3A Public Hearing 2006 1204 - Laserfiche WebLink

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GEOTECNIA Project Number: 060805 2888 Darius Way, San Leandro October 27,2006 Page 11 retaining wall footing. This may require excavations to replace the clay soils with select fill in the active zone. The long-term lateral earth pressures for the condition where the backfill within the active zone consists of the in-situ very highly expansive clays would be significantly higher (2 to 3 times) than the values given below for select backfill due to the creeping nature ofthe clays. Some of the excavated clays may be re-used as backfill for the upper 12 inches (above the select backfill) to reduce the potential for infiltration of surface water where concrete is not provided. Yielding retaining walls which are free to rotate at the top at least 0.1 percent of the wall height should be designed to resist static "active" lateral earth pressures equivalent to those exerted by a fluid weighing 40 pef where the backfill is flatter than 4: 1, and 50 pcf for backfill at a 2: 1 slope. Retaining walls restrained from movement at the top should be designed to resist "at-rest" equivalent fluid pressures equivalent to those exerted by a fluid weighing 55 pcf where the backfill is flatter than 4: 1, and 75 peffor backfill at a 2:1 slope. For intermediate backfill slopes, the lateral equivalent fluid weights may be obtained by interpolating between the above values. Backfill slopes steeper than 2: 1 are not recommended. The actual condition of the wall may range between active and at-rest. Where the wall is more rigid, such as near comers or buttresses, the wall may approach at-rest conditions. Elsewhere, the wall may approach active conditions. The designer should use the most appropriate condition for each section ofthe wall, or one single value between the values for active and at- rest depending on how much of the wall is closer to active or at-rest conditions. In addition to lateral earth pressures, retaining walls must be designed to resist horizontal pressures that may be generated by surcharge loads applied at or near the ground surface. Where an imaginary 2H: 1 V (30-degree) plane projected downward from the outennost edge of a surcharge load or foundation intersects a retaining wall, that portion of the wall below the intersection should be designed for an additional horizontal thrust from a unifonn pressure equivalent to one-third and one-half of the maximum anticipated surcharge load for active and at-rest conditions, respectively. For different types of surcharge loads, we can provide the appropriate lateral surcharge pressures on retaining walls once the geometry is defined. Wall backfill should be spread in level lifts not exceeding 8 inches in thickness. Each lift should be brought to at least optimum moisture content and compacted to not less than 90 percent relative compaction, per ASTM 01557. Retaining walls may yield slightly during backfilling. Therefore, walls should be properly braced during the backfilling operations. Where migration of moisture through retaining walls would be detrimental or undesirable, retaining walls should be waterproofed as specified by the project architect or structural engmeer. Retaining walls should be supported on foundations designed in accordance with the recommendations presented above. A minimum factor of safety of 1.5 against overturning and sliding should be used in the design of retaining walls.