Bearing capacity failure envelopes of foundations with skirts subjected to combined loading

Abstract

The bearing capacity of the foundation is reduced in combination with horizontal loads and moments, and can be further reduced when torsion moment is applied. Therefore, torsion moment must be taken into account when calculating the loading capacity. This is particularly relevant for offshore foundations (underwater structures), since torsion is often of considerable size.The aim of this thesis is to make a numerical study of a rectangular mudmat foundation subjected to vertical loading, horizontal loading and torsion moment. The numerical calculations were mainly calculated by the finite element program PLAXIS 3D. Meanwhile, hand calculations of bearing capacities with Janbu and Davis & Booker methods are also carried out for comparison. The numerical studies were limited to undrained loading condition, where the undrained shear strength increases linearly with depth for a rectangular mudmat foundation; a) with outer skirts only b) with both outer and inner skirtsVertical and horizontal bearing capacities of mudmat foundation from finite element program PLAXIS 3D correspond well with the hand calculation results by Janbu method. However, the vertical bearing capacities from PLAXIS 3D are slightly higher than Janbu method, with a differencial value of 460kN(Model 1) and 80 kN(Model 2), corresponding to a differencial of 6% and 1%, respectively. It may have several explanations:? The results of finite element programs is dependent on network element and element type. Theoretically, the analyses of model with more fine meshes and smaller average element size would generate more accurate results. ? PLAXIS 3D bulids a real 3-dimensional model, taking into consideration the 3D effections. However, Janbu method is based on the plane strain. However, torsional bearing capacity from PLAXIS 3D is almost twice the value determined from hand calculations, which may because of the conservative calculation method or some possible parametrical errors in PLAXIS 3D. By intergrating the FEM analyses package PLAXIS 3D with the Swipe test procedure of loading, the failure envelopes of mudmat foundation and approximating expressions are investigated. Through numerical computations and comparative analyses based on FEM, the two-dimensional failure envelopes of mudmat foundation are established by using proposed method to evaluate the stability of foundation under combined loadings. These results could be utilized to provide vital reference for the design and construction of mudmat foundation.