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Project No. JB-569 20 June 2006 Page 11 installing a subdrain system is not possible, the retaining walls should be designed for full hydrostatic loading. We recommend that the foundation walls be waterproofed. The retaining wall drainage blanket should extend the full height of the wall to within 12 inches of the top of backfill. The drain should consist of a minimum thickness of 12 inches of drain rock surrounded by an approved non-woven geotextile. Class 2 Permeable material is an acceptable equivalent for drain rock and geotextile. A 4-inch perforated PVC pipe should be placed at the base of the drain, bedded in a minimum of 2 inches of drain rock, the pipe should be installed with the perforations facing down and sloped to drain at a minimum grade of 1 percent. Outside of the drainage system, the drain should be extended with solid pipe to an acceptable discharge point. 6.5 Concrete Slabs-on-Grade If the earthwork recommendations are complied with, concrete slabs-on-grade may be protected from unwanted moisture vapor by an underlainment of a 4-inch thick capillary break of Class 2 drain rock, clean '/z by 3/<-inch crushed drain rock, or pea gravel. Class 2 base rock may not be used as the capillary break. If the potential for a damp slab is undesirable or if moisture sensitive floor coverings are used, we recommend that a membrane of 10-mil minimum thickness be placed on the drain rock and overlain by a minimum of 2 inches of clean sand to assist in the proper curing of the slab. The select material or sand should be moistened but not saturated prior to placement of concrete. It should be noted that the intention of the membrane is to limit moisture transmission through the slab, not to eliminate moisture transmission through the slab. A membrane will not eliminate moisture transmission which can cause mold growth. The membrane must be constructed properly to effectively limit moisture transmission. Proper construction includes sealing the perimeter of the membrane as well as all seams and penetrations. If greater resistance to moisture transmission is desired, we recommend sealing the slab with an approved concrete sealant. We also recommend reducing the water-cement ratio of the concrete mix design for slabs as low as possible to help further reduce the potential for moisture passing through the slab. The structural engineer should determine the final requirements of the concrete mix design. We recommend concrete slab-on-grade floors and exterior flatwork be a minimum thickness of 4 inches. The actual slab thickness must be determined by the project structural engineer. ~Korbmacher Engineering, Inc.