This paper presents a new generalized e!ective stress model, referred to as MIT-S1, which is capable of predicting the rate independent, e!ective stress}strain}strength behaviour of uncemented soils over a wide range of con"ning pressures and densities. Freshly deposited sand specimens compressed from di!erent initial formation densities approach a unique condition at high stress levels, referred to as the limiting compression curve (LCC), which is linear in a double logarithmic void ratio, e, mean e!ective stress space, p. The model describes irrecoverable, plastic strains which develop throughout "rst loading using a simple four-parameter elasto-plastic model. The shear sti!ness and strength properties of sands in the LCC regime can be normalized by the e!ective con"ning pressure and hence can be uni"ed qualitatively, with the well-known behaviour of clays that are normally consolidated from a slurry condition along the virgin consolidation line (VCL). At lower con"ning pressures, the model characterizes the e!ects of formation density and fabric on the shear behaviour of sands through a number of key features: (a) void ratio is treated as a separate state variable in the incrementally linearized elasto-plastic formulation: (b) kinematic hardening describing the evolution of anisotropic stress}strain properties: (c) an aperture hardening function controls dilation as a function of &formation density'; and (d) the use of a single lemniscate-shaped yield surface with non-associated #ow. These features enable the model to describe characteristic transitions from dilative to contractive shear response of sands as the con"ning pressure increases. This paper summarizes the procedures used to select input parameters for clays and sands, while a companion paper compares model predictions with measured data to illustrate the model capability for describing the shear behaviour of clays and sands.