Research On Failure Mechanism And 3-D Strength Criterion Of Hard Rock In Deep Ground Engineering

With the rapid development of underground engineering construction,deep underground space has become a land resource that countries are scrambling to explore and develop.The deep hard rock is usually hosted in true triaxial high stress(σ1>σ2>σ3)environment,therefore,in this thesis,true triaxial tests for hard rock are conducted.Methods for quantifying the rock failure mechanism are proposed,and the criterion for discriminating rock failure mechanism is established.Methods for determining the crack initiation and propagation stresses are proposed,and intrinsic strength properties of hard rock fracture are revealed.A 3-D form function in the stress space is fit,and a 3-D Hoek-Brown strength criterion for hard rock is built.By applying the criterion for discriminating rock failure mechanism and the 3-D Hoek-Brown strength criterion for hard rock to the excavation of the deep diversion tunnel of N-J hydropower plant,the validity of the criterion for discriminating rock failure mechanism and the 3-D Hoek-Brown strength criterion for hard rock is verified,thus providing theoretical support and scientific basis for accurate early warning and effective prevention of hazard in deep hard rock engineering.The main research contents are as follow:(1)By conducting the true triaxial fracture tests,the macroscopic failure modes of hard rock are presented.According to the multiscale fracture characteristics,the mechanisms of 7 types of basic fractures are determined.Based on the variation trends of tensile fracture number and proportion are established,and the influences of stress environment and flaw dip angle on the failure mechanism of hard rock are analyzed qualitatively.(2)By adopting the laser scanning technology,the 3-D morphology of fracture surface of hard rock is reconstructed.Based on statistical methods and Hurst exponent,the intrinsic correlation between the 3-D morphology of the fracture surface and the fracture mechanism is revealed.The surface area method is specified as a more effective method for quantifying the 3-D morphology of fracture surface of hard rock.The validity of fractal dimension for evaluating the fracture mechanism of hard rock is verified,and an indicator for quantify the fracture mechanism of hard rock is proposed based on fractal dimension.(3)Based on the indicator for quantify the fracture mechanism of hard rock and the imagesegmentation technology,methods for quantifying the rock failure mechanism are proposed,and the influences of stress environment and pre-existing flaw on the rock failure mechanism are analyzed quantificationally.By analyzing the parameters of AE signal,the validity of methods for quantifying the rock failure mechanism is verified.Based on the quantification results of hard rock failure mechanism,a criterion for discriminating rock failure mechanism is created.This criterion can be applied to determine the hazard mechanism of deep hard rock engineering.(4)Based on the completed stress-strain curves and the evolution characteristics of AE signalunder,the methods for identifying the crack initiation and damage stresses are proposed.The influences of stress environment and pre-existing flaw on the strength characteristics of hard rock are analyzed,and the intrinsic strength properties of hard rock fracture are revealed.(5)By carrying out the true triaxial test for hard rock using a novel loading path,which maintains constant Lode angle throughout the test,the effects of intermediate principal stress,mean stress,and Lode angle on the hard rock strength are completely investigated.A 3-D shape function is proposed,and a 3-D Hoek-Brown strength criterion for hard rock is established.The verification shows that the 3-D Hoek-Brown strength criterion can precisely predict the true triaxial strength of hard rock,and can be applied to predict the hazard in deep hard rock engineering.(6)Taking N-J hydropower plant in Pakistan as engineering background,by secondary development,the criterion for discriminating rock failure mechanism and the 3-D Hoek-Brown strength criterion for hard rock are programmed,and the excavation of the deep diversion tunnel are simulated.By comparing the simulation results with the actual damage form of tunnel on site,the validity of the criterion for discriminating rock failure mechanism and the 3-D HoekBrown strength criterion for hard rock is verified,and the stability and the failure mechanism of surrounding rock in deep high stress hard rock tunnel are analyzed.