Study On Seepage And Slip Instability Mechanism Of Rough Fractured Rock Mass Under The Action Of Fluid-deformation Coupling

For various rock mass engineering(mining engineering,geothermal mining,bridges and tunnels,etc.),the natural rock mass on which it relies is often in a state of natural balance.Due to the influence of excavation disturbance or other external factors,the original natural fissures and secondary fissures in the rock mass will initiate,expand and even penetrate on a large scale.These fissures are not only weak surfaces in the rock mass,but also the main water storage space and migration channel of groundwater.Under the stimulation of repeated mining stress,further compression closure or shear slip occurs.It is easy to cause the imbalance of the stress field and seepage field,which poses a huge threat to the stability of rock mass engineering.In addition,the heterogeneity and anisotropy of the fracture geometry will make the fluid-deformation interaction process more complicated.Therefore,grasping the evolution law of mechanics and seepage characteristics of rough fractured rock mass under the action of fluid-deformation coupling and clarifying its slip instability mechanism have important application value for the prevention and control of groundwater and the stability control of rock mass engineering.Based on this,in this paper,considering the influence of roughness and fracture aperture on its fluid-solid coupling process,a 45° inclined single through-through rough crack was prepared by loading a standard specimen cushioned with steel bars.Using a combination method of theoretical analysis,physical experiments and numerical simulations,the evolution law of the mechanics and seepage characteristics of the fracture specimens in the seepage test in the full stress-strain process and the internal relationship between them were deeply studied.Then,the slip instability mechanism of the fractured rock mass model under the action of unilateral water pressure and overlying rock pressure is studied by using 3D discrete element program.The main research contents and conclusions are as follows:(1)From a theoretical point of view,the mechanism of fluid-structure interaction is systematically studied.Based on the effective stress principle of saturated soil,the effective stress principle for rough fractured rock mass is proposed;and based on the cubic law and the relationship between external stress and fracture deformation,the permeability equation after fracture deformation under normal load is deduced.(2)By carrying out the seepage test of the fractured sandstone under the condition of different confining pressure and orthogonal combination of seepage pressure in the full stress-strain process,the evolution law of fractured rock mass mechanics and permeability under different stress states is obtained.The research shows that there is a trade-off relationship between the mechanics and permeability of the specimen,and the influence mechanisms of confining pressure and osmotic pressure on the seepage and deformation instability of the specimen are completely different.(3)According to the corresponding relationship between the fracture permeability and its failure stage during the test,the intrinsic relationship between the deformation instability of the specimen and its flow capacity is expounded.The test results show that: in the full stress-strain process,the fracture first undergoes compression closure and then shear slip occurs,and the fracture permeability also undergoes five changing stages of "reduction-stabilization-increase-redecrease-restabilization" accordingly,which is closely related to the compression closing process of the fissure and the process of contact and shear wear of the asperities on the fissure surface.(4)Considering the effects of confining pressure and osmotic pressure on the mechanics and seepage characteristics of the specimen,the difference between confining pressure and average water pressure in the fracture is called effective confining pressure.The results show that the initial permeability of the fracture decreases exponentially with the increase of effective confining pressure,while the mechanical properties of the specimen increase exponentially with the increase of effective confining pressure,which is closely related to the influence of confining pressure on the strength influence coefficient k value.(5)The 3D discrete element prgram is used to numerically simulate the fractured rock mass model under the action of unilateral hydraulic pressure and overlying rock pressure,revealing the sliding instability mechanism of the fractured rock mass under the action of fluid-deformation coupling.The research shows that when the fluid flows stably in the fracture,the stress state of the fracture surface will gradually migrate from the stable area to the failure area,resulting in the shear failure of the fractured rock mass along the direction of the fracture surface from the area with higher pore pressure to the area with lower pore pressure.And on the failure development line,the area with high pore pressure will fail instantaneously,while in the area with no pore pressure or low pore pressure,the shear stress will rise slowly in a step-like manner and the shear failure will occur with hysteresis.