Response of Vertically Loaded Centrifuge Suction Caisson Models in Soft Clay

Abstract

Abstract Tidal current turbines are designed to interact with flowing water. This interaction results in a significant horizontal load and corresponding overturning moment that can be resisted through a compression and tension mechanism on opposing foundation legs if the foundations are configured as a multipod. This study investigates the response of suction caisson foundations in soft clay with an aspect ratio of one to this compression and tension loading by means of centrifuge testing. Suction caisson models with an aspect ratio of one were fabricated out of T6-6160 aluminum at a scale of 1:90 and tested in kaolin testbeds. The kaolin testbeds were consolidated from slurry under an effective stress matching the effective stress at the mid depth of the testbed when spun at 90-g. A series of monotonic vertical load tests were conducted at 90-g on a beam centrifuge using a vertical stepper motor. The series of load tests comprised of undrained monotonic loading phases conducted both prior and post long period cyclic load phases corresponding to the longer period components of tidal current velocity fluctuations. Geotechnical centrifuge testing allows for the investigation of suction caisson response under loading conditions applicable to tidal current turbine structures under realistic stress and stress distritbutions by artificially increasing the gravity. The response of the suction caisson under vertical monotonic loading without any prior cycling provided a baseline case for determination of the effects of long period cycling (i.e. semi-diurnal tidal current fluctuations) on future monotonic undrained loading. The load tests showed that the effects of long period cyclic loading on the degradation of the undrained monotonic capacity were dependent on the amplitude of the cyclic loading. The cyclic strain thresholds that caused a significant decrease in the post cyclic monotonic capacity were not the same for compression and uplift loading mechanisms. This indicates that serviceability constraints will play a critical role in tidal current turbine foundation design. Ensuring reliable foundation design will allow for cost efficiency during construction and over the lifetime of the tidal current turbine structure. As tidal current turbine technology, at this time, is primarily in the prototyping stage, it is key to understand the response of the foundations to these loading conditions in order to increase the cost efficiency of the overall structure.