This paper presents the results of a series of laboratory tests performed on model piles in dense sand to investigate the axial response and tensile capacity. The test schedule was designed to impose varying cyclic loads followed by static tensile tests to measure the improvement or degradation in pile capacity. The degree of degradation was calculated by using a simplified axial degradation model, a formulation that encompasses average and cyclic loads along with a prediction of the number of cycle to failure of the pile. The results indicated that few intensity cyclic loading rapidly leads to shakedown behaviour. The model predicted no capacity degradation during low-intensity cycling at low load levels with minimal degradation in the high-intensity cyclic loading tests. Measurements of tensile capacity after high intensity compressive cycles, which showed varying amounts of degradation, indicate that very low cyclic load levels need careful consideration in any degradation formulation. Load levels that did not cross a threshold, dependent on the load application history, showed little or no degradation. This threshold will be higher if low load level cycling has improved capacity and lower if degradation has occurred in previous loading cycles.
A large body of literature is available on the subject of loading of piles used for offshore structures of different types, such as oil and gas platforms. In the case of sand, cyclic loading is expected ultimately to reduce the axial capacity of the pile itself. As noted by Abdel-Rahman and Achmus (2011), there is a need for a method of calculating axial pile capacity degradation with regard to load magnitude and number of load cycles. This is important in relation to the improvement of economic design methods and the number of wind farms proposed to be constructed in the coming years.
