Study On Strength Criterions And Damage Characteristics Of Rocks

Rock has initial damage with various fissures,inducing scattering strengths of specimens under the uniaxial compression and low confining pressure compression.With the increase of confining pressure,when the friction bearing capacity of fissure increases up to the cohesion of the intact rock in its neighborhood,the fissure would not slide,which has little effect on rock strength.Only under high confining pressure,would the initial damage weaken the impact on the real strength of rock specimen.Uniaxial and conventional triaxial compressive strengths of various kinds of rock specimens were analyzed with different criterions to explain the effect of initial damage.The fitting results of 56 intact sandstone specimens indicate that the Hoek-Brown criterion,generalized Hoek-Brown criterion and exponential criterion all have self-consistent,but the difference among them is that the strength deviation calculated by exponential critetion would reduce with the rise of confining pressure,while the others are just opposite;the normal parabolic criterion can be applied to fit strengths at high pressure of intact granular rocks like sandstone,and the parameter in the criterion is the ideal uniaxial compressive strength,of which the difference to the practical test datum shows the initial damage in rock.Linear variation of strengths under low confining pressure is usually induced by fissures with specific dip angle,and not applicable to calculate the internal friction coefficient with the Coulomb criterion.Furthermore,plastic deformation under axial compression results in cohesions of marble specimens losing from low to high,while the thermal damage causes cohesions of marble specimens losing in full scale.The conventional triaxial strengths of various rocks,if isotropic in macroscopic,are 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 fissure.Therefore,the nonlinear characteristics of rock strength affected by fissures could be explained,and the real cohesion and the damage of rock are also evaluated.Results have some guidance to evaluation of engineering rock strength and analysis of rock damage.