Vertical cyclic loading of piled foundations in expansive clays

In the design of onshore wind turbines constructed in swelling clays, the use of grouped-piled foundations is an attractive option. The large lateral forces to which the turbine is subjected, results in significant vertical cyclic loading of the piles. This study presents the results of a centrifuge test, conducted to analyse the response of scaled reinforced-concrete piles subjected to vertical cyclic loading. This study highlights the difficulties of performing cyclic testing on expansive clays in the centrifuge. Emphasis is placed on various facets of the model design which include the preparation of instrumented concrete piles cast “in situ”, incorporating the use of a mix design and steel reinforcement, scaled for purpose. A description is then provided of the system used to perform load-controlled cyclic loading, through the use of a low-cost, open-source control system based on the use of a Raspberry Pi microcomputer. Results indicate that cyclic testing conducted in swelling clays is problematic since the magnitude of swell (and of swell-induced softening) is constantly changing throughout the duration of the test. Measurements from the instrumented concrete piles indicate that meaningful results can be obtained if strain gauges are attached to the reinforcement cage, rather than to the outside of the pile as is the more conventional approach. Finally, the low-cost, load and displacement control system developed at the University of Pretoria (termed Fly-by-Pi) is shown to provide almost perfect load control despite soil stiffness continuously changing due to cyclic loading and swell-induced softening.