Study On Characteristics Of Shotcrete Under Sulfate Attack And Its Influence On Tunnel Stability

With the continuous development of China's economy,transportation infrastructure has played a positive role in promoting the national economy.However,it has also been found that during the construction of the tunnel,especially in the construction of urban tunnels,it is often encountered that the underground environment is chemically corroded due to urban garbage pollution.Under such conditions,the tunnel lining is easily affected by the corrosive factors in the environment and cannot reach its proper mechanical properties.This puts the tunnel at a certain security risk.In recent years,tunnel lining corrosion damage caused by environmental problems such as urban pollution has become a hot spot in tunnel research.This paper relies on the natural science fund project “Study on the corrosion mechanism of landfill leachate to tunnel support structure under multi-field coupling(Project No.: 51576091)”.The shotcrete in the initial support structure of the tunnel was selected as the research object.According to the results of tunnel water chemical analysis,a major pollutant,sulfate,was used as the corrosion factor.Comprehensive research methods and methods using on-site “spraying large board” sampling,laboratory experiments,statistical analysis,theoretical analysis,and indoor computer simulation.Study on the deterioration of mechanical properties of tunnel shotcrete under sulfate corrosion environment and the effect on the overall stability of tunnel after deterioration.The main research work and results are as follows:(1)The mechanical experiments of early shotcrete were carried out and the internal corrosion factors of sulfate were introduced in the experiment.The experimental results show that the change of curing time has significant effects on the mechanical properties of shotcrete,including elastic modulus and peak strength.The different types of sulphate corrosion(chemical and physical)have a significant impact on the mechanical properties of the material.Through the fitting of experimental data,the formula of the mechanical parameters of early shotcrete changes with time,that is,the time hardening function of mechanical parameters is obtained.For early shotcrete subjected to internal corrosion of sulfate,not only the final hardening property of the material is damaged,but also the hardening law of the mechanical properties of the material is affected by sulfate corrosion.The effect can be expressed by adding a parameter representative of the sulfate concentration to the time hardening function.(2)The long-term immersion corrosion test of sprayed concrete was carried out.Combined with the mechanical test after corrosion of the material and the results of electron microscopy,the following conclusions were drawn.The type of sulphate corrosion of shotcrete can be divided into two types according to the concentration of sulphate solution,namely chemical sulphate attack(CSA)and physical sulphate attack(PSA).The mechanical properties of shotcrete under these two types of corrosion show different changes in both the early hardening process and the long-term corrosion weakening process.(3)The mechanical mechanism of the effects of chemical corrosion and physical corrosion of shotcrete on sulfate was analyzed by the theory of damage stress of materials.The results show that in the early stage of corrosion,although the two types of corrosion have not caused significant changes in the mechanical parameters of the material,the damage stress of the material has different trends.This means that the two types of sulfate corrosion have different damage to the material.Chemical attack is not obvious to the material in the early stage of corrosion,and it shows an exponential increase as the corrosion time increases.Physical attack,from the early stage of corrosion,shows a rapid linear trend in material damage.(4)Shotcrete exhibits obvious elastoplastic failure characteristics under long-term sulfate immersion corrosion conditions.According to the results of the indoor triaxial test,the Mohr-Coulomb mechanical parameters of the material were fitted to obtain the function of the mechanical parameters corresponding to the physical corrosion concentration and corrosion time of the sulfate,and the constants in the function were calibrated.(5)The nonlinear elastoplastic constitutive model of shotcrete is obtained by adding the time hardening function of the material to the traditional Duncan-Chang model and the strain softening model based on the M-C yield criterion.The formula for solving the model parameters is given.The model is verified by the indoor mechanical test results of shotcrete.It is considered that the elastoplastic constitutive relation can basically describe the stress-strain condition of the shotcrete material compared with the experimental value.On this basis,the damage evolution equation and damage constitutive model of long-term immersion corrosion of sprayed concrete were established by introducing two types of corrosion,chemical corrosion and physical corrosion.(6)The finite element software was used to simulate the hardening process of the shotcrete during tunnel construction and the material weakening process under long-term corrosion of sulfate.The results show that it is more realistic to consider the hardening process of shotcrete during tunnel excavation.There is a significant correlation between the change of the characteristics of the sprayed concrete material caused by long-term corrosion of sulfate and the initial support effect of tunnel shotcrete.The change of stress and displacement of the tunnel indicates that the chemical corrosion and physical corrosion of sulfate have different effects on the stability of the tunnel.