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Floor Slabs <br />We have assumed that some of the proposed building structures may be constructed with concrete slab-on- <br />grade floor systems. The on-site soils and bedrock materials, with the exception of the existing fill <br />materials, are suitable to support slab-on-grade construction. However, floor slabs present a very difficult <br />problem where swelling materials aze present neaz floor slab elevation because sufficient dead load cannot <br />be imposed on them to resist the uplift pressure generated when the materials are wetted and expand. <br />Based on the moisture-volume change characteristics of the clays and claystone-shale bedrock encountered <br />at this site, we recommend that structural floor systems over well-ventilated crawlspaces be used in the <br />proposed buildings. If the client elects to construct concrete slab-on-grade floor systems, we recommend <br />that the following special design and construction precautions be followed so that the amount of movement <br />in the floor slabs can be reduced, if the clays become wetted. <br />The following measures should be taken to reduce the damage, which could result from movement should <br />the underslab clays or bedrock materials be subjected to moisture changes. <br />1) The floor slabs should be sepazated from all beazing walls, columns and their foundation supports <br />with a positive slip joint. We recommend the use of yz-inch thick cellotex or impregnated felt. <br />2) Interior non-bearing partition walls resting on the floor slabs should be provided with a slip joint, <br />preferably at the bottom, so that in the event the floor slab moves, this movement is not <br />transmitted to the upper structure. This detail is also important for wallboazd and doorframes and <br />is shown in Figure #11. <br />3) A minimum 12-inch gravel layer should be provided beneath all floor slabs to act as a capillary <br />break and to help distribute pressures. Prior to placing the gravel, the excavation should be <br />shaped so that if water does get under the slab, i[ will flow to the low point of the excavation. In <br />addition, all of the existing fill materials should be removed prior to placement of the underslab <br />gravels or new structural fill materials. The installation of interior drains may also be advisable to <br />permit the rapid removal of water from under the slab azeas. <br />4) The floor slabs should be provided with control joints placed a maximum of 12 feet on center in <br />each direction to help control shrinkage cracking. The location of the joints should be carefully <br />checked to assure that the natural, unavoidable cracking will be controlled. The depth of [he <br />control joints should be a minimum of i/s the thickness of the slab. <br />5) The underslab soils should be kept as close as possible to their in-situ moisture content. Excessive <br />wetting or drying of these soils prior to placement of the floor slab could result in differential <br />movement after the slabs aze constructed. <br />6) It has been our experience that [he risk of floor slab movement can be reduced by removing a[ <br />least 3 feet of the expansive materials and replacing [hem with a well compacted, non-expansive <br />fill. If this is done or if fills are required to bring the underslab soils to the desired grade, the fill <br />lob Number: 07-7442 NWCC, Inc. Page6 <br />