Influence of intermediate principal stress on dilation and strength characteristics of sandstone

Based on the self-made “ multi-functional true triaxial fluid-solid coupling experiment system”,an experi- ment to investigate the strength of sandstone under true triaxial compression(σ1 ﹥ σ2 ﹥ σ3 )for different coefficients of the intermediate principal stress and maintaining the volume stress unchanged was carried out. The authors have dis- cussed the deformation,strength and shear failure plane feature of sandstone under different coefficients of intermediate principal stress. The research result shows that the starting point of dilatancy decreases with the increase of the coeffi- cient b of intermediate principal stress under true triaxial compression. The dilatancy exists an obvious stage under ex- perimental conditions,with the increase of maximum principal stress,the volume strain increases linearly at the steady compressing stage and increases as a quadratic function at the accelerating stage with the strain rate increases linearly. The strength of sandstone decreases with the coefficient b increase and coincides with the Modified Lade Criterion un- der the condition of keeping volume stress unchanged. In this experimental test,the rock specimens generally devel- oped a typical type of symmetrical double shear failure and the angle of shear plane increases linearly with the coeffi- cient b increase.