Soil–water–air fully coupling finite element analysis of slope failure in unsaturated ground

In this paper, a program of the finite element method (FEM), named as SOFT, using a finite element–finite difference scheme (FE–FD) for soil–water–air three-phase coupling problems, has been developed based on a rational and simple constitutive model for unsaturated soil proposed by Zhang and Ikariya (2011). In the program, similar to the works by Uzuoka et al. (2009) and Oka et al. (2010b), the FE–FD formulation in saturated condition of soil–water coupling problem, proposed by Oka et al. (1994), has been extended to unsaturated condition in soil–water–air fully coupling scheme, taking the saturation as a state variable. In order to verify the availability of the proposed numerical method, triaxial tests on unsaturated silty clay under fully undrained and unvented conditions, conducted by Oka et al. (2010a), are firstly simulated by the proposed method. The development of pore air pressure and pore water pressure measured in the specimen can be reproduced well by the proposed method. Furthermore, model tests on slope failure in unsaturated Shirasu, carried out by Kitamura et al. (2007), are also simulated by the same numerical method. From the simulation it is known that the slope failure behavior of the model ground observed in the tests can be described, on the whole, with satisfactory accuracy