Undrained conditions of unsaturated soil using an integral form of the constraints and full Newton-Raphson procedure.

The undrained condition is the initial stage of a consolidation problem for unsaturated soil and for the particular case of saturated soil. The undrained behaviour is simulated in a coupled formulation using a small time step; several iterations are necessary due to convergence difficulties. A simple procedure, which does not introduce the time factor but is compatible with the formulation of a consolidation problem, is to formulate the undrained problem using the integral form of the constraints. The undrained constraints derived from physical consideration and a general relationship in terms of net stresses and pore pressures are added to the equation governing the equilibrium of an element of unsaturated soil through an integral form of the constraints. The linearisation process results in set of highly non-linear algebraic equations with net stresses, pore water pressures and pore air pressures as unknowns. The algorithm solving these equations using a full Newton procedure is presented for use in a finite element program.