Mechanical And Acoustic Emission Tests Of Plagiogranite Under Different Stress Paths

There are essential differences in the mechanical characteristics of rock mass under loading and unloading conditions.Most of the previous studies in the field of rock mechanics were based on the mechanical conditions of loading,but in the field of geotechnical engineering,the mechanical properties of the rock are mostly unloading,such as foundation engineering or rock slope excavation,tunnels and underground chamber excavation.In recent years,research on rock unloading has made some progress,but there are still several problems.:1)The lack of in-depth study on the fracture mechanism of unloading rocks and the lack of comparative analysis of the fracture mechanisms under different unloading paths;2)The lack of research on rock burst characteristics and the evolution of mechanical parameters during unloading;3)There are few researches on rock damage degradation and the variation of damage factors in the cyclic process.In view of the above problems,this paper takes the granite of a dam site of a hydropower station in Jinsha River as the research object,and takes the conditions of water(natural and saturated)and confining pressure as variables to carry out the “three-axis circulation plus loading and unloading” and “the confining pressure classification and unloading."The mechanical tests of the two unloading paths have also been compared with the "conventional triaxial compression test." The strength characteristics,deformation and fracture mechanism,acoustic emission characteristics and damage evolution mechanism of granite during stress-strain are discussed.The main research results are as follows:(1)In conventional triaxial tests,the peak strength,peak strain,elastic modulus,and Poisson's ratio of granite are approximately linear with the confining pressure;the shear strength parameters(c,?)of saturated rocks are smaller than those of natural rocks,c It decreased by 10.8%,decreased by 7.5%,and the deformation parameter(?,peak intensity)was higher than that of the natural sample.It shows that the water-saturating effect degrades the mechanical properties of rocks and reduces their bearing capacity and brittleness.The shear strength parameters of rock under the cyclic triaxial path are not much different from the conventional triaxial rock,and the cyclic load does not change its mechanical properties.The internal friction angle of the rock under the unloading confining pressure path is slightly higher than the conventional triaxial and triaxial cycles,increasing by 2.5%-9.7%.The cohesive force c is significantly lower than that of the other two,decreasing by 18%-41%.This is because Under the circumstance of unloading confining pressure,the fracture mechanism of shear fracture of rock is caused.(2)Under the conventional triaxial and triaxial cycles,the fracture mechanism of granite is significantly affected by the confining pressure,and the low confining pressure is dominated by tensile failure.With the increase of confining pressure,it gradually changes to shearing or shear fracture,and then turn into a shear fracture or a pressure shear fracture.Under the unloading confining pressure path,the low confining pressure is dominated by tensile and shear properties,and the tensile characteristics are obvious.As the confining pressure rises,it gradually transforms into a conjugated X-shear fracture or a conjugated X-type shear fracture.In general,the cracking of the rock is even more serious.(3)Under the three-axis circulation path,as the elastic modulus of the cycle increases first and then decreases,the Poisson's ratio first increases slowly,after the rapid increase,the residual strain first decreases sharply,then fluctuates,and the evolution laws of the three are reflected.The internal micro-cracks are accelerated during the cyclic loading of rock,and the mechanical properties are accelerated and degraded,and the expansion is more pronounced and the strain is rapidly adjusted.In the unloading confining pressure test,using the formula of Gao Chunyu to calculate the deformation modulus and Poisson's ratio has a good effect,and the deformation modulus and Poisson's ratio curve increase sharply in the final stage of unloading,indicating that the rock is close to the limit of destruction.(4)Acoustic emission reflects the deformation and fracture of rock.It is reasonable to classify the deformation phase according to the characteristics of acoustic emission parameters.The damage curve based on acoustic emission quantitatively reflects the damage characteristics of the rock in the test.The rock damage model based on the parameters of acoustic emission can better reflect the damage evolution of rocks in the conventional triaxial path.