Prestressing effect evaluation for a grouted shaft: a case study

Pressure-grouting the shaft tip has been proven to be an effective method to improve the performance of drilled shafts. In this paper, the prestressing effect during grout injection is introduced and studied. The mechanism whereby preloading affects the end bearing and side friction is illustrated and detailed. The analysis indicates that the prestressing effect will lead to the improvement of side friction due to the development of negative side friction. The tip resistance of the grouted shaft can be mobilised at a much smaller displacement, owing to the prestressing effect. A field case study is reported to further demonstrate the influence of the preloading. The prestressing effect on a post-grouted drilled shaft bearing on weathered sandstone is evaluated. The grouting pressures during and after grout injection are measured to study the variation of the locked pressures. Moreover, the measured side friction and tip resistance provide further validation of the proposed mechanism.