Borehole Roughness: A Key Parameter in Predicting Rock Socket Side Resistance

Predicting the performance of rock socketed bored piles has traditionally been a difficult task. The results given by conventional methods of predicting shaft resistance, based on empirical correlations with intact rock strength, have proven to be poor when used outside the specific data set for which they were developed. Researchers have long recognised that socket roughness is an important parameter. However, attempts to incorporate roughness parameters as an integral part of design have been unsuccessful. Recent advances at Monash University have led to a wholly theoretical design method which incorporates roughness as a key parameter. The Monash method is broadly applicable, but its use requires a prediction of socket roughness to be made. At present, rock socket roughness is poorly understood. A better understanding of socket roughness production across a variety of drilling conditions is crucial for the full potential of this new approach to be realised. Using the Monash method, historical load test data has been re-analysed to evaluate the contribution of socket roughness to pile capacity. The results of the study suggest upper and lower bounds on socket roughness, and confirm that roughness variations are particularly significant in low to medium strength rock. A borehole profiler is being developed which will be used in an imminent field investigation of socket roughness. The aim of this investigation will be to develop socket roughness guidelines for use in design.