Study On Strength Parameters Of Rock Mass Under High Stress

With the development of hydropower resources in southwest China , the rock mechanics problem of rock mass with high geo-stress is becoming increasingly prominent . Because of the difference of geo-stress environment between deep-buried rock mass engineering and shallow rock engineering , the basic mechanical properties and engineering response of rock mass are also significantly distinct , leading to a significant difference of disaster mechanism,engineering stability and design theory . Therefore , it is necessary to research the rock mass mechanical parameters on the surrounding rock with high geo-stress and the corresponding method of rock mass mechanical parameters .This paper combine the project"The Research of Rock Mass Mechanical Parameters on the Surrounding Rock with High Geo-stress of Jinping II Hydropower Station", in laboratory , we have done rock triaxial tests under different loading or unloading paths ; in field , we have done rock mass direct shear tests and true triaxial tests , at the same time , based on rock mass fine structure description , we estimate rock mass strength parameters by using Hoek-Brown strength criterion , by comparing with the strength parameters which got in different ways , we analyze the impact of stress path and confining pressure on rock mass strength parameters , research the strength parameters of rock under high stress . Some conclusions are summarized as the following :⑴Rock under various stress paths have confining pressure effect . The confining pressure have a greater impact on the axial bearing capacity of rock , the pre-peak unloading confining pressure is the most sensitive , followed by the post-peak unloading confining pressure , and the conventional triaxial loading is the least sensitive .⑵The rock strength and strength parameters under different stress paths are distinct , conventional triaxial loading test >pre-peak unloading confining pressure test>post-peak unloading confining pressure test .⑶Structural plane direct shear test show that the peak shear strength and normal stress have a very clear nonlinear relationship at high stress , and for the structural planes of the same character , as the normal stress increases , the friction coefficient decreases and cohesion increases . ⑷The rock mass strength parameters under high stress are significantly different of which under conventional stress , not only the sizes of strength parameters , but also the characteristics of strength parameters , the rock mass strength under high stress show obvious nonlinear characteristics .⑸The results of rock mass triaxial unloading test show that under low stress level (normal stress <25MPa) , the Mohr-Columb strength criterion and the Hoek-Brown strength criterion have little difference , but under high stress level (normal stress >25MPa) , the Hoek-Brown strength criterion is more reasonable than the Mohr-Columb strength criterion .⑹The stress state of rock mass samples have a greater impact on the rock mass strength parameters . The rock mass triaxial unloading test is in three directions stress state , the direct shear test is in two directions . The friction coefficient is almost the same between rock mass triaxial unloading test and direct shear test , but the rock mass cohesion of triaxial unloading test is significantly greater than direct shear test .⑺The rock mass strength is related to the sample size , exists scale effect . The rock mass strength parameters acquired by rock triaxial test,rock mass triaxial unloading test and the estimation with Hoek-Brown strength criterion show that with the increase of rock mass size , the friction coefficient and cohesion in general decreases .