Stability And Safety Thickness Study Of Surrounding Rock For Tunnel Construction In Water Rich Karst Area

With the continuous investment of Chinese transportation infrastructure,the field of tunnel and underground engineering develops rapidly.It is inevitable to build in the western region of China with complex geological terrain,and the problem of karst disaster is becoming increasingly serious.During the construction of karst tunnel,the collapse,mud and water inrush are often caused due to the instability of surrounding rock,which has a great impact on tunnel construction.Therefore,how to ensure the stability of surrounding rock and reasonably determine the safety distance is particularly important.Taking Yangzong tunnel as the research case,stability of surrounding rock and safety distance between karst cave and tunnel are studied through the methods of theoretical analysis and numerical calculation,so as to provide reference for the safe construction and disaster prevention in water rich karst tunnel.The research work and achievements are as follows:（1）This paper expounds the formation and development mechanism of karst.The development of karst is mainly affected by stratum lithology,geological structure,topography,groundwater and natural conditions.Karst types are divided according to different morphological characteristics.Dozens of typical geological disaster cases of karst tunnels are analyzed statistically,and the disaster modes of karst tunnels are divided according to different damage characteristics.Taking Yangzong tunnel as a case,the general situation of engineering geology,meteorological and hydrogeological conditions,karst distribution and characteristics are comprehensively studied,which provides a basis for the study of karst tunnel construction and disaster prevention.（2）Based on the bending and shear strength criteria,the formula of safety distance is established when the karst cave is located at the top,bottom and side of the tunnel.The orthogonal test（48 test schemes）is carried out by using the numerical simulation software,and the formula of the safety distance between the tunnel and karst cave at different positions is established by regression analysis.The results show that the safety distance increases with the increase of surrounding rock level,cave water pressure and cave size,and its comprehensive influence degree can be ranked as follows:surrounding rock level>cave water pressure>cave size.The safety distance prediction model is applied to Yangzong tunnel,and the results show that the safety distance predicted based on strength theory is conservative,and the safety distance based on numerical test is consistent with the designed value.（3）FLAC^3Dsoftware is used to establish a three-dimensional numerical model to study the influence of tunnel excavation on the stability of surrounding rock when the karst cave is located in front of the tunnel.The stress distribution characteristics of surrounding rock are that the surrounding rock is compressed as a whole,the surrounding rock of vault and invert are in low stress area,and the surrounding rock from arch waist to the wall foot are in high stress area.The displacement distribution characteristics of surrounding rock are that the surrounding rock of vault subsides downward,invert rises upward,and the tunnel face moves to the free face.The pore water pressure distribution characteristics of surrounding rock are that the surrounding rock near the tunnel face is in the low value area of pore water pressure,and the seepage phenomenon of tunnel face is obvious.The stress,displacement and pore water pressure of surrounding rock on the monitoring section will have a sharp mutation in a certain range of excavation.The higher the grade of tunnel surrounding rock（1～4）,the greater the stress release rate and displacement of surrounding rock,the earlier the plastic zone is connected,and the safety distance is 2～7m.The higher the water pressure of karst cave（0.6MPa～1.8MPa）,the greater the stress release rate of surrounding rock,the greater the axial displacement,the greater the dissipation rate of pore water pressure,and the earlier the plastic zone is connected,the safety distance is 3～6m.The results show that the face excavation method is easier to lead to the instability of surrounding rock than the bench excavation method,and the safety distance is 4～6m.（4）From the perspective of karst disaster prevention,the geological detection technology,safety monitoring technology,risk assessment method and disaster early warning method of karst tunnel are studied.From the perspective of karst disaster treatment,this paper studies the grouting water plugging technology,drainage pressure technology and bypass crossing technology.Based on the case of Yangzong tunnel,according to its geological conditions and karst development characteristics,the specific preventive measures such as advanced geological prediction,construction optimization,monitoring measurement and disaster warning are analyzed,and the specific treatment measures are analyzed according to different water inflow,different types and locations of karst caves.