Study On Thedeformation And Failure Mechanism Of Tunnel Surrounding Rock In Fault Zone

With the increasing construction of tunnels and underground works in China,the construction often requires crossing the unfavorable geological areas,such as fault zone,clay interlayer,rich water karst and soft rock section lot.And the rock mass in the fault zone is characterized by high degree of joint fracture and easy deformation along the discrete surface,which will easily lead to rock slope instability and even the collapse of the tunnel as whole construction disasters in this formation in the tunnel construction.Based on the Nanwan Tunnel project,this paper focuses on the stability of the surrounding rock of the tunnel.Firstly,the physical and mechanical characteristics and deformation and failure mechanism of the rock are analyzed by field sampling and indoor test of the tunnel fault zone.The geometrical characteristics of joints and the influence of mechanical parameters on the physical and mechanical properties and deformation and failure mechanism of the rock mass by the prefabricated jointed rock specimen made of cement mortar.And the simulation and analysis of the above-mentioned indoor rock test and joint rock mass model are carried out by using the discrete element software PFC2 D.Finally,the excavation of the fault zone of the Nanwan Tunnel is simulated to study the influence of the relative position of the fault zone and the tunnel on the fracture propagation law of tunnel surrounding rock and the deformation failures mechanism.The details are as follows:(1)The uniaxial,triaxial and cyclic loading tests were carried out to obtain the effects of different confining pressures and repeated loads on the mechanical properties and deformation and failure mechanism of the rock.The experimental results show that the increase of the confining pressure increases the peak strength and elastic modulus of the rock,and the peak strength of the rock specimen under repeated loading is obviously decreased.(2)The joint rock specimen model was made through the use of cement mortar and other materials with the indoor model test to get the influence of the joint angle,the length and the type of fillers on the mechanical properties and the failure mode of the rock mass.The experimental results show that the peak strength of the rock mass is the lowest when the joint angle is about 45°,and the failure mode is the shear failure.The existence of the joint filler improves the elastic modulus and peak strength of the joint rock mass.(3)The simulation experiment about the uniaxial and triaxial simulations ofsimulated rock specimens was carried out by discrete element software PFC2 D,and the reliability of rock mechanics and deformation mechanism were studied and compared with laboratory tests with the fracture of rock obtained and the distribution law extended;and then the uniaxial loading simulation experiment of different sizes of rock was carried out to extend the influence of rock size change on the mechanical properties and failure modes of rock.Finally,the simulation experiments of jointed rock mass with different angles and length are carried out,and the results are compared with the joint model test to verify the reliability of the mechanical properties and deformation failure mechanism of jointed rock mass.The simulation results show that the crack propagation curve of the rock is "S" type under the external load,and the number of cracks is increased rapidly in the short time after the rock reaches the peak strength.With the increase of the aspect ratio of rock specimen,the peak intensity of rock mass decreases and the elastic modulus increases obviously.(4)The fractal zone of Nanwan tunnel fault is simulated by discrete element software PFC2 D,and the relative position of fault and tunnel is studied.The influence of the relative position of fault and tunnel on the deformation law and the law of crack propagation as well as the influence of bolt support and lining support on stability of surrounding rock is studied.Finally,the deformation and failure law of the surrounding rock of the tunnel under the condition of unsupported measures are analyzed by PFC2 D simulation,and the influence of the change of the relative position of fault and tunnel on the surrounding rock failure pattern and the crack propagation distribution rule is studied.The simulation results show that the initial support is favorable to the stability of the surrounding rock and controls the initiation and expansion of the crack,and the fault has the greatest effect on the stability of the tunnel when the fault intersects with the tunnel excavation surface.Moreover the surrounding rock of the tunnel is easy to slip along the joint surface.The destruction of the surrounding rock is influenced by time effect.