3A Public Hearing 2006 1204 - Laserfiche WebLink

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GEOTECNIA Project Number: 060805 2888 Darius Way, San Leandro October 27,2006 Page 8 by the geotechn~~~l 'engineer during pier installation. The foundation contractor should provide a unit cost for piers that extend deeper (additional charge) or are shallower (cost deduction) than the assumed depths. During excavations for the piers, the contractor may encounter difficult excavation conditions locally in the bedrock, especially in the immediate vicinity of Boring B. 7, where practical refusal conditions were encountered at a depth of only about 4 feet. The piers should be spaced at least three pier diameters center to center, and the above minimum recommended pier depth should be checked against the required depths to resi~taxialloads. The required pier depth should be the longest of the above-recommended minimum penetration or the depth required to resist axial loads. The designer should assume that the upper 3 feet of the soil or highly weathered bedrock layer around the piers would exert lateral creep loads, applied over two pier diameters. The creep loads may be estimated using an equivalent fluid weight ofl20 pounds per cubic foot (pct). Lateral creep pressures should also be applied against underground portions of grade bearns, where applicable. Piers should be designed for maximum allowable skin friction values of 500 pounds per square foot (pst), for combined dead plus sustained live load. This value, which may be used for both downward and uplift loads and includes a safety factor of2.0, may be increased by one-third for totall()~ds_, including the effects of seismic or wind forces. Skin friction should be disregarded in theu.oDe~ 8: feet of the Diers due to the expansive soil conditions as discussed below. and end bearing should be neglected. The weight of foundation concrete extending below grade may be disregarded for downward loads, but should be added to the skin friction capacity for uplift loads. The soils around the upper 4 feet of the piers may either act in uplift or downdrag during either the swelling or shrinking phases of the expansive soil cycle, respectively. We estimate that the potential uplift/downdrag force in the upper 4 feet of the piers would be about 18 kips per pier for an 18-inch-diarneter pier. This value does not need to be added to the structural load on the pier since we assumed that the portion of the pier between depths of 4 and 8 feet would resist potential downdrag in the upper 6 feet of the pier. 'The pier design should consider both the uplift and downdrag conditions, and the amount of reinforcement in the upper portions of the piers should be checked for the condition when the upper 4 feet of the piers is subject to uplift from the surrounding expansive soils so that the piers do not fail in tension. The_pl~~4 improv~ments s1.!pQ9rted on drilled piers are anticipated to settle less than % inch. Differential settlements are anticipated to be less than ~ inch over a 20-foot span. To reduce uplift pressures from the swelling expansive soils on grade beams, we recommend that a minimum 3-inch void be cast between the bottoms of the grade bearns/existing footings and the subgrade using a void-forming product such as SureVoid, Econ-O-Void, or equivalent. Otherwise, the piers would need to be deeper or wider to resist anticipated uplift pressures on the grade beams that could be on the order of2,500 to 3,400 psffor a PI of38. If the grade beams