Formulation of a unified constitutive model for clays and sands

This paper presents a new generalized effective stress model, referred to a s MIT-S1, which is capable of predicting the rate independent, effective st ress-strain-strength behaviour of uncemented soils over a wide range of con fining pressures and densities. Freshly deposited sand specimens compressed from different initial formation densities approach a unique condition at high stress levels, referred to as the limiting compression curve (LCC), wh ich is linear in a double logarithmic void ratio, e, mean effective stress space, p'. The model describes irrecoverable, plastic strains which develop throughout first loading using a simple four-parameter elasto-plastic mode l. The shear stiffness and strength properties of sands in the LCC regime c an be normalized by the effective confining pressure and hence can be unifi ed qualitatively, with the well-known behaviour of clays that are normally consolidated from a slurry condition along the virgin consolidation line (V CL). At lower confining pressures, the model characterizes the effects of f ormation density and fabric on the shear behaviour of sands through a numbe r of key features: (a) void ratio is treated as a separate state variable i n the incrementally linearized elasto-plastic formulation: (b) kinematic ha rdening describing the evolution of anisotropic stress-strain properties: ( c) an aperture hardening function controls dilation as a function of 'forma tion density'; and (d) the use of a single lemniscate-shaped yield surface with non-associated how. These features enable the model to describe charac teristic transitions from dilative to contractive shear response of sands a s the confining pressure increases. This paper summarizes the procedures us ed to select input parameters for clays and sands, while a companion paper compares model predictions with measured data to illustrate the model capab ility for describing the shear behaviour of clays and sands. Copyright (C) 1999 John Wiley & Sons, Ltd.