No abstract

This paper describes a series of centrifuge tests on the cyclic lateral behaviour of a tetrapod piled jacket foundation (TPJ) in Malaysian kaolin clay. The dimensions and axial and bending stiffness of the model foundation were determined in accordance with a prototype TPJ foundation built in the Guishan offshore wind farm in Guangdong Province of China. Both one- and two-way cyclic loadings with differing load magnitudes were applied. The rotational, lateral and vertical displacements of the TPJ and the corner piles, the distribution of internal forces along piles and the lateral pile–soil interactions were scrutinised. It was shown that settlement of the model foundation is dominant during both one- and two-way loading, and even the upwind piles that are expected to be pulled up exhibit considerable downward displacement except for the very first few cycles of loading. In addition, a two-way loading that is applied after one-way loading could generate considerable reversal accumulative displacements, both rotationally and laterally, on the TPJ foundation, provided that the load magnitudes are comparable. The reduction in lateral pile–soil resistance with the number of loading cycles is quantified by means of a degradation factor, which appears to be significantly higher than that in sand.

This paper presents a programme of model tests on the lateral behaviour of tetrapod caisson-founded jacket (TCJ) structures in silt. Loading tests on the TCJ as well as on a mono-caisson were conducted at different rates. In addition to the load–displacement response of the TCJ, the vertical, horizontal and overturning forces at the top of the corner caissons were also measured by in-house-designed load cells. For monotonic loading, the resisting moment provided by corner caissons takes up only 8.7% of the total overturning moment acting on the TCJ. The bearing capacity of the TCJ accounting for caisson skirt friction (Hc) is 154% greater than when assessed without considering it (H0) and, more importantly, the inclination needed in mobilising Hc is still within the allowable limit. For cyclic loading, simultaneous upward, excessive rotation of the jacket is developed when the maximum load amplitude (Hmax) exceeds Hc. Cyclic loading with the minimum load amplitude (Hmin) below H0 proves to be desirable for the control of accumulative rotation, due to the essentially two-way loading characteristics of upwind caissons.