Long-term response evolution of monopod suction caissons in sand for offshore wind applications

Cyclic loading features in many applications. Questions important for design include: Does the monotonic capacity increase or decrease following cyclic loading? How does foundation rotation, stiffness and damping evolve? This is investigated here for suction caissons in sand, looking to applications as foundations for offshore wind turbines where changing stiffness, capacity and accumulated rotation can be critical, and soil damping is being looked at more closely. The problem is investigated experimentally through a series of single gravity monopod caisson tests in saturated sand subjected to unidirectional or multidirectional cyclic loading with between 360,000 and 10⁶ cycles applied in each test. Results from the unidirectional tests are consistent with previous experimental studies, whilst also demonstrating the expected changes in damping ratio during cyclic loading for a monopod caisson in sand. The multidirectional tests reveal more significant and potentially important findings, particularly on the very significant increases in unloading stiffness and damping ratio associated with load direction changes.