Research On Bearing Capacity And Failure Mode Of Different Support Structures In High Ground Stress Tunnels

With the continuous development of tunnel construction in China,an increasing number of tunnels need to pass through areas with high-stress weak surrounding rocks,which presents significant challenges for tunnel excavation and support work.This article focuses on the excavation and support process of high-stress soft rock tunnels and carries out excavation and overloading tests on the surrounding rock of high-stress tunnels.The study investigates the influence of medium-length and extra-long anchor rods on the bearing capacity of combined support structures.The main content and achievements of this paper are as follows:(1)Based on the geological conditions of high-stress soft rocks,tunnel excavation tests were conducted on medium-length anchor rod combined support structures and extralong anchor rod combined support structures.The study analyzed the arch crown settlement and surrounding pressure evolution laws of the two combined support structures during the upper and lower bench excavation method.The results show that:(1)After tunnel excavation,the initial stress field of the surrounding rock is disrupted,and the surrounding pressure fluctuates rapidly and increases continuously during excavation.The upper and lower bench excavations have significant differences in the disturbance of the surrounding pressure.For the extra-long and medium-length anchor rod combined support structures,the proportion of the surrounding pressure change in the total surrounding pressure change is 86.61% and 86.95% during the upper bench excavation,respectively.The disturbance of the upper bench excavation to the surrounding rock is much greater than that of the lower bench excavation.(2)The surrounding pressure changes after the completion of excavation for the extra-long and medium-length anchor rod combined support structures are 6.12 k Pa and 7.15 k Pa,respectively,which account for 15.91% and 18.59% of the initial stress(38.46 k Pa),indicating that the surrounding rock has a certain bearing capacity during excavation.(3)The excavation-induced surrounding pressure evolution laws of the two combined support structures are basically the same.The excavation-surrounding pressure curve stabilizes rapidly after the support structure is completed.This indicates that both combined support structures can effectively resist the pressure of the surrounding rock,and the range of the excavation’s influence on the surrounding rock disturbance is one excavation step.(4)The excavation-displacement laws of the two excavation conditions are basically the same.After monitoring,the excavation-displacement curve before the section has obvious fluctuations,and the arch crown displacement gradually stabilizes when it is far away from the monitoring section.However,the excavation-displacement curve of the extra-long anchor rod combined support structure is flatter,and the final deformation is also smaller.(2)The tunnel after excavation was subjected to overload testing to analyze the stressstrain evolution laws,deformation laws,and failure process of the support structure during the loading process.The results showed that:(1)When the ground stress is less than 64.1k Pa,the deformation,lateral pressure,and internal force evolution laws of the two combined support structures are very similar,and the effect of the long anchor rod on the bearing capacity of the support structure is not significant.After the ground stress exceeds 64.1k Pa,the improvement effect of the long anchor rod on the support structure gradually appears,and the improvement effect becomes more and more obvious with the increase of ground stress.After the ground stress exceeds 102.56 k Pa,the improvement effect of the long anchor rod gradually diminishes.(2)Both types of support structures experience a drop in confinement during loading,but the drop in confinement of the combined support structure with ultra-long anchor rods is lower,and the confinement value at the final failure is higher.(3)Both types of combined support structures exhibit a flat shape at failure,but the arch deformation of the combined support structure with ultra-long anchor rods is reduced by 19.26% at the crown and 27.67% at the spring line,and the failure occurs later,with a smaller range and less severity.(4)The buckling time of the arch of the combined support structure with ultra-long anchor rods is later,and the effective bearing capacity of the structure is 51.28 k Pa,while the effective bearing capacity of the combined support structure with ultra-long anchor rods is 64.1k Pa,an improvement of 24.27%.