Experimental Investigations of the Cyclic Response of Suction Caissons in Sand
Authors: Byrne B.W. , Houlsby G.T. 
Published: October 2008

Suction caissons are a relatively new design concept being considered for use as foundations in a wide variety of offshore applications. Unlike many other offshore developments, there is no onshore equivalent of suction caissons to use in the development of design guidelines. This contrasts with the development of offshore piling theory. It is therefore essential to identify key behavioural patterns and important mechanisms that govern capacity under a wide variety of loading regimes. This would allow the establishment of a broad framework of response and thus focus subsequent site or project specific investigations.

This paper is intended to add to this framework by detailing key results from an experimental investigation into the response of the foundation subjected to a variety of cyclic loading regimes. The model tests are conducted at 1g using a complex loading apparatus capable of applying independent control on vertical, horizontal and moment loading at cyclic rates of up to 1Hz. The foundation is embedded in oilsaturated sand in a medium to a very dense state, and the typical period of cycling is such that offshore loading conditions are modelled.

Important areas studied during the testing programme were (i) cyclic loading of the foundation into tension, (ii) cyclic horizontal and moment loading under constant vertical loads, and, (iii) the relationship between cyclic loading and monotonic loading. These experimental results provide powerful insights into load-displacement relationships that could lead to the development of simple analytical or numerical models.

Introduction

Offshore structures are subjected to large loads from wind and waves which result in complex loading on the foundation. Wave loading is typically periodic in nature with a dominant period of 15-20 seconds being typical during a large storm. The response of foundations on sand under these loading conditions is best described as partially drained. There is sufficient time during the loading cycle for small amounts of fluid movement and volume change to occur within the soil matrix before the application of the next part of the cycle.

One of the critical responses of a caisson is considered to be to tensile loading. Large strains may occur before the maximum tensile capacity is mobilised or cavitation of the fluid occurs13. The serviceability of the platform probably requires deformations to be kept much smaller. The performance of caissons under repeated (cyclic) loading of variable amplitude is therefore of vital importance in design.

The loading on the foundation is usually idealised to the general planar loading condition comprising of {V, M/2R, H} components as set out by Butterfield et al1. The drained response of the foundation to this three degree-of-freedom loading has been studied in detail for flat footings8 and caisson footings4. The behaviour of the footing can be encapsulated within a work hardening plasticity theory with the advantage that the footings are represented by a 'macro-model' which may be incorporated within structural analyses5,6.