Investigation On Rock Failure Criterion Under Combined Compression-shear Load And Gently Inclined Pillar Strength Model

The underground rock mass engineering structures often encounter the combined compression-shear loading,resulting in high shear stress concentration and poor structural stability,resulting in hidden dangers.In mining engineering,gently inclined pillar is a typical supporting structure under combined compression-shear loading.Due to the influence of pillar inclination,different the combination of compression and shear load has different the bearing capacity of pillar.At present,there is a lack of effective method to calculate the bearing capacity of gently inclined pillar,which increases the difficulty of pillar safety management.Thus,it is urgent to carry out further research on the safety and stability of gently inclined pillar.The deformation and strength characteristics of rock under combined compression-shear loading were investigated by the laboratory test,numerical simulation and theoretical analysis,and the rock failure criterion varying with compression-shear ratio under compression-shear load was established.Based on the failure criterion of rock under compression-shear loading,the strength model of gently inclined pillar was established by introducing the initial stress ratio and pillar inclination angle.The combined compression-shear loading test of rock was carried out,and the failure law and strength characteristics of rock are revealed.A rock compression-shear test device was designed to control the inclination angle(loading angle)to obtain different compression-shear ratio,and the compression-shear test of rock under different compression-shear ratio is realized.Three kinds of samples with height to width ratio of 1.0,1.5 and 2.0 and cross section of 50 mm × 50 mm were prepared.Compression-shear tests were carried out under different compression-shear ratios,the strength and failure law of the samples were obtained,and the distribution law of strength with loading angle was fitted.The results show that with the increase of the loading angle,the specimen gradually changes from axial splitting failure to shear failure,and the strength decreases gradually,showing the dependence on the loading angle.The failure criterion of rock under compression-shear loading was established.Based on Mohr strength theory,the stress circle equation characterizing the stress state of rock under compression-shear loading was derived.The analysis shows that the ultimate stress of rock under compression-shear load is less than that under uniaxial compression.Considering the influence of loading angle on rock strength,the rock failure criteria corresponding to linear,parabolic and hyperbolic Mohr criteria were established,respectively.The failure criterion based on linear Mohr criterion includes three parameters which are uniaxial compressive strength,internal friction angle and Poisson's ratio.The failure criterion is used to predict the strength of rock under different compression-shear ratios,and the results are in good agreement with the test.The failure mechanism and stress evolution path of gently inclined pillar were revealed.Aiming at the gently inclined pillar under compression-shear load,the numerical simulation method was used to study the deformation and failure characteristics of pillar with different dip angle and width-height ratio.The effects of pillar width-height ratio and dip angle on pillar deformation,strength characteristics and failure mode were analyzed.It is revealed that the gently inclined pillar is dominated by progressive shear and supplemented by tension,and the shape of the damaged pillar is asymmetrical hourglass.In Mohr stress space,the effects of mining ratio,pillar dip angle and initial stress ratio on pillar stress state are analyzed,and it is revealed that the stress evolution paths controlled by pillar load,dip angle and initial stress ratio,which are linear,arc and curved respectively.A strength model of gently inclined pillar including pillar inclination and initial stress ratio was established.Based on the failure criterion of rock under compression-shear loading,two methods for establishing the strength model of gently inclined pillar were proposed.First,the strength parameters of the vertical pillar strength model were modified by using the rock failure criterion under compression-shear load,and the gently inclined pillar strength model was constructed.Second,by using the derivation of rock failure criterion under compression-shear load,the factors of pillar inclination and initial stress ratio were introduced into the equation of average stress circle of pillar,and the strength model of gently inclined pillar was established based on rock strength theory.The comparative analysis shows that the strength models established by the two methods are consistent,which are composed of vertical pillar strength model and dimensionless coefficient considering initial stress ratio and pillar inclination angle.The bearing capacity of gently inclined pillar is successfully predicted by using the strength model of gently inclined pillar.The established strength model of gently inclined pillar under compression-shear expands the application field of rock failure criterion and enriches the strength theory of rock mass.The established strength model realizes the accurate calculation of pillar bearing capacity under different compression-shear ratios,which provides theoretical support for pillar stability analysis and safety management.