A new method for predicting the ultimate shaft resistance of rock-socketed drilled shafts

Abstract

Data from 187 loading tests on rock-socketed shafts are used to develop a new relationship between the ultimate shaft resistance fsu and unconfined compressive strength σc of intact rock. The adhesion factor αq of the shaft resistance for different types of rock and the relationship between αq and the embedment ratio are discussed. Furthermore, because the shafts are socketed in the natural rock mass, as well as the aforementioned correlation with intact rock, three new relationships between fsu and the unconfined compressive strength σcm of the rock mass are proposed by considering the rock quality designation, which is related to discontinuity of the rock mass. Statistical methods are used to evaluate and verify the existing methods and newly proposed relationships. Results obtained using the new relationship based on σc are found to be more accurate than those obtained using existing methods, and when the rock quality designation is available, results obtained using σcm are closer to measured values than those based on σc. The value of αq of the discussed rocks decreases with σc. Mudstone and shale have relatively high values of αq with more dispersion, whereas αq values for gneiss, basalt and granite are small overall with less dispersion.