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Abstract The cohesion and friction of rock do not act at the same location simultaneously, therefore，the linear Coulomb criterion does not describe the rock strength accurately. The friction within fissures is related to the confining pressure. A fissure will not slide as the friction approaches the cohesion of intact rock in its neighborhood. And thus，the range of dip angle of fissures affecting the strength of specimen decreases gradually with the increase of confining pressure. The linear variation of the strength under the low confining pressure is usually induced from the fissures with the specific dip angle，and is not applicable to calculate the internal friction coefficient in the Coulomb criterion. The natural fissures with various dip angles in sandstones result in the scattered uniaxial compressive strength under the low confining pressure. The strength varies regularly with the confining pressure，and their relationship can be described with the exponential criterion. The plastic deformation due to the axial compression results in the cohesion losing from low to high in marble specimens，while the thermal damage in marble causes the cohesion losing in full scale. Freezing enhances the cohesion but has no influence on the internal friction. The conventional triaxial strength of macroscopically isotropic rock is perfectly fitted with the exponential criterion，from which the initial influence coefficient of confining pressure on strength is applicable to estimate the friction coefficient of fissures.
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2016, Vol. 35 
