| dc.description.abstract | Drilled shaft foundations are commonly used to support traffic signal mast arm structures because of their ability to resist large lateral, overturning, and torsional loads. Although current design procedures generally evaluate lateral and torsional loading independently, field observations and previous research indicate that these loading modes may interact, particularly under extreme wind events. This study investigates the behavior of drilled shaft foundations subjected to pure torsional, pure lateral, and combined lateral-torsional loading through a comprehensive program of full-scale testing and numerical modeling.
Seven full-scale load tests were performed on two 36-in-diameter drilled shafts constructed within a controlled geotechnical chamber at Auburn University. The testing program included pure torsional loading, pure lateral loading, combined lateral-torsional loading, and sequential loading cases in two distinct phases. In Phase I, shafts were constructed in place through excavation and concrete placement. In Phase II, the shafts were exhumed, reinstalled, and surrounded by recompacted sand to evaluate the influence of construction method and shaft-soil interface conditions. Load, displacement, rotation, and strain measurements were collected throughout the testing program.
The experimental results showed that torsional and lateral behavior are strongly influenced by loading sequence and shaft installation conditions. Pure torsional tests in both phases yielded similar ultimate torsional capacities of approximately 145-148 kip-ft; however, the recompacted Phase II configuration exhibited substantially higher initial torsional stiffness. Combined lateral-torsional loading reduced the mobilized torsional stiffness and altered the progression of resistance mobilization, requiring larger rotations to achieve the same torque levels observed under pure torsion. Sequential loading tests indicated that prior torsional loading reduced lateral stiffness and delayed the mobilization of resistance, although differences in ultimate lateral capacity were less pronounced.
Numerical models were developed using FB-MultiPier and back-calculated against the measured load-displacement and torque-rotation responses. The best-fit estimation demonstrated that stiffness-related parameters, particularly shear modulus and horizontal subgrade reaction, were highly sensitive to construction effects, loading history, and stress path. Strength-related parameters, including unit weight, friction angle, and torsional shear resistance, exhibited comparatively smaller adjustments. The results demonstrate that conventional uncoupled design approaches do not adequately represent the behavior of drilled shafts under realistic loading conditions and highlight the need to consider construction effects, shaft-soil interface conditions, and load interactions in foundation design for mast arm structures. | en_US |
