Modelling the behaviour of unsaturated soil using an elastoplastic constitutive model

A numerical model of the stress-strain behaviour of unsaturated soil is presented. An extended critical state elastoplastic constitutive model based on net stress and suction is solved via the finite-element method. Numerical techniques are proposed which allow for the development of an elastoviscoplastic solution algorithm within the context of a transient finite-element analysis. The simultaneous solution of the coupled partial differential equations provides variations of net stress, suction, specific volume and degree of saturation in an unsaturated soil. Several examples are then analysed which illustrate typical features of the behaviour of unsaturated soil samples. Expected patterns of behaviour are reproduced and good correlation is obtained between numerical results and an alternative solution of the theoretical model. Comparisons of numerical predictions with results obtained from two suction-controlled experiments on compacted kaolin and sandy clay are then presented. Good correlation is again achieved, yielding confidence in the validity of the numerical algorithm. Finally, the solution of a boundary value problem is attempted, namely the prediction of seasonal ground movement. Comparison of numerical results with field observed data shows good correlation is achieved. The ability of the new approach to solve boundary problems of geotechnical significance is thus illustrated.