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Mr. Gary Gleason, P.E. 11 January 17, 2001 <br /> GME Projeet No. 9256 <br /> Floor Slab Subgrade <br /> We recommend that backfill around the new foundations and in underslab <br /> utility trenches be select imported granular soil . This material should <br /> be placed in loose lifts about 6 inches thick, and compacted with <br /> manually-operated vibratory plate compactors to at least 95� . We <br /> recommend that large towed or self-propelled vibratory compactors not be <br /> used within 4 feet of new foundation walls, to avoid damaging these <br /> walls . <br /> Based on a subgrade prepared with fill placed during mass grading, and <br /> with compacted fill placed in foundation and utility trenches as <br /> described above, the floor slab may be cast on-grade. We recommend that <br /> the Structural Engineer use a modulus of subgrade reaction of 175 pounds <br /> per cubic inch to design the floor slab thickness and reinforcement . <br /> Because of the silty and semi-cohesive nature of some of the subsoils <br /> encountered in our borings, we recommend that a vapor barrier be placed � <br /> under the floor slab. The purpose of a vapor barrier is to reduce �the <br /> potential for the upward migration of water vapor from the soil into and <br /> through the concrete slab. Water vapor migrating upward through the slab <br /> can damage floor. coverings such as tile, carpeting, or wood. One method <br /> for installing a vapor barrier is given in Part 2, Section 302, of the <br /> Manual of Concrete Practice of the American Concrete Institute. However, <br /> OM!CONSULTAMTS.INC. <br />