Prediction of Side Resistance in Poor Quality Rock: RQD vs. GSI

There are many factors that influence the side resistance prediction of drilled shafts in poor quality rock. As part of a bridge replacement project in Oregon, we evaluated the performance of various design methodologies for predicting the side resistance in poor quality rock. In particular, we looked at the side resistance predicted in poor quality rock from the rock mass strength developed using Rock Quality Designation (RQD) as well as the Geological Strength Index (GSI) developed by Hoek and Brown. We predicted the capacities of drilled shafts at several bents along the bridge alignment and compared the estimated bedrock socket required to resist the structural loading. This paper summarizes the determination of rock mass quality using the RQD and GSI methods, describes the prediction methods used for determining the side resistance in poor quality rock, presents the comparison of RQD versus GSI predicted design values, and evaluates the performance of the predictive methods and discusses the difference in cost associated with the drilled shaft lengths predicted by each method. Based on the results of this study, using the RQD methods without any specific laboratory strength or deformation test results may under predict the ultimate shaft resistance within poor quality rock, i.e., RQD < 60 percent.