Rock Strength Criterion Considering End Friction Effect

Mohr-Coulomb strength criterion is widely used to evaluate the stability of rock mass engineering,but the effect of end friction is not considered.Because of the inconsistency between the deformation of the pressure head and rock sample,friction will occur between the pressure head and the end of rock sample.The rock sample inevitably receives the influence of the end friction effect.The results directly fitted by M-C criterion do not only reflect the relationship between confining pressure and peak strength.Taking limestone sandstone as the research object,the end friction effect of rock bearing capacity is studied through conventional uniaxial compression test,triaxial compression test,triaxial confining pressure unloading test and uniaxial compression test under different end friction factors.The main conclusions are as follows:The results show that the uniaxial strength at different sizes and the triaxial compressive strength under the same confining pressure increase with the increase of the end friction factor.The existence of the critical friction factor makes the failure mode of the uniaxial compression specimen of different sizes change from shear to tensile,and most of the triaxial compression failure is shear failure,and the failure angle of the specimen increases with the end friction.Based on the test results,linear strength criterion named End-Friction(E-F)strength criterion including the end friction factor is proposed.The E-F strength criterion can reflect the strength characteristics of the rock material itself.The peak strength and the confining pressure are fitted by the conventional triaxial test data,and the calculation formula is established.The strength envelope of the rock material itself can be obtained just by the end friction factor K value.The results show that under the same end friction factor and initial confining pressure,the greater the unloading rate of confining pressure was,the lower the failure confining pressure and the greater the differential stress were;under the same confining pressure unloading rate and initial confining pressure,the greater the end friction factor was,the lower the failure confining pressure and the greater the differential stress were;Friction angle ? and Cohesion c of rock specimen increased linearly with the increase of end friction factor as a whole;With the increase of confining pressure unloading rate,the Friction angle ? and Cohesion c of the rock specimen increased on the whole with a decreased increasing rate.An End-Friction(E-F)unloading strength criterion for rock including end friction factor and unloading rate was proposed,which can describe the strength of rock specimen failed in confining pressure unloading condition through fitting the relationships between the peak strength and failure confining pressure of rock specimens.If the end friction factor K is known,the E-F unloading failure strength envelope of rock specimens under different confining pressure unloading rates v can be drew out,then the strength parameters of rock specimens under different confining pressure unloading rates can be obtained.E-F strength criterion and E-F unloading strength criterion can well reflect the strength characteristics of rock materials under different end friction factors and confining pressure unloading rates,which is worth popularizing and applying in rock engineering practice.