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

Data from 187 loading tests on rock-socketed shafts are used to develop a new relationship between the ultimate shaft resistance f(su), and unconfined compressive strength a c of intact rock. The adhesion factor alpha(q) of the shaft resistance for different types of rock and the relationship between alpha(q) 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 f(su), and the unconfined compressive strength a sigma(cm), 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 sigma(c) are found to be more accurate than those obtained using existing methods, and when the rock quality designation is available, results obtained using sigma(cm) are closer to measured values than those based on sigma(c). The value of alpha(q) of the discussed rocks decreases with sigma(c). Mudstone and shale have relatively high values of alpha(q) with more dispersion, whereas alpha(q) values for gneiss, basalt and granite are small overall with less dispersion.