Research On Crack Evolution Law Of Secondary Lining Of Operating Tunnel

The secondary lining of the tunnel is subjected to external loads during operation,resulting in a variety of diseases,such as concrete deblocking,cracking,etc.Among them,crack is the most common disease of the lining,which can endanger the safe operation of the tunnel in severe cases.Therefore,applying various external loads to linings with different cracking degrees to study the stress and deformation of the lining under external loads,and to know the expansion process of initial cracks.These are very important to the safety judgment of the tunnel.Firstly,using the extended finite element method in ABAQUS to simulate the lining crack test.Comparing the crack propagation process,the lining deformation trend with the test results to verifies the reliability of using the extended finite element to simulate the lining crack.Furthermore,studying the stress distribution,deformation,crack initiation and propagation process of the secondary lining with different cracking states when subjected to the relaxation load of the vault surrounding rock.Under this load,the inside of the horseshoe-shaped lining vault and the outside of the vault are stretched,and the outside of the vault and the inside of the vault are compressed.And multiple cracks occur in the complete lining,which is in contrast to the complete oblate lining that only generates a single crack in the existing research.When the surrounding rock relaxation load range is expanded from 20° to60°,the critical cracking load of the lining structure is reduced by about 75%,and the ultimate bearing capacity is reduced by about 78%.Under the same surrounding rock relaxation load range,the initial crack depth of the lining becomes deeper,the smaller the corresponding critical cracking load and ultimate bearing capacity of the lining.When the initial crack depth is 0.5 times of the thickness of the lining section,the critical cracking load and ultimate bearing capacity of the cracked lining are reduced by 80% and 78% respectively.Secondly,analyze the influence of train static and dynamic load on the lining.The applied loads mainly change the deformation and stress distribution of the lining wall foot and invert,and the dynamic load has a greater impact on the lining.Change the axle load and speed of the train to find that when the speed is 100 km/h or less and the train axle load is 25 t,the vertical acceleration at the lining invert is lower than the specified acceleration limit.When the axle loads are increased to 30 t and 35 t,the vertical accelerations of the inverted arch exceed the specification limit by about16% and 35%;when the driving speed is 120 km/h,the vertical accelerations of the invert arch under the three axle load values are greater than the specification limit.Therefore,it is necessary to reasonably set the axle load and speed of the train in the actual project to ensure the safety of the structure.For cracked linings,the section safety factor decreases with the increase of the crack depth.When the initial crack depth is 0.5 times the section thickness,the section safety factor is reduced by about57.6% compared with the complete lining.Finally,the aerodynamic effect caused by the high-speed train entering the tunnel,the combined effect of air pressure and the dynamic load of the train are simulated.Select three monitoring points on the lining vault,side wall and foot of the wall.When the aerodynamic effect manifests as pressure,the compressive stress values at the three monitoring points increase;When the aerodynamic effect manifests as tension,the compressive stress values at the three monitoring points decrease.The faster the train travels,the more obvious the air pressure changes due to aerodynamic effects,which could lead to greater acceleration of the lining structure.Under the simultaneous action of aerodynamic effect and train dynamic load,the stress changes at the vault and side wall are similar to the stress changes under only aerodynamic effect,but the stress change at the foot of the wall is obvious,which is 20 times the stress only under aerodynamic effect.The above shows that the effect of train vibration on the lining stress is more significant.Under the two cyclic external loads,the deeper the initial crack depth,the smaller the number of loads required for its further expansion.At the same crack depth,when there is a dynamic train load,the number of loads required to crack the lining is about 25% of that when only aerodynamic effect occurs,indicating that the dynamic train load has a greater adverse effect on the lining than the aerodynamic effect.