 1  COMPUTATIONAL MECHANICS WCCM VI in conjunction with APCOM’04, Sept. 5-10, 2004, Beijing, China  2004 Tsinghua University Press & Springer-Verlag Evaluation of Foundation Stiffness Using the Scaled Boundary Finite Element Method James P. Doherty 1 *, Andrew J. Deeks 1 , Guy T. Houlsby 2 1 School of Civil & Resource Engineering, The University of Western Australia, 35 Stirling Highway, Crawley, WA 6009, Australia 2 Department of Engineering Science, University of Oxford, Parks Road, Oxford, OX1 3PJ, UK e-mail: doherty@cyllene.uwa.edu.au, deeks@civil.uwa.edu.au, guy.houlsby@eng.ox.ac.uk Abstract In this paper the effect of caisson skirt flexibility on the overall load-displacement response of a caisson foundation subjected to vertical, horizontal and moment loads is investigated. The study is carried out numerically using the scaled boundary finite-element method, to represent an elastic half-space, combined with shell finite-elements, to represent the caisson’s skirt. The flexibility of the caisson’s skirt is shown to have a significant influence on the load-displacement response, and charts are presented relating the dimensionless stiffness of the foundation to the relative stiffness of the skirt. Key words: scaled boundary finite-element method, caisson foundation, shell element INTRODUCTION Caissons are a novel foundation type that may be used in place of traditional piling methods for founding or anchoring offshore oil and gas production facilities. It has also been proposed that they could provide an economical alternative to piled or gravity based foundations for supporting offshore wind turbines [1]. However, extensive research is required to confirm their applicability. Caisson foundations are circular in plan, with radius R, and consist of a lid, which rests on the soil surface, and a skirt, which penetrates the soil to a depth d, as shown in the Figure 1. The principal advantages of caisson foundations is partly due to their relative light weight and ease of fabrication, but primarily due to the quicker and simpler installation process, which involves pumping water out from the inside of the caisson once a sufficient seal is made between the rim of the skirt and the soil. The suction assists the installation by temporarily reducing the resistance of the soil, as well as increasing the net downward force on the caisson [1]. As offshore structures bear relatively high horizontal forces arising from environmental factors, such as winds, waves and currents, their foundations are subjected to high horizontal loads and overturning moments, which may rapidly fluctuate. The behaviour of the structure under these loading regimes is critically linked to the behaviour of its foundations. It is therefore essential that realistic estimates of the load-displacement response of caisson foundations be made to carry out a reliable structural assessment. The elastic response of a caisson foundation subject to vertical (V), (H) and (M) loading can be expressed in matrix form as:           V GR 2 H GR 2 M GR 3 =         K V 0 0 0 K H K MH 0 K MH K M       w R u R θ M (1)