| With the development of economy and society,the demand for energy and mineral resources is increasing day by day,and many deep underground projects such as water conservancy and hydropower,transportation projects,and coal mining are gradually increasing.Deep underground projects have become an important part of the national innovation driven strategic development,and the development of underground engineering construction towards deep areas has become an inevitable trend.In this paper,based on a detailed analysis of the test data of the disturbed stress field of the Phase II project of the Jinping Underground Laboratory,the variation laws of the disturbed stress field under different construction processes are interpreted,and the variation laws and patterns of the principal stress values and directions are analyzed.Through comparison and analysis of in-situ borehole photography of rock masses,the influence of perturbation stress changes on damage and fracture of surrounding rock is revealed,and the quantitative value of perturbation factors in the Hock Brown strength criterion is discussed based on measured perturbation stress data.The main research contents and achievements of this paper are as follows:(1)Deeply analyzed the real situation of three-dimensional disturbed stress changes during the excavation process of the Jinping Underground Laboratory Phase II Project,studied and summarized the changes in stress fields at different depths of measurement points and different excavation time points in the rock body,explored the characteristics of spatial and temporal changes in disturbed stress during different excavation stages,and revealed the impact of changes in stress fields under deep rock excavation disturbances on rock mass damage and fracture,The research results show that:(1)The variation of disturbed stress value is affected by initial crustal stress,tectonics,advance distance of working face,depth of measuring point and excavation method,basically showing a trend that the deeper the depth of measuring point,the smaller the degree of stress change.The maximum impact range of the excavation process of the middle pilot tunnel on the radial rock mass of the chamber is about 12.5m,and the maximum impact range of the expansion process of the side wall on the radial rock mass of the chamber is about 8.5m.Disturbed stress has a significant face effect,with significant differences in principal stress changes under different face advancing distances.The influence range of excavation process in the axial direction of the chamber is between 15 m and 20 m.(2)The response degree and response mode of the disturbance stress to excavation blasting vary in different excavation stages.The disturbance stress in the middle pilot tunnel excavation stage will have a significant "ladder like" increase with the progress of excavation blasting,and the stress value of each "ladder like" increase will gradually increase with the increase of blasting times;During the expansion stage of the side wall,the disturbance stress will also exhibit a significant steep rise(drop)phenomenon at the time node of excavation and blasting,and the disturbance stress will exhibit a similar "U-shaped" change.(3)During the excavation stage of the middle pilot tunnel,the maximum principal stress continuously increases,and the radial unloading and tangential stress concentration caused by excavation result in the formation of a tensile stress zone in the shallow surrounding rock.A large number of tensile cracks develop and connect with each other in the rock near the free face,resulting in a series of tensile damages,such as V-shaped peeling damage and wedgeshaped plate damage of the rock mass.During the expansion excavation stage of the side wall,under the long-term effect of stress,the cracks in the surrounding rock continue to develop and expand,while some cracks and structural planes open,resulting in a reduction in the bearing capacity of the surrounding rock.When the accumulated damage reaches the critical point of failure,the surrounding rock is damaged,and the three-dimensional principal stress is significantly reduced.Therefore,measures should be taken based on the stress monitoring situation.When the side wall is expanded to the monitoring section,some cracks are re closed under the action of pressure.At this time,both the maximum principal stress and the intermediate principal stress in the rock body have rebounded,but the minimum principal stress value is still decreasing.The shear stress in the rock body increases sharply,leading to the development of a large number of shear cracks and inducing shear failure of the wedge body of the rock body.(2)Through in-depth analysis of the test data of the disturbed stress field of the Phase II project of the Jinping Underground Laboratory,the variation rules of the disturbed stress direction under different construction processes are interpreted,the mode and rotation rules of the disturbed stress rotation are summarized,and the impact of the change in the main stress direction on the damage and fracture of the rock mass is revealed through comparative analysis of in-situ borehole photography of the rock mass.The research results show that:(1)Disturbed stress rotation modes are mainly divided into two modes: stress rotation and stress gyration.The excavation stage of the middle pilot tunnel is the most active stage in which the disturbance stress direction rotates.During the process of advancing the excavation face towards the monitoring section,the rotation mode of the main stress direction is mainly stress gyration.During the process of advancing the excavation face away from the monitoring section,the change mode of the main stress direction is mainly stress rotation.The position with the largest rotation amplitude of the main stress direction is at the measuring point 4.5 meters away from the side wall;During the expansion excavation stage of the side wall,the change mode of the main stress direction is mainly stress rotation,and there are multiple special stress rotation modes of large angle stress gyrations.The position with the largest rotation amplitude of the main stress direction is at the measuring point 2.5 meters away from the side wall.(2)The change in the direction of disturbed stress directly affects the shape and trend of rock mass fracture development.Stress with large angle rotation can easily induce tension fractures and tension shear mixed fractures that deviate from the original fracture plane.In fracture morphology,it is manifested as the bifurcation of the original fractures and special fracture morphology with "protruding angles",which easily leads to the mutual penetration of surrounding fractures,leading to V-shaped peeling failure and wedge-shaped plate shaped failure of the rock mass.When rotating at a small angle,it is more likely to induce shear fractures with concave and convex shapes whose fracture surface deflection angle is equivalent to the stress rotation angle,and form a similar network of shear dislocation zones that are staggered with each other.The stress with large angle gyration often forms intersecting "X" type shear fractures,which easily lead to wedge shear failure of rock mass,leading to wedge collapse and shear type rock burst disasters.(3)Based on the test data of disturbed stress field,a disturbance degree evaluation index based on equivalent stress is proposed to explore the changes in the disturbance degree of rock mass during excavation.The value of the disturbance factor D in the Hock Brown strength criterion is quantified based on the second stress deviator invariant.The quantified value of the disturbance factor D is fast and convenient to obtain,and the calculated value of the disturbance factor D changes with the change of the disturbance stress.Compared with the subjective empirical value,it is more authentic,and more accurate rock mechanics parameters can be calculated accordingly. |
PDF Full Text Request