Shear strength of drilled shafts with minor flaws

Experimental and computational studies were performed to assess the reduction in shear capacity of drilled shafts having minor flaws. The considered flaws included voids or soil inclusions occupying 15% of the cross-sectional area of the shaft and mis-positioned reinforcing steel cage. Such flaws are often produced in construction but either go undetected by NDE or appear as uncertain anomalies on NDE records. Limited resources and data did not permit performing a probabilistic analysis to determine appropriate reliability-based resistance factors. However, the selected tests and deterministic parametric studies were sufficient to develop a general understanding of the phenomena involved and to suggest provisional values of resistance factors to account for the effect on shearing resistance of unknown construction flaws. The results showed that presence of minor undetectable flaws in drilled shafts can decrease the shear strength of the shafts by about 5% if the void flaw is within the concrete cover. However, larger reductions in shear strength can be expected if multiple flaws exist in shafts and if corrosion of the lateral steel reinforcement can occur in the vicinity of a void flaw. Hence, it is recommended that a reduction factor equal to 0.90 be used as a multiplier to the strength reduction factor to account for undetectable voids. This factor needs to be reduced to 0.75 if corrosion of the reinforcement is expected to happen during the life of the foundation.