Civil Engineering Association

Development of Supplemental Resistance Method for the Design of Drilled Shaft Rock Sockets

Rock Socketed Drilled Shafts have been used on a variety of projects, especially bridges, in areas with bedrock close to the surface. The large loads that these foundation structures can resist make them more practical than alternatives, such as pile groups, in certain situations. However, the current conservative design practices are based on the performance of Rock Socketed Drilled Shafts in soft rock formations. The current design practice for axial capacity often neglects one of the two resisting forces, usually end bearing. In areas, such as New England, with good quality hard rock, Rock Socketed Drilled Shafts have been found to have ultimate axial capacities 7-25 times the predicted value. The goal of this research is to develop a design method that utilizes the full potential of Rock Socketed Drilled Shafts in hard rock. A finite element model using constitutive relationships and surface interactions was created to replicate Rock Socketed Drilled Shafts in hard rock. The model was calibrated by duplicating results from five Osterberg load tests on shafts in hard rock. For the purpose of this research hard rock is classified as having an unconfined strength greater than 30 MPa. After calibrating the model, the model was used to show performance of shafts of various sizes founded in rock of two different qualities. The performance results were used to develop a design method for Rock Socketed Drilled Shafts based on service limit criteria. This research revealed that portions of both resistance forces, end bearing and side shear, can be used together in the design of Rock Socketed Drilled Shafts if service limit state criteria are considered. This method applies to well cleaned sockets and primarily hard rock. This method can be used with equations for nominal resistance as well as Osterberg tests.