CYCLIC AXIAL LOADING OF DRILLED SHAFTS IN COHESIVE SOIL

Medium- and large-size model drilled shafts up to 174 mm in diameter were constructed in laboratory-prepared cohesive soil deposits. The models were subjected to cyclic axial loading, and the resulting displacements and changes in capacity were determined, while the in-situ stresses at the foundation/soil interface were measured. The applied loading was designed to simulate realistic windstorm events. which are an important source of cyclic loading for foundations. Shaft response was found to depend critically on the type of cyclic loading. One-way uplift, in which the direction of loading is always upward, caused large permanent displacements at high load levels but only small displacements at moderate load levels, while capacity reductions were modest. Two-way loading, in which the direction of loading reverses twice every cycle, caused minimal response at low load levels but a sudden degradation in displacement response at moderate load levels, with an associated substantial reduction in capacity. Critical levels of repeated load, below which shaft response remained stable, were established; and the mechanisms responsible for capacity reduction are discussed.