Study On The Water Inrush Mechanism Of Deep And Long Tunnel In Karst Areas

With the deepening of the western development of strategy,the national “the 13 th Five-Year Plan” and the “Belt and Road” initiative,major constructions such as transportation,water conservancy and hydropower have ushered new opportunities for development,which greatly promotes the construction of deep and long tunnel.China has become the country with the largest scale and difficulty in tunnel construction in the world.A large number of deep and long tunnel projects are gradually developing to the western mountains and the karst area in the extreme complex terrain.A series of karst-related disasters,such as water inrush,mud inrush,might be encountered during tunnel construction in karst terrain,which seriously threaten the safety of personnel and equipment at the construction site and cause serious economic losses to the country.The important reason for the difficulty of the major tunnel water inrush disaster control is that the occurrence and evolution mechanism of water inrush is extremely complex,and a clear and systematic understanding has not yet been formed,making it difficult to effectively guide the prevention and control of disasters.In this thesis,water inrush mechanism of deep and long tunnels in karst areas is taken as the main object of this study.Based on a deep and long tunnel project in China,through engineering site survey,model test,numerical simulation and theoretical analysis,the evolution process of water inrush,the failure mechanism of outburst-prevention rock mass and the criteria for disaster occurrence of deep and long tunnels in karst areas under the combined action of excavation disturbance and karst cave water pressure are deeply studied.There are some research achievements which process a certain theoretical value and engineering significance to the tunnel construction.The main works and achievements of this dissertation are listed as follows:(1)According to the relevant cases and literatures of water inrush disasters in karst areas,the types and influencing factors of water inrush disasters in deep and long tunnels in karst areas were summarized,and its disaster-causing mechanism was analyzed from the micro and macro perspectives.According to the development characteristics of karst caves,underground morphology,scale and positional relationship with the tunnel,a generalized mechanical model of the tunnel-karst cave system is established,which lays the foundation for quantitative research on the mechanism of water inrush in deep and long tunnels in karst areas.(2)The devices for making single karst cave and karst cave-structural plane were independently designed,and the problem that single cave,single structure surface and cave-structure surface were difficult to make in model test was solved,which provides a new idea for the construction of complex geological conditions in model tests.(3)Based on the water inrush disaster engineering example of a long tunnel in China,the model test of water inrush of the long tunnel under the geological condition of the single cave and the combined geological condition of the cavern-fissure zone was carried out,the whole process of rock destruction and water inrush occurred in the cavern by the joint action of excavation disturbance and cavern water pressure was simulated,and the changes in the precursor information such as displacement,stress,and seepage pressure occurred in the process of disaster change were obtained,which revealed the mechanism of disaster change in the whole process of rock destabilization.(4)In order to verify the validity of the model test,the secondary development of MatDEM discrete element software based on MATLAB language proposed a new method to construct single fissures and random fissure groups,solved the problem of generating different fissures,and provided an effective numerical simulation method for the simulation of deep and long tunnel burst water under the compound geological conditions of the cavern-fissure zone.The numerical simulation results show that the magnitude and change trend of displacement,stress,and seepage pressure were in good agreement with the model test results,which verified the validity of the model test.(5)A theoretical calculation method was proposed for solving the construction elasticity problem of circular tunnels in infinite elastic space in elliptical cavern geological conditions at arbitrary locations by using the Cauchy integral method and the Schwarz alternating method in the complex variable function,and an explicit expression was obtained for the analytical solution of the stress in the surrounding rock after tunnel excavation under the above geological conditions.Based on the explicit expression of the analytical solution of the surrounding rock stress,a calculation method combining Griffith’s strength criterion and MATLAB programming was further proposed to calculate the theoretical value of the minimum anti-burst rock thickness between small and medium scale hidden caverns and tunnels,and the theoretical value of the minimum anti-burst rock thickness was obtained.An engineering example verifies the rationality of the calculation method.(6)Based on the characteristic analysis of the water inrush process of the relying project,a conceptual model of the evolution process of water inrush hazard in deeply buried tunnels under cavern-fissure zone composite geological conditions was proposed,and the conceptual model was divided into four stages,the initial balance stage,the original fissure expansion stage,the water inrush channel formation stage,the water barrier rock rupture,and destabilization stage.The simplified mechanical model and criterion of water inrush in deep-buried tunnels under the compound geological environment of the karst cave-fissure zone were established by fracture mechanics,and the critical water pressure calculation formulas of pull-shear fracture water inrush and pressure-shear fracture burst water in deep long tunnels were obtained respectively.The engineering example show that the theoretical derivation of the critical water pressure calculation formula has good applicability and correctness.