Study On Fracture Failure Mechanism Of Fractured Rock Mass Under Steady Hydraulic Pressure

There are many tunnels and underground projects in China,and the hydrogeological and structural forms are complex and the construction is difficult.The water and mud inrush disaster is an important factor restricting the construction of underground projects.Rich water,high pressure and unfavorable geology are the main geological conditions to induce water and mud inrush.It is easy to form water inrush and mud inrush channel under the combined influence of seepage pressure and engineering disturbance,and disasters are caused.Therefore,the study of macro mechanical characteristics and fracture failure mechanism of rock mass affected by external load under the action of stable seepage water pressure has important scientific research and practical engineering significance for the prevention and control of water and mud inrush disaster in tunnel and underground engineering.Based on the self-developed stable hydraulic pressure test device,the effect of hydraulic pressure stability in the transient process of crack initiation and propagation under external load is realized.The uniaxial compression and cyclic loading and unloading tests of the fissure specimens under the action of stable hydraulic pressure were carried out.The mechanical characteristics and initial crack propagation law of the fissure specimens under the action of stable hydraulic pressure were analyzed.Based on the energy criterion,the energy mechanism and damage characteristics of the fissure specimens under the influence of stable hydraulic pressure and fissure dip angle during the cyclic loading and unloading process were explored.The main achievements are as follows:(1)The uniaxial compression test results of the fissure specimens under the combined influence of the fissure opening and the inclination angle show that: with the increase of fissure opening,the variation law of peak strength of fissure specimens with the increase of fissure dip angle changes from decreasing first and then increasing to increasing gradually,and the minimum value of peak strength corresponds to the fissure dip angle from 45° To 0° deviation.By using the numerical analysis software of particle flow,combined with the displacement of prefabricated fissure and the evolution characteristics of force chain near its neighborhood,it is found that there is flexural deformation on the surface of prefabricated fissure;Under the condition of small opening,the contact closure phenomenon of prefabricated fissure surface appears.The order of prefabricated fissure closure and tensile crack development is the main reason that the fissureopening affects the fracture strength.(2)With the decrease of fissure opening,the crack initiation position of horizontal fissure specimens shifts from near the middle of prefabricated fissure to the tip of prefabricated fissure under uniaxial compression.In order to further explain this phenomenon,combined with numerical simulation,the displacement characteristics of prefabricated fissure surface are analyzed.The results show that: During the loading process,the prefabricated fissure surface of the specimen with an opening degree of 0.4mm and below exhibits the prefabricated fissure surface closure,and the smaller the fissure opening,the earlier the contact time of the fissure surface;A fixed beam model is established to analyze the crack initiation mechanism of horizontal fissure specimens with different opening degrees from the perspective of flexural deformation,and the numerical model is used to verify the results.(3)The critical stress of composite fracture of open fracture under the action of hydraulic pressure is deduced.When other conditions are the same,the greater the hydraulic pressure,the lower the initiation strength of the fissure specimens;the greater the confining pressure,the higher the initiation strength;with the increase of the fissure inclination,the initiation strength of the fissure specimens first decreases and then increases.According to the principle of maximum circumferential stress,the theory of maximum energy release rate and the theory of strain energy density,the fissure specimens initiation characteristics of water-force coupling are discussed and analyzed.The mode I stress intensity factor at the tip of the wing crack is calculated by superposition of the stress intensity factor at the tip of the wing crack and the stress intensity factor at the tip of the single wing crack.According to the fracture criterion,the critical hydraulic pressure of wing crack propagation under compression shear is obtained.(4)Uniaxial compression fracture test of fissure specimens under1 MPa steady hydraulic pressure was carried out.The hydraulic pressure and softening caused by seepage water accelerate the crack initiation and propagation,and weaken the peak strength and elastic modulus of the fissure specimens in varying degrees;The initial crack propagation length of crack initiation is longer under hydraulic pressure.The acoustic emission ringing count and the total number of ringing counts present different evolution characteristics in the initial compaction stage,elastic deformation stage,plastic deformation stage and failure stage of the fissure specimens.Finally,based on the particle flow software PFC2 D,a fluid-solid coupling experiment was carried out to analyze the fissure model stress-strain curve,the number evolution characteristics of microcracks,and the force-chain evolution mechanism near the prefabricated fissure surface in the initial stage and the crack initiation stage.(5)The uniaxial cyclic loading and unloading test results of fissure specimens under stable osmotic water pressure show that: The fissure specimens produces the largest amount of vertical irreversible deformation in the first cycle stage,and then a steep drop occurs in the second cycle,and then tends to be stable.the irreversible deformation amount under hydraulic pressure is greater than that under no hydraulic pressure;Analysis of damage characteristics based on irreversible strain.As the number of cycles increases,the damage parameters of the fissure specimens gradually decrease,the damage variable gradually increases,and the damage gradually accumulates.Analyzing the energy mechanism of the fissure specimens in the cycle stage from the energy point of view and found that: Under the action of hydraulic pressure,the dissipated energy generated in the cycle stage of the fissure specimens is more.As the number of cycles increases,the internal energy dissipation of the fractured rock mass gradually decreases;The proportion of dissipated energy is higher than the proportion of elastic energy in the first cycle,the energy dissipation of fissure specimens is the main part,and the proportion of subsequent cycle energy dissipation is greatly reduced,and a large amount of elastic strain energy is released;Defining the damage variable generated during the cycle of fissure specimens based on the dissipated energy can reasonably describe the damage evolution characteristics of the fractured rock mass during the cyclic loading process.