This paper presents an experimental evaluation of a simple critical state model and its extension in modelling the stress-strain behaviour of sand under a wide range of confining pressures and initial void ratios under both drained and undrained loading conditions. The critical state model concerned is known as CASM developed by Yu [1,2] and CASM is a relatively simple model as it only requires seven model constants, five of which are the same as those used in the modified Cam clay model. All these constants have clear physical meanings and can be easily determined from the results of triaxial tests as demonstrated in this paper. A key advantage of CASM over many other existing critical state models lies on its unified nature as it can be used to model the behaviour of both clay and sand. In addition, the paper also shows how CASM can be further extended in order to simulate a particular undrained behaviour of loose sand, which is the reappearance of hardening after the material experiences a peak shear stress followed by a softening response.