Mechanism And Long-term Safety Evaluation Of Water Damage In Operating Karst Tunnels

Considering China’s swiftly advancing transportation sector,the enduring operational safety of the lifeline tunnels is paramount.Water damage from the evolution of karsts affecting the surrounding rocks and tunnel structures causes significant concerns for these services.In this article,we spotlight intriguing water damage instances in Guangdong’s northern karstic region.Looking at these issues in light of the Shuang’ao and Yangbang Tunnels on the Beijing Hong Kong Macao Expressway,we’re conducting an in-depth investigation,utilizing engineering inspection,theoretical dissection,lab testing,and digital simulation for understanding how water damages evolve in active karst tunnels and their significance to operate these tunnels.We’ve engineered experimental cracking of linings and theoretical analyses,set up degradation models such as concrete and steel bar,and simulated long-term tunnel operations safety with incorporating karst evolution and lining decay,offering preventative measures against water damage in karstic caves.Key research content and achievements include:(1)Based on the characteristics of karst development and seasonal monsoon precipitation in northern Guangdong’s karst region,20 operational karst tunnels and 3prominent tunnel water hazard instances were examined.Analysis revealed that tunnel water hazards escalated primarily from May to July,Guangdong’s rainy season,when precipitation was intense and directly linked to severe rain;Karst was formed along the tunnel,exhibiting surface karst elements like debrisfalls,sinkholes,karst depressions and valleys,and underground karst features like under slab channels,underground karst cavities(caves)and underground rivers(springs).Limestone is the main lithology of the water damage in the operating tunnel,and the cracks are mainly circular cracks at the tunnel side wall and arch waist,up to 91.2%,and most of the water damage occurs at the tunnel side wall and inverted arch.The complexity of geological conditions and optimal negative terrain,coupled with elevated local water pressure due to heavy rainfall,deteriorated tunnel lining and drainage system failure under unique karst hydrological circumstances are the primary causes of karst tunnel water calamity.(2)Performing an analysis of the karst strata and operational tunnel composite system and their respective water hazard influences,it is evident that the water hazard endured by the karst stratum and operational tunnel progresses in a spiraling manner mirroring incidents such as lining cracking,water leakage,deformation,and even water and mud gushes.Exploring the karst development impact on the operational tunnel,two facets are considered: first,the macrocosmic effects of the karst geologic environment on the tunnel are evaluated based on surface subsidence,lithological boundary,geological structure,and karst passage;secondly,the meso-microcosmic influence of surrounding rock deterioration is factored in to evaluate the rock wall instability and the critical safety distance of water hazard on the rock wall.And the critical condition of joint fissure failure under conditions of fracture propagation failure and shear slip failure,as well as the critical safety distance between the existing water-enriched cavern and the tunnel,are calculated.(3)In view of the groundwater environment of the karst tunnel,the chemical composition of the groundwater and the corroded lining of the diseased tunnel is analyzed,and the concrete deterioration under the action of sulfate corrosion and dry-wet alternation,as well as the reinforcement corrosion deterioration under the action of sulfate and chloride compound corrosion are comprehensively considered,the tunnel lining deterioration test is designed and carried out.The change of surface morphology,mass,corrosion depth,relative dynamic modulus of elasticity and macroscopic mechanical properties of concrete members,the surface morphology of reinforced concrete members,the macroscopic crack law of concrete under corrosion expansion of steel bars and the corrosion characteristics of steel bars in reinforced concrete members are obtained.The test results show that the curves of mass change,corrosion depth change,relative dynamic modulus of elasticity and macro-mechanical properties of concrete members verify each other,and the four curves all show a slight upward trend in the early stage and a downward trend in the late stage.This can be categorized into two stages that encompass the "compacting effect"(or performance improvement phase)in the earlier period and the "cracking damage effect"(or performance deterioration phase)in the later period.The test results reveal that the reduction of concrete strength and the corrosion cracking of steel bars are the main reasons for the deterioration of the lining.(4)Based on the assumption of uniform corrosion of steel bars and the theory of thick-walled cylinder,considering that the two sides of the tunnel lining contact with surrounding rock and air respectively,the influence coefficient of surrounding rock parameters is used to unify the two contact modes,and the theoretical cracking model of thick-walled cylinder is improved,and the cracking model of concrete cover of tunnel lining is established.According To the elastic-plastic deformation theory of concrete,the deformation of lining concrete undergoes three stages of elastic,elastic-plastic and plastic limit,which are crack initiation(elastic),partial cracking(elastic-plastic)and complete cracking(plastic limit).The cracking model of concrete cover is simplified as a plane axisymmetric thick-walled cylinder problem,and the whole process analysis is carried out,and the formulas of stress,displacement and crack of lining concrete at different stages are derived.The Yangpeng Tunnel is taken as an example to verify the effectiveness of the model.Through the analysis of the influence coefficient of the surrounding rock parameters,it is theoretically verified that the incompactness or cavity between the lining and the surrounding rock will accelerate the deterioration of the lining.(5)Recognizing the direct recharge of surface water into the karst cavity via direct connection,the karst water enhances the rock wall and tunnel lining within conduits or weak fracture zones,triggering local high head pressure.The pipe-type karst form is summarized,and a generalized model of water disaster in operative karst tunnels is presented and numerically simulated.Based on the lining deterioration test and damage mechanics theory,the regression function analysis is carried out through the concrete compressive strength degradation curve and corrosion depth variation curve,and the time-varying deterioration model of lining materials(concrete,steel bar)is established,and the numerical calculation is carried out.The results show that the influence of lining deterioration on tunnel displacement and stress is significantly greater than that of surrounding rock deterioration.Taking the tunnel in service for 50 years as an example,the increase of vault settlement,invert bottom heave and haunch horizontal displacement caused by the deterioration of lining is 2.2 times,3.1times and 1.7 times of that caused by the deterioration of surrounding rock,respectively;while the increase of maximum vertical stress and maximum horizontal stress caused by lining deterioration is 6.2 times and 6.1 times of that caused by surrounding rock deterioration,respectively.According to the particularity of karst strata and the actual situation of operating tunnels,the prevention and control principle of karst tunnel water disaster is put forward on the basis of the general principle of tunnel water prevention and drainage,which is "combining prevention,drainage,interception and blocking,giving priority to drainage,local blocking to prevent accumulation".Based on tunnel traffic operation and disposal objectives,two types of karst water hazards and their disposal processes are suggested,along with water hazard prevention suggestions.