Abstract:The geotechnical properties of decomposed granite soil are very different depending on the property of parent rock and the degree of weathering. This kind of soil is unstable physically and chemically. In this paper,in order to evaluate the stress-strain behavious of decomposed granite soil,the mechanical properties are first investigated and discussed by carrying out a series of triaxial compression tests,and then an isotropic elasto-plastic model is proposed. The model contains eight experimental parameters which are not only easily determined from a few conventional triaxial tests but also have a clear physical meaning. The predicted behaviours for drained triaxial tests of a few kinds of compacted decomposed granite soil are in good agreement with experimental data.