Abstract : Depending on stress state and loading conditions, instability in loose sand can develop at a shear stress level much lower than the Mohr-Coulomb failure criterion. This type of failure mode, very different from strain-softening in dense sand, can lead to catastrophic collapse of earth structures. An appropriate constitutive model is therefore needed that can capture the correct failure modes and accurately predict the stress-strain response for both dense and loose sands. In this paper, a micromechanical approach developed earlier for the modelling of granular material behaviour is adopted. The constitutive model is utilised to simulate undrained triaxial, constant-q and proportional strain tests on loose sand, and numerical results are compared with experimental results. The model is capable of accurately reproducing the macroscopic experimental tests, and is also able to provide microstructural information. This enables the observation of instability at the particle scale, and provides a better understanding of the linkage between particle-scale instability and instability at the assembly level.