Full-scale coupled torsion and lateral response of mast arm drilled shaft foundations

Abstract

Drilled shaft foundations supporting mast arm assemblies are subjected to significant torsion and lateral load during severe wind loading (e.g., hurricane). Past centrifuge studies in granular soils suggest that the design of such foundations should be performed for a coupled load case, since the lateral resistance is considerably reduced by the concurrent application of torsion. However, current design practice still considers lateral load and torsion independently due to the lack of field verification of centrifuge results. This paper reports on a full-scale test program to investigate the coupled load behavior of drilled shafts. A novel load test setup (with a heavy-duty mast arm assembly) and instrumentation were used for the combined torsion and lateral loading (e.g., wind loading). The study revealed a significant reduction in lateral resistance due to the influence of torque as observed from previous centrifuge studies. Torsional resistance was reduced (approximately 20%) by the impact of lateral load when compared with the anticipated torsional resistance based on unit skin friction values, derived from the axial load test results (i.e., no influence of lateral load). A comparison of measured torsional resistance during the combined loading with the predicted values using different approaches was also made. O’Neill and Hassan’s beta (β) method (sand) and alpha method (clay) are found to predict the torsional resistance reasonably well (±10%), while all other methods based on the standard penetration test and cone penetration test considered in the study overpredicted or underpredicted the resistance.