Study On Macro-meso Failure Mechanism Of Deep Buried Granite Under Strong Unloading Of Excavation

In the topic of surrounding rock stability of high-stress underground engineering,the stress-type brittle failure of deep buried hard rock is one of the serious threat factors.Starting from the macro-meso failure mode in the field,in order to study the essence of failure characteristics of deep buried granite under strong unloading,the macro-meso deformation failure mechanism of deep buried granite is obtained from both macro and meso-scale tests.Firstly,MTS815 electro-hydraulic servo rock test system is used to carry out conventional triaxial and cyclic loading and unloading tests under confining pressures of 10MPa,30MPa,40MPa and 50MPa on granite drilled from underground caverns of a hydropower station,and the corresponding stress-strain curves and macroscopic deformation and failure characteristics are obtained.The results show that:(1)under two stress paths,granite samples under each confining pressure show obvious brittle failure characteristics;(2)under the two stress paths,the peak strength and crack damage stress increase linearly with confining pressure;Elastic modulus and cracking stress increase first and then decrease with confining pressure.Poisson's ratio increases first and then remains unchanged or decreases with confining pressure.Under the same confining pressure,the peak strength,crack initiation stress,crack damage stress and Poisson's ratio of the specimen under cyclic loading and unloading are generally larger than those under conventional triaxial,while the unloading elastic modulus is smaller than that under conventional triaxial;(3)under conventional triaxial and cyclic loading and unloading stress paths,the macroscopic failure of the specimen is mainly shear failure.Then,scanning electron microscope and energy dispersive spectrometer were used to scan the failure section,and the microscopic failure characteristics of conventional triaxial and cyclic loading and unloading under confining pressure of 10MPa,30MPa,40MPa and 50MPa were obtained.The results are as follows:(1)the failure modes of crystals with different mineral compositions in the deep-buried granite do not differ with the changes of stress path and confining pressure factors;(2)albite and potash feldspar in the deep-buried granite are prone to transgranular fracture,while Shi Ying is prone to intergranular fractureThe macro-and micro deformation and failure rules of granite samples revealed by the study have significant reference value for the selection of rock mass mechanical model for the stability of surrounding rock of deep underground engineering,the evolution rules of mechanical parameters with damage variables and the formulation of surrounding rock support countermeasures.