fluid pressures to be used on this project. <br />• 26 <br />