Under certain loading conditions, loose sand can develop instability at a shear stress level much lower than the critical state failure line. To analyze these types of problems, we have adopted the micromechanics model developed by Chang and Hicher for modelling granular material behaviour. The stress-strain relationship for a granular assembly is determined by integrating the behaviour of the inter-particle contacts in all orientations. The constitutive model is applied to simulate undrained triaxial, constant-q and proportional strain tests on loose Hostun sand. Experimental results are used to evaluate how well the model can capture the modes of instability at the assembly level. The notion of control variables is discussed according to these different loading conditions.