Based on the mechanical tests of fine siltstone which were conducted under different confining pressures,the effect of confining pressure to peak strength and failure form was analyzed,and the law between tension-shear deformation and confining pressure of whole process was researched.The study results showed as follows.The relationship curve of peak strength and confining pressure of fine siltstone is a polygonal line which has a inflexion.When confining pressure is less than the σ3L of inflexion, the failure of fine siltstone is controlled by shear deformation,and its strength is influenced by tension deformation,so it will be less than the strength calculated by the Coulomb rule,and its rupture angle is larger than (45°+φ/2). When the confining pressure is higher than the σ3L,the failure of fine siltstone is only controlled by shear deformation,its strength and rupture angle are as same as the Coulomb rule gives.Based on this,the index ft/fγ which is a reflection of tension deformation and shear deformation in the failure process was given,and the strength rule of fine siltstone under different confining pressures was constituted,which accorded with fact.
