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Aucust 28, 1991 BALBI$CHANG ASSOCIATES <br /> File No. 1525-69 <br /> The recommended pier capacities were determined based on soil stresses only. <br /> Individual piers should be structurally designed to support both horizontal and vertical <br /> loads. All drilled piers should be connected by pier caps or grade beams which will <br /> serve to transmit loads to the piers and to provide lateral restraint. Pier caps and <br /> grade beams should be properly reinforced to transmit both vertical and horizontal <br /> loads. <br /> E. Concrete Slab-on-Grade Floor <br /> For concrete slab-on-grade construction, the subgrade soils should provide a relatively good <br /> foundation after proper site preparation. Structural details of the concrete slab-on-grade floor, <br /> such as the floor thickness and reinforcement, location and pattern of construction joints, <br /> should be determined by the architect / engineer of the project. <br /> Concrete slab-on-grade should be constructed on at least four inches of clean gravel br <br /> crushed rock for uniform supportMaterial used to support the slabs should be compacted <br /> to at least 90 percent of maximum dry density and at or near optimum moisture content as <br /> determined by ASTM D1557-78 standard test method. We recommend that the slab <br /> supporting material be placed as soon as the subgrade soil is compacted to prevent drying of <br /> the soil. If the sub-rade soil is allowed to dry out prior to the construction of concrete slabs- <br /> on-grade, the subgrade soils should be pre-saturated by spraying or flooding and the process <br /> verified by the geotechnical engineer. <br /> For slab-on-grade. with moisture sensitive surfacing, such as carpet or vinyl tile, we <br /> recommend the slab supporting material be consisted of drain rocks. Also recommended is <br /> an impermeable membrane placed over the rock to prevent migration of moisture vapor <br /> through the concrete"slabs.- In order-to-promote a more uniform curing of the slab and to <br /> provide protection of the vapor membrane, itis advisable that one to two inches of moist fine <br /> sand be placed on top of the membrane prior to placement of the slab concrete. <br /> F. Lateral Earth Pressure <br /> Lateral earth pressures will be imposed on all below-ground structures, including walls and <br /> foundations. The recommended lateral earth pressure values for horizontal backfill are <br /> presented in Table on next page as equivalent fluid densities. <br /> f <br /> Page 12 of 17 <br />