Mechanism Of Karst Filling Deterioration And Lining Crack Of Operating Tunnel Under Dry-wet Cycle

Our country's soluble rock strata are widely distributed,accounting for nearly onethird of the country's land area.The special environment and geological forces have formed the filling karst geology with unique physical and mechanical properties.With the large-scale construction of tunnel projects in karst areas,more and more tunnels pass through the filling karst geology.The existence of the filled karst area will not only affect the stability of the surrounding rock,but also affect the stress state of the lining structure,and bring potential safety hazards to the construction and operation of the tunnel.In recent years,operating tunnels in karst areas have entered a period of high incidence of disease.Most of the disease locations are located in filled karst areas,especially in areas with significant seasonal rainfall.Diseases such as lining cracks and water leakage are more significant,which seriously threaten the safety of passing vehicles.Seasonal precipitation makes the surrounding rock of the filling karst area in an alternate dry and wet environment,and the strength of the surrounding rock is significantly degraded under the action of the long-term dry-wet cycle,and the external load acting on the lining changes significantly,which will then cause the lining to crack and damage.With the increasingly prominent problems of operating tunnel diseases and prevention and control,it is urgent to carry out research on the mechanism and treatment technology of lining cracks in filled karst areas under the action of dry-wet cycles,so as to provide theoretical basis and technical support for the construction and operation of tunnels in filled karst areas.Based on the key scientific and technological project of Jiangxi Provincial Department of Communications "Research on water inrush mechanism and comprehensive prevention and control technology of karst tunnel during operation",taking nanshibi tunnel of Wuji expressway as the engineering background,this paper comprehensively adopts the methods of field investigation,indoor test,theoretical analysis and model test to study the strength deterioration mechanism of karst backfill under dry wet cycle.A series of studies have been carried out on the calculation method of surrounding rock pressure in filling karst area and the stress characteristics of lining structure under dry and wet cyclic load.The main research results and conclusions are as follows:(1)Based on the types,characteristics and distribution of lining cracks during the operation period of the karst tunnel,combined with the filling karst incubation environment and occurrence characteristics,the main controlling factors of the lining cracks were determined,and the catastrophe model of lining crack in the filling karst area of the operating tunnel was established.(2)Taking the Nanshibi tunnel karst filling as the object,the strength indicators of the karst filling under the influence of the dry-wet cycle,including the cohesive force and internal friction angle,are studied.The study found that the cohesion and internal friction angle both decrease with the increase of the number of dry and wet cycles,and tend to be stable after a certain number of cycles.The influence of dry and wet cycles on cohesion is greater than the angle of internal friction.The degradation coefficient of the strength index with the dry-wet cycle is used to quantitatively study the degradation law of the karst filling under the dry-wet cycle,and the cohesive force degradation model of the karst filling and the internal friction angle degradation model are established.(3)Based on the theory of limit equilibrium,the calculation formulas for karst filling loose surrounding rock pressure are deduced for the two working conditions of tunnel full penetration of karst cave and tunnel top exposed karst cave.Combined with the model of the strength degradation of the karst filling under the action of the dry-wet cycle,the change law of the surrounding rock pressure of the tunnel filling in the karst area with the operating life,that is,the number of dry-wet cycles,is obtained:During the operation period,the pressure of the surrounding rock in the dry and wet seasons increases year by year,and the increase in the dry season is greater than that in the wet season.The pressure of the surrounding rock stabilizes after 6 years of operation;the pressure of the surrounding rock fluctuates sharply with the alternating dry and wet,causing damage to the lining structure.The cycle of loading and unloading accelerates the cracking process.(4)A new type of tunnel lining direct loading platform was developed to simulate the external load change of the lining under the influence of dry and wet cycles,and the lining crack model test was designed and carried out to study the internal force change and deformation during the cracking process of the lining structure under cyclic loading.The characteristics and the law of occurrence and development of cracks reveal the cracking mechanism of tunnel filling lining structure in karst area under the action of dry and wet cycle.(5)On the basis of the above-mentioned research results,a grouting reinforcement test for degraded karst fillings was carried out to verify the validity of the wet-dry cycle strength index degradation model,and a grouting suitable for filling karst areas in operating tunnels was proposed.Water-blocking reinforcement technology,which has been successfully applied to the Nanshibi Tunnel lining disease treatment project,can provide a certain reference for similar projects.