Study On The Concrete Thermal Stress And Crack Control Of A Tunnel Final Lining

The secondary lining of the tunnel will be deformed due to temperature changes at the beginning of pouring,temperature stress occurs when deformation is not constrained by large constraints,the greater the degree of constraint,the greater the temperature stress,this is one of the important reasons for the early circumferential and longitudinal cracks in the secondary lining.Some cracks exist on the surface of the structure,and some cracks are penetrated,especially the penetrating cracks are the most harmful to the structure.Penetrating cracks can easily lead to leakage of water,steel corrosion,etc.,seriously threatening the durability of secondary lining.Therefore,the study on the constrained tensile stress caused by the early shrinkage deformation of secondary lining concrete is of great significance for evaluating the characteristics and trend of secondary lining cracking.According to the calculation formula of the early temperature stress of the tunnel secondary lining established by the theory of elastic foundation beam,it is found that if the position of the outer surface of the secondary lining is subject to the same degree of restraint,The maximum temperature stress will occur at the center of the lining ring,which is the most likely crack at this location,However,some actual engineering cracks do not appear in the vault position.Therefore,it is necessary to study the position of the maximum temperature stress of the secondary lining from the perspective of restraint based on the field measured data.And the simulation analysis model of temperature and temperature stress of secondary lining is established by finite element method.The temperature stress and crack development law of secondary lining under different restraint conditions are analyzed.The main research results obtained are as follows:(1)The early temperature stress calculation formula of tunnel secondary lining lining established according to the theory of elastic foundation beam fully reflects the influencing factors of early temperature stress,considering the stress relaxation caused by the shrinkage and creep effect of early concrete under beam conditions.The calculation results can be used to evaluate the early cracking characteristics of the secondary lining.(2)Early temperature field and stress analysis model of secondary lining was established by using finite element software,the model can simulate the early temperature and stress changes of secondary lining under different constraints.The simulation results are in good agreement with the calculated results based on the measured data,can be used for early temperature stress prediction and cracking risk assessment of secondary lining.(3)The position of the through crack appears at the maximum extent of the secondary lining constraint.When the secondary lining vault has a void phenomenon,the maximum position of the secondary lining curvature is most constrained.(4)When the waterproof sheet and the geotextile are laid between the outer surface of the secondary lining and the initial support,the outer surface of the secondary lining is cracked and the inner surface is not cracked.When the outer surface of the secondary lining is in direct contact with the initial support,the secondary lining will have a through crack.(5)Without changing the concrete mix ratio and labeling,the method of reducing the concrete pouring temperature combined with the reduction of the template constraint as much as possible can effectively and conveniently control the development of the crack,and can achieve secondary lining without cracks.If the secondary lining is in direct contact with the initial support,even if this measure is taken,the secondary lining will have a through crack.