Behaviour of finned piles in sand under lateral loading

Abstract

Reviewing the development of offshore wind farms, large-diameter monopiles have been widely used as foundations for offshore wind structures. Unlike onshore foundations which are mainly used to transmit vertical load into the ground, offshore foundations are usually subjected to large environmental loads from wind, wave and current which could exceed 30% of their gravity load. In order to improve the lateral resistance of monopiles, a finned pile has been proposed. Empirical and Numerical methods were used to simulate pile head lateral load and displacement (P-Y) curves, and the efficiency of fins under static loading has been estimated. Pile soil response along the pile was predicted based on the distribution of deflection, bending moment, shear force and soil resistance. Three-dimensional charts from FEM analysis represent pile and soil responses especially the soil reaction around the fins. To compare the lateral resistances of a monopile and of finned piles with various fin dimensions, 1 G model tests were carried out. Tests were conducted in a 1 cubic metre steel tank filled with dry dense sand. Based on the results of ultimate lateral loads, fin efficiency under static and repeated loadings was determined. A modified relationship of load deflection behaviour has been suggested. Small-scale lateral cyclic load tests were performed in order to determine the effect of fin length on the lateral displacement of laterally loaded piles. Ten thousand cycles were used in each test to represent twenty years of environmental loading on offshore structures. Variables included the magnitude, frequency and direction ofthe load, the pile tip condition and the fin length. The efficiency of fins was evaluated by measuring the reduction of displacement of the pile head. The relationship between maximum load and displacement established from lateral load-displacement curves demonstrates that fins have significant impact on vertical and horizontal displacements. Piles subjected to combined loads were tested, and the failure envelopes of normalised combined loads represent the lateral resistance increase resulted from the use of finned piles. Under combined cyclic loading with various load features, a finned pile showed better performance in lateral resistance than a monopile. In order to achieve the optimum fin efficiency, the ideal fin width should be equal to half of the pile diameter and the fin length should be equal to half of the pile length. Outcomes from this research provide concepts for laterally loaded piles and useful parameters for the design of finned piles. The device of cyclic loading system and the use of 3D finite element method (FEM) can be applied in the future study of finned piles.