Study On The Shear Behaviors Of Rocks Under Normal Unloading Or Tensile Stress

Excavation in rocks and some earth surface processes,such as incised valley,excavation disturbance,seismic load,as well as in the cases of transtension fault,overhanging rock,may change the stress state of rock masses in the disturbed zone.In these cases,rocks are failed under unloading stress or tensile-shear stress,or be failed in the conversion process from under compressive-shear stress to tensile-shear stress.Shear failure is one of the foundational failure patterns of rocks.However,owing to its discontinuity and heterogeneity,the mechanical behaviors of rock under loading and unloading conditions are different,as well as under compression-shear and tensionshear.In other words,the loading path has a significant impact on the behaviors of rock.To study the shear mechanics of rock under normal unloading stress and normal tensile stress,this article,relied on the National Natural Science Foundation of China: An initiation by mega-earthquakes(41472245),investigated the energy evolution,strength criterion and mesoscopic fracture mechanism under normal unloading and tensile stresses by using the methods of laboratory test,discrete element simulation and theoretical analysis.The main work and conclusions of this study are as follows:(1)A direct shear test method was proposed to model the shear failure of rock under unloading normal stress.The unloading tests were carried out on the sandstone,which is widely distributed in China.The results showed that the unloading normal stress and initial stresses lead to a reduction of cohesion and an increase of internal frictional angle.Moreover,the cohesion and the internal frictional angle increases and decreases respectively as the initial normal stress increases.The failure normal displacement and failure shear displacement had an obvious growth with the increase of the initial shear stress.However,the unloading normal displacement reduces gradually as the initial shear stress increases.The unloading shear displacement is not sensitive to the initial stresses,i.e.,the correlation between the unloading shear displacement and initial stresses is weak.The relationship between the initial stresses and the failure normal stress can be described by using a quadric surface.(2)A method was proposed for the calculation of the strain energy based on the loading processes.In the process of unloading in the unloading test,the variation of the total energy done by normal and shear stresses showed different characteristics.When the initial stresses are at a high level,the total energy are tend to increase while the total energy tend to decrease gradually.During loading shear stress,part of the energy input by shear stress will be released in the form of negative work in the normal direction.When the rock sample was failed,the energy done by normal stress will less than zero in more cases when the initial stresses are at la low level while that will larger than zero in more cases when the initial stresses are at a high level.The magnitude of work done by normal force can reflect the intensity of dilatancy during test.The converted energy is always within a certain range and whether the rock sample is damaged or not can be judged by the converted energy.When the converted energy is less than the lower limit of that range,the rock sample is not failed while the rock sample has been damaged when the converted energy is greater than the upper limit of that range.(3)A simple double shear device for direct tensile-shear test was designed and different rocks were selected to carry out the direct tensile-shear tests using this device.Compared to the normal compressive stress,the shear stiffness and peak shear displacement are more sensitive to normal tensile stress.For the strength criterion,the Mohr-Coulomb criterion is more suitable in the compressive region while the Hooke-Brown criterion is more suitable in the tensile region.Based on this achievement,a new piecewise criterion was proposed,for which the Mohr-Coulomb criterion is used in the compressive region and the Hooke-Brown criterion is used in the tensile region.As the normal tensile stress decreases,the shear failure characteristics become more and more obvious and the failure of rock is transformed from tensile failure to tensile-shear failure.(4)Direct tensile-shear tests were carried out on limestone,gneiss and basalt.Results showed that there are three change laws in the curve of normal displacement: type I is a gradual decrease,type II is a gradual increase and type III is to increase first and then decrease.The type I variation occurred only in the test on the limestone and only when the normal tensile stress is at a low level.The type III variation occurred in the tests on all the three types of rocks,but more occurred in the tests on the limestone.It occurs more when the normal tensile stress is at a low level while the type II variation occurred more frequently when the normal tensile stress is at a high level.The shear stiffness of the limestone and gneiss increases linearly and nonlinearly respectively while that of the basalt remained almost a constant.The yield surface of the parabolic criterion in 3-D stress space is an elliptic paraboloid.(5)Particle flow code is adopt to simulate the microscope mechanics of rock under normal tensile stress.The shear failure ratio was used to describe the conversion process from tensile failure to tensile-shear failure.The conversion process was proved to be linear and continuous.When the normal stress is at a lower level,the initial crack is shear crack while that is tensile crack when the normal stress is at a higher level.The initial crack is the inducer of the failure of rock sample.A failure criterion,which revealed the microphysical significance of the parameters in the Hooke-Brown criterion,was proposed based on the mesoscopic fracture mechanism.