The paper investigates the performance of various time stepping schemes for coupled displacement and pore pressure analysis. A number of alternative forms of the automatic time stepping method proposed by Sloan and Abbo (1999a) are also presented. These alternative schemes use different updates for the displacements and pore pressures and also adopt different starting conditions for the iterations. The automatic schemes are compared with an implicit h-method, as well as an explicit method, through analysis of a variety of problems involving undrained loading, drained loading, and consolidation for Mohr-Coulomb and critical state models. As expected, the numerical results confirm that the explicit scheme is neither accurate nor robust. Although the implicit h-method is accurate and fast, it fails to give a solution in a number of cases where the time step is large. The automatic schemes are shown to be accurate, fast and generally robust. Two of the automatic schemes proposed never fail to furnish a solution for the cases considered. In addition, all the automatic schemes are able to constrain the time-stepping (temporal integration) error in the displacements and pore pressures to lie near a prescribed tolerance, provided the iteration error tolerance is properly chosen. For complex soil models, it is important that the latter is set sufficiently small in order for the schemes to be able to constrain the time-stepping error to lie within a prescribed tolerance.
IntroductionIn geotechnical engineering, deformation is often coupled with the flow of pore fluid. Finite element analysis of geotechnical problems therefore often uses a mixed formulation where the nodal variables are displacements and pore pressures. Such a formulation eventually leads to a set of stiff, non-linear ordinary differential equations in terms of these variables. One critical issue for solving such a system accurately is the choice of time discretisation. In the literature, as well as in commercial software, the discretisation in the time domain is usually carried out using the generalised trapezoidal method (also called the h-method).With the h-method, all time dependent variables are estimated at some intermediate point within the interval, depending on the actual value of h. The non-iterative explicit method, which estimates time dependent variables at the start of the interval, is usually avoided due to its poor stability properties. Therefore, the h-method is usually used in an implicit form with the need for iterations in each time step. The key disadvantage of the conventional h-method is the need for a trial-and-error process to find an accurate and efficient sequence of time steps. This is not a trivial task in practice, as coupled consolidation problems often span long durations with vary large gradients in pore pressures and displacements at certain times.In an effort to address the problems associated with coupled consolidation analysis, Sloan and Abbo (1999a) proposed an automatic time stepping scheme. This attempts to choose the...