Stability Of Surrounding Rocks Of The Large Deep-buried Tunnel In Fault Fracture Zone

Southwest China is characterized by mountainous terrains.During the construction of tunnels that go through the mountains,these buried structures will experience serious landslide,pass through fault fracture zone,encounter debris flow.These structures will also suffer several construction accidents,e.g.,lining cracking and sinking,roof collapse and roof fall.Among them,the fault is one of the relatively common adverse geological phenomena that cause engineering accidents such as tunnel collapse,water inrush,and mud inrush,etc.Therefore,during the excavation of large deep-buried tunnels in the fracture zone,the study of surrounding rock stability is of great significance for the construction safety of underground engineering,and helpful to predict some accidents and propose control measures.In this paper,based on the project of main transformer tunnel of Wudong Hydropower Station,ABAQUS finite element software is used to analyze the stability of surrounding rock of large deep-buried tunnel in fault zone,from which the displacement variation and stress distribution of surrounding rock are obtained.The influence of the characteristics of fault fracture zone,construction method,tunnel section shape and other factors on surrounding rock deformation is analyzed.The main research results are as follows:(1)When the fault passes through the arch foot of the tunnel,the convergent displacement and settlement of surrounding rock are smaller than those at the arch top,shoulder and waist,so the tunnel excavation on the upper fault is more conducive to the stability of the surrounding rock.The stress value and settlement displacement value of tunnel surrounding rock increase first,then decrease and then increase after passing the faults with different dip angles.At 0° and 90°,the settlement of vault and the uplift of arch bottom are great,which is not conducive to tunnel excavation,and the supports should be strengthened.(2)The stress distribution of surrounding rock of tunnels with different sections is roughly the same,and the stress concentration occurs in vault,arch shoulder,arch waist,arch foot and invert.When the circular and straight-wall circular arch tunnels passing the fault zone,the vault settlement value and invert uplift value of surrounding rock are smaller than those of the three-center arch tunnel,and the variation trend is more moderate.Considering the utilization of tunnel section,the tunnel can be excavated with straight-wall circular arch section.(3)There are great differences between step-excavation method and full-section method in construction process.Considering the efficiency and safety of tunnel excavation,the step-excavation method is superior to the full section excavation method.(4)According to the geological conditions of the main transformer tunnel of Wudong Hydropower Station,select the appropriate construction method.The stress and deformation of surrounding rock during tunnel excavation are analyzed by using the numerical simulation,and the settlement of tunnel structure is within the allowable range of construction safety.At the same time,the simulation results are basicallty consistent with the monitoring data.The rationality of the tunnel construction and support of the main transformer chamber is confirmed.