Analysis And Numerical Simulation Of Diversion Tunnel Lining Structure Defects Under Long-term Service

To address the issue of regional water resource scarcity,China has constructed numerous water diversion tunnel projects since the 1970 s.However,with the passage of time and aging of structures,many lining structures of these tunnels have experienced issues such as cracks,voids,and water seepage,posing serious threats to their structural safety.Water diversion tunnels,being buried underground and having longer blind operation periods,receive less attention compared to other hydraulic structures.Currently,there is a lack of clear understanding regarding the laws of tunnel diseases and their impacts on structural performance,making it difficult to accurately assess the actual working condition of lining structures.Therefore,conducting safety stability analysis and comprehensive evaluation of long-term operating water diversion tunnels is of significant engineering value to ensure their safe operation.Based on a large-scale inter-regional water diversion tunnel project in Shenzhen,which has been in operation for over 20 years.The research methods employed in this study include current condition investigation,specialized testing,and three-dimensional numerical simulation,to study the distribution patterns of diseases in concrete lining,analyze the causes of differences in crack density,and analyze the mechanical characteristics of the lining.The main innovative research findings are as follows:(1)Based on the investigation of defect distribution patterns on site,the study reveals the patterns of occurrence,types,quantities,and severity of defects in concrete lining of water diversion tunnels.The study finds that lining cracks are the most common and severe defects in tunnels,and proposes "crack density" as an index for evaluating the severity of lining cracks in tunnels.The results show that crack density varies with different surrounding rock grades.(2)A comprehensive evaluation of the safety and stability of a long-serving water diversion tunnel was conducted using rebound and core extraction methods to analyze the compressive strength of concrete,as well as ground penetrating radar(GPR)to analyze lining and surrounding rock voids.The influence factors on the variation in crack density in different sections of the tunnel were investigated from the perspectives of material properties and external loads.The results indicated that,except for groundwater quality not meeting Class Ⅲwater quality standards,the engineering quality of the water diversion tunnel met requirements and the overall operational condition was satisfactory.Weak geological formations,concrete properties,and external water pressure were identified as factors affecting the variation in lining crack density in different sections of the tunnel under long-term service conditions.(3)To further reveal the controlling factors of crack density differences in water diversion tunnels under different surrounding rock grades,three-dimensional numerical simulation analysis of typical sections of lining structures is conducted.The results show that the lining structure overall meets the requirements for safe operation,but there is stress concentration at the arch foot of the lining,indicating potential safety hazards.The main controlling factors causing differences in crack density are initial ground stress,lining thickness,and external water pressure.(4)Based on field investigations and statistical analysis of the occurrence and distribution patterns of cracks in tunnel linings,this study utilized the weak layer elements in Ansys finite element software to construct a three-dimensional numerical model of cracked linings.The influence of changes in crack locations on the safety of lining structures was evaluated.The results indicated that the bearing characteristics of cracked linings met the requirements for safe operation.Among the cracks,shoulder cracks exhibited the highest tensile stress values and ranges,posing the greatest threat to the structure.Crown cracks showed the highest shear strain increment,indicating a higher risk of sliding and potential dangers of collapse and block falling,which could compromise the safety of the lining structure.The research findings presented in this article contribute to a thorough understanding of the damage patterns and formation mechanisms of lining structures in long-term service diversion tunnels,the mechanical characteristics of lining structures under varying surrounding rock conditions,and the load-bearing characteristics of lining structures with crack defects.These findings provide valuable theoretical references for further research on the mechanical properties of reinforced concrete linings in long-term service diversion tunnels,as well as the prevention and mitigation of damage to lining structures in such tunnels.