||Because the offshore wind turbine pile foundations such as that jacket and tripod are subjected to the WTG loading and long-term wind and wave loading transmitted from upper structure to lower substructure, the generated cyclic axial compression and tension, result in pile groups of difference settlement, affecting the stability of operation of the wind turbine. Therefore to investigate different pile behavior under cyclic axial load, is the destination of this study.|
In this study, air pluviation method of sample was adopted to prepare a dense homogeneous dry sand specimen with a relative density of 60%, and 1-g model pile load test was conducted. When conducting axial cyclic load test, the loading which approach 1/3 of ultimate static in the axial load capacity was applied to simulate the normal conditions under safety factor of 3.In the static load tests it was, found that near bottom of the pile the sand relative density significantly affects the pile bearing capacity, the ultimate static load test total was totally 10 groups, with recording the relative density of sand layers in order to observe the variation of pile bearing capacity, and to ensure the consistent in preparation of sand samples in the future. Six unidirectional cyclic axial load test were conducted, in which three tests were under cyclic axial compression, and three tests under cyclic axial tension. The magnitude of loading were 1/3Pu, 1/6Pu and 1/12Pu, and the number of cycles were 10000, 10000 and 20000 respectively.
The results represent that the larger number of axial load cycles apply, the smaller equivalent stiffness of pile head was obtained as well as pile head displacement increases with an increasing number of cycles. Under the same condition of axial load cycles, the accumulated residual displacement of pile foundation under cyclic axial compression was larger than cyclic axial tension case. When increasing number of cycles, the pile shaft friction decreases. But the bearing capacity of pile tip increases.
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