A methodology for numerical modelling of the drained and undrained behaviour of an unsaturated soil has been developed and implemented in finite element software; an elasto-plastic constitutive model is used to represent the soil behaviour. In drained problems the suction is known and is either constant, for example during a very slow application of a load, or varying in a predetermined manner, for example due to a fluctuating groundwater level. The prediction of drained behaviour requires the simultaneous solution of equations involving net stress, suction and strain only. For undrained loading the suction varies to satisfy the constant water content requirement and the problem requires the solution of equations involving net stress, suction, strain and water content (or specific water volume). The constant water content condition can be introduced either at the level of the constitutive equation or at the level of the equilibrium equation. The first method resutls in a non-symmetric stiffness matrix and hence the use of a non-symmetric solver to obtain a solution to the equilibrium equations. The second method is preferable as symmetric matrices are maintained. Introduction of hte undrained constraint into the equilibrium equations results in additional terms which necessitate use of an iterative solution technique; sufficient iterations are performed to ensure that both the equilibrium and undrained conditions are satisfied. This paper describes the methodology and its implementation in a finite element program.
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