Mechanical Response And Fracture Characteristics Of Tunnel Surrounding Rock Considering Structural Plane And Principal Stress Direction

With the development of traffic tunnel,water conservancy and hydropower,mining engineering,nuclear waste storage,carbon dioxide geological storage and other projects to the deep,the instability and failure of tunnel surrounding rock under high ground in-situ stress is involved in these processes,and the failure often affects the deep surrounding rock.As a non-linear,heterogeneous and anisotropic geological body,rock mass will be distributed with structural planes such as cracks,weak interlayer and joints with different occurrence under the action of geological structure.The existence of structural planes will significantly reduce the bearing capacity of surrounding rock and have a significant impact on the stress distribution and failure characteristics.In addition,the surrounding rock will also show different mechanical response and failure characteristics with the change of principal stress direction in the process of tunnel excavation,which will bring severe deformation control and monitoring problems to the construction of underground engineering.In this paper,the instability of surrounding rock under the influence of structural plane(represented by crack and weak interlayer)and principal stress direction were studied.Firstly,the mechanical response and failure evolution characteristics of complete surrounding rock at different distances from the tunnel boundary were explored by using the self-developed true triaxial fluid structure coupling test system;Further,the surrounding rock near the tunnel boundary was taken as the research object,and based on the composition of the actual rock mass and the change of stress state during tunnel excavation,the influences of structural plane characteristics(crack length and inclination,weak interlayer thickness and inclination)and principal stress direction on the mechanics and failure characteristics of surrounding rock were studied;Finally,the FDEM simulation method based on cohesive element was used to study the spatio-temporal evolution process of the failure of surrounding rock under the influence of structural plane and principal stress direction,and the corresponding stability control measures were put forward.The main research results are as follows:(1)The mechanical response and failure evolution characteristics of complete surrounding rock at different distances from the tunnel boundary were studied.It was found that with the increase of the distance from the tunnel boundary,the surrounding rock gradually changed from brittleness to ductility,and the sensitivity of failure characteristic stress near the tunnel boundary to radial stress was greater than that far away from the boundary.Based on the rock failure characteristics under different radial stress constraints,a "four element" zoning of surrounding rock failure from the tunnel boundary to the original rock stress area was proposed: tension splitting failure area,tension shear composite failure area,shear failure area and micro shear failure area.(2)The crack and weak interlayer were taken as two type of structural planes,the experimental of the influence of structural plane on the mechanics and failure characteristics of surrounding rock under true triaxial stress was carried out.The test results of surrounding rock with crack showed that the failure mode and bearing capacity of surrounding rock near the tunnel boundary were mainly affected by the crack length.Under different fracture lengths and dip angles,the fractal dimension of new fractures had a critical value,which made the number and the complexity of distribution of new fractures minimum.(3)The test results of surrounding rock with weak interlayer showed that when the dip angle of weak interlayer was between 0 ° ～ 90 °,the tensile shear mixed fractures along the interlayer were mainly formed after the failure of surrounding rock near the tunnel boundary,the bearing capacity of surrounding rock and the fractal dimension of new fractures gradually decreased with the increase of the dip angle;The surrounding rock far from the boundary always showed shear fracture across the interlayer,which reflects that the radial stress played a leading role in the failure mode of deep surrounding rock.With the increase of the thickness of weak interlayer,the failure of surrounding rock was gradually concentrated in the weak interlayer,and its bearing capacity and the fractal dimension of new fractures were gradually reduced.In additione,the shear slip deformation of the rock block above the weak interlayer increases linearly.(4)The experimental of the influence of the principal stress direction on the mechanics and failure characteristics was carried out,and the analytical solution of the surrounding rock stress considering the principal stress direction and the distance from the tunnel boundary was established.It was found that with the increase of the angle between the principal stress direction and the tunnel,the maximum tangential stress around the tunnel gradually changed to the direction of the minimum horizontal principal stress,and its value gradually increased,which leaded to the failure of surrounding rock more easily,and reduced the strength enhancement effect caused by the same section size of the tunnel.However,the damage range gradually decreased with the increase of the angle.(5)The plastic zone model of the surrounding rock under different principal stress directions were established.It was found that with the increase of the angle between the principal stress direction and the tunnel,the plastic zone of surrounding rock was gradually close to the bottom and roof,and the range of plastic zone gradually decreased.However,the maximum depth of the plastic zone was always parallel to or nearly parallel to the direction of the minimum horizontal principal stress.When the principal stress direction was kept constant,the maximum depth of plastic zone shifted to the minimum horizontal principal stress direction with the increase of the vertical principal stress.(6)The influence of structural plane and principal stress direction on the failure of surrounding rock was simulated.The results showed that when the angle between the principal stress direction and the tunnel side was less than 90 °,the failure characteristics of surrounding rock was significantly affected by the structural plane;when the principal stress direction was perpendicular to the tunnel side,the failure of surrounding rock was mainly controlled by the principal stress direction.Based on the influence of structural plane and principal stress direction,the corresponding instability control technical of “structural plane strengthening + local priority strengthening support + full-face anchor net combined support” was proposed.