Reliability Analysis of Extrapolated Ultimate Load of Drilled Shafts Embedded in Weathered Rock

In general, a drilled shaft embedded in weathered rock has a large load bearing capacity. Therefore, most of the load tests are performed only up to the load level that confirms the design load capacity of the pile, and stopped much before the ultimate load of the pile is reached. If a reliable ultimate load value can be extracted from the non-failed load test data, it will be possible to greatly improve the economical and design efficiencies of a pile. The main purpose of this study is to propose a method for judging the reliability of the ultimate loads of piles obtained from extrapolated load test data. To this aim, ten static load test data were obtained from testing of piles to their failures from three different field sites. For each load-displacement curve, loading was assumed to be known, as being 25%, 50%, 60%, 70%, 80%, and 90% of the actual pile bearing capacity. The limited known data were then extrapolated using the hyperbolic function, and the ultimate capacity of a pile was re-determined by the Davisson method (1972). Statistical analysis of the reliability of the re-evaluated ultimate loads was carried out. The results showed that if the ratio of the maximum-available displacement to the predicted displacement at the extrapolated ultimate load exceeded 0.6, the ultimate load may be regarded as reliable, having less than a conservative 20% error on average. The applicability of the proposed method for reliability evaluation was also verified against static load test data of driven piles.