Research On The Performance And Evaluation Method Of Underground Structures At Contaminated Sites

Soil contamination-induced property variation and chemical corrosion can seriously threaten the service performance of underground structures in contaminated sites.As underground structures are buried in soil and ground water,the physical and mechanical characteristics of surrounding soils and the bearing capacity of the structures will be changed when the site environment is polluted.It is significant to clarify the evolution behaviour of performance of underground structures under the coupled condition of soil and structure corrosion deterioration and propose proper evaluation methods for the long-term service performance of underground structures in contaminated sites.This study,which focuses on the performance of underground structures in contaminated sites,is funded by the Key Project of Natural Science Foundation of China(41330641).The evolution behaviour and evaluation method of the performance of the underground structure in contaminated sites was systematically studied through theoretical analysis,laboratory tests,and numerical simulation.The major findings are concluded as follows:(1)The research progress on the performance of structures in salt contaminated sites as well as the evaluation methods were systematically summarized.The engineering characteristics of salt contaminated soil,the migration law of pollutants in structure concrete and surrounding strata,and the corrosion deterioration of structures were analyzed.The existing problems was critically reviewed.(2)The mechanism of microstructure of salt contaminated soil was examined,and the engineering characteristics of salt contaminated clayey soil with different pollutants and concentrations were studied.The results indicate,with the increase of salt contamination concentration,the pore volume decreases,the particle contact becomes tighter,and the agglomeration phenomenon of soil particles appeared.With the increase of the concentration of salt contaminates,the liquid limit and plastic index of clayey soil increase,and the compression coefficient decreases,the compressibility of both silty clay and clay decreases due to salt contamination.With the increase of salt contaminants concentration,the electrical resistivity of clayey soil decreases accordingly.In this study,the relationship between c,?,a_1-2 and the salt contaminants concentration is established.The plastic index,compression coefficient,cohesion and internal friction angle of contaminated soil are highly correlated with electrical resistivity,and the sensitivity of electrical resistivity parameters to salt contamination is stronger than that of physical and mechanical parameters.(3)Laboratory immersion test of reinforced concrete in chloride and sulfate solution was carried out to study the influence of crack width,depth,and corrosion time on the compressive strength and corrosion of reinforcement in corrosive salt environment,and the calculation method of time-varying damage degree of concrete with crack in sulfate corrosion was established.The results indicate that the existence of cracks in concrete structure changes the mode of structural compression failure,and the specimen becomes more fractured when subjected to compression failure after sulfate attack,the compressive strength of the specimen with 1mm and 3mm width cracks are reduced by 13%under the immersion conditions.The cracks reduce the compressive strength of concrete specimens,the compressive strength of the cracked specimen is reduced by 10%under the test conditions.Moreover,vertical cracks perform a greater influence on the compressive strength of specimens than horizontal cracks.The increase of crack width and depth accelerates the corrosion of reinforcement in concrete,and the effect of crack with width less than 0.05mm on the corrosion of reinforcement is not obvious.(4)By establishing the numerical model of chloride ion migration in cracked concrete and the strata surrounding the tunnel,the time-varying influence of crack width and hydrostatic pressure on chloride ion migration into underground structure concrete was revealed,the influence of leakage rate of lining crack and position of contaminant leakage on contaminant transport field in surrounding strata of lining was obtained.The results show that cracks with a width larger than0.1mm and hydrostatic pressure can accelerate the migration of chloride ion in concrete,and the frontier of chloride ion migration is deeper in the crack.The groundwater flow field in surrounding strata of the underground circular tunnel can be calculated by the solution of a steady irrotational flow of incompressible fluid around a cylinder.The pollutants are more likely to accumulated at the place towards the source side of pollutant plume,and the total pollutant concentration is higher.The horizontal and vertical distance between the leaking point of pollutants and the tunnel has a great influence on the concentration field of pollutants in the strata around the tunnel lining.When the leakage rate of a single crack/joint of lining reaches 5L/d,there is an obvious influence on the concentration distribution of pollutants around the lining.Five different pollutant distribution patterns in the surrounding stratum are summarized based on the numerical simulation results.(5)Based on the numerical model of underground shield tunnel and stratum under different contamination conditions,the long-term evolution behaviour of internal force of tunnel lining,longitudinal uneven longitudinal settlement,and convergence deformation of lining section under the coupling effect of soil corrosion–structural deterioration were further studied.The research results indicate that the influence degree of salt contaminates on the internal force of lining structure is positively related to the influence degree on soil and lining structure performance,and the axial force is most affected.Longitudinal settlement/uplift,uneven settlement/uplift of tunnel and convergence deformation of lining section increase with deterioration of soil and lining structure performance in salt contaminated environment.The variation of structural internal force,settlement and section convergence increase with corrosion time.The non-uniform distribution of pollutants in the surrounding stratum has a certain influence on the internal force of lining structure and the convergence deformation of lining section.For the bending moment and axial force of the lining structure,when the concentration of pollutants in the upper part of the lining is higher,the bending moment at the upper part of the lining is smaller than the uniform condition.The axial force distribution at lining vault and arch bottom increases on the side with high concentration of pollutants and decreases on the side with low concentration.The variation law of axial force at arch waist with negative bending moment is contrary to this,and the shear force of arch shoulder and lower part of arch waist is more affected.The non-uniform distribution of pollutants has effect on the vertical convergence deformation and transverse convergence deformation of the lining section,the convergence deformation increases with the pollutant concentration,and the horizontal ellipse of the deformation of the lining changes into an oblique ellipse with a certain angle to the horizontal line.(6)The performance evaluation methods of the underwater and underground tunnel in salt contaminated sites are established by using fuzzy-AHP method and reliability method,respectively.Based on the Nanchang Honggu Tunnel project,the health monitoring system for the immersed tunnel in the inland river is designed and constructed.Furthermore,the monitoring,detection contents,and methods for deteriorating of structural material are proposed.In view of the potential environmental corrosion that the tunnel maybe encounter in the future,environmental factor and structural material deterioration factors including thickness of the siltation,concentration of pollutants,loss rate of reinforcement section,the strength of concrete,and thickness of tube section are included.The index evaluation criteria and the evaluation model of the fuzzy-comprehensive evaluation method for the immersed tunnel in corrosion environment are established.Aiming at the reliability calculation of lining section of underground tunnel in polluted and eroded environment during long-term service,the failure modes of bearing capacity of assembled shield tunnel lining are divided into main section failure and joint failure.The bearing capacity functions of lining structure are constructed,and the calculation method of section failure probability under conditional probability are given.Considering the change of structural mechanical characteristics after lining material deteriorated and stratum polluted,Monte Carlo simulation is used to calculate the probability of full sectional compressed and partial sectional compressed of the lining section as well as the height of the concrete compression zone of the section as the basis of calculation of section load effect.The joint is equivalent to a non-homogeneous continuous beam structure,and the calculation model of concrete stress and bolt tension of joint structure connected with double-layer bolts is derived.The proposed method is verified based on the Shanghai Dapulu Tunnel project.