Combined Lateral and Vertical Loading of Caisson Foundations

Abstract

Abstract This paper summarises aspects of a four-year study of caisson foundations under the action of combined vertical, horizontal and moment loading. The study included physical (centrifuge) modelling and numerical analysis (finite element, upper and lower bound analysis). Caissons with skirt lengths of 40 to 50 % of the foundation diameter were modelled, with physical tests conducted in either kaolin clay, calcareous silt or calcareous sand, under either undrained or partially drained loading conditions. In addition, laboratory tests (simple shear, triaxial compression and extension) were undertaken to evaluate element response under different loading paths. The results presented in this paper have implications for the design of caisson foundations in low permeability clays and silts, focusing on the response to lateral and vertical loading. General numerical solutions (presented in the form of design charts) for bearing capacity, tension capacity and lateral capacity are compared to results from experimental testing. In addition, the observed installation resistance is discussed, as is the potential for significant increases in bearing and lateral capacity arising from the application of preload. Finally, caisson performance under combined vertical - lateral loading is examined, and the results from a series of model tests are used to investigate empirically the form of a yield envelope for caisson foundations, and the potential use of an associated flow rule to define plastic deformation at yield. Introduction Over the last two decades, offshore oil and gas development has moved towards design in increasingly deep water and hostile loading environments. Coupled with fluctuating oil prices and the development of marginal fields, this has led foundation engineers to challenge traditional design methods for founding offshore structures (such as piled foundations). In turn, this has led to the development and increased use of caisson (or skirted) foundation systems. The idea of caisson foundations originated from gravity base platforms in the North Sea, where skirts were employed to transfer vertical bearing loads below mudline level to more competent material. The generic term, caisson foundation, may be applied to a family of alternatives including:Skirted gravity base structures, such as the Gullfaks C platform installed in the North Sea in 1989 (Tjelta et al1, Tjelta2).Suction anchors (used as alternatives to drag anchors), as installed at the Laminaria field in 1999 in the Timor Sea (Randolph et al3).Caisson foundations for taut leg mooring systems, such as installed in the North Sea for the Snorre tension leg platform in 1992 (Anderson et al4).Caisson foundations for jacket structures or jackup rigs, such as described by Erbrich and Tjelta5 for the Draupner E platform, installed in dense sand in the North Sea in 1994.Caisson templates for subsea structures, as described by Aas and Anderson6. The use of caisson foundations for either taut leg mooring systems or jacket structures (where skirt lengths may be in the order of 10 to 50% of the foundation diameter) has been the focus of a detailed research project at The University of Western Australia (UWA), and is the subject of this paper.