Study On Durability Influence And Countermeasures Of Train-Induced Load On Cast-in-Situ Concrete In Underlying Tunnel

At present,many cities in China have a large demand for the construction of urban rail transit systems and utility tunnels,which means that a large number of newly-built tunnel projects that pass underneath railways and roads will inevitably appear.During the construction of these projects,if cast-in-situ lining is adopted,the train dynamic load in normal operation will inevitably continue to affect the cast-in-place formwork concrete.At an early age,the concrete slurry in the formwork will be subjected to continuous and intermittent train vibration.Vibration loads will affect the inside cement-stone structure during curing,and change the continuity and cohesion.Excessive vibration will make the internal defects expand and connect,thereby affecting the performance of the concrete.Therefore,it is necessary to study the effect of railway dynamic load on the durability of cast-in-situ concrete in tunnels.In this paper,relying on the new large-section tunnel of Chengdu Metro Line 1 to pass underneath the existing railway,numerical simulation,laboratory experiments,and theoretical analysis were comprehensively used to evaluate changes in macroscopic impermeability performance and microscopic pore structure of concrete after being subjected to train dynamic load at an early age.The corresponding countermeasure of materials was also studied.The main research contents and results of the paper are as follows:(1)The tunnel-stratum-track calculation model was established to study the influence of railway dynamic load on the train-induced response of the underlying tunnel structure.It was determined that the cross-section vault was the most unfavorable response position.The longitudinal and vertical influence range of train-induced vibration on tunnel structure was proposed,and the division interval of the range was proposed based on the distance from the intersection area and the depth of the tunnel.(2)Relying on the actual project,mimicking actual conditions to design and carry out the vibration experiment of early age concrete,using the macro-penetration experiment and the microscopic pore structure experiment to study the vibrated concrete,the various rules of the permeability of concrete under different vibration levels,such as water permeability resistance,capillary water absorption performance,and electric flux,were obtained,and the dynamic load influence intervals based on different impermeability properties were determined.Based on the dynamic load influence interval and sensitivity of various impermeability properties,it was determined the quasi-critical point where the durability of concrete was affected by train vibration,divided the gain and damage interval of the dynamic load impact durability performance,and established a permeability-based durability evaluation of vibrated concrete.(3)The changes in concrete micromorphology and pore structure under different vibration levels were studied.Combined with qualitative analysis of cement stone morphology and quantitative analysis of pore structure changes,the effect of dynamic load on the changes of large,medium,and small pores of concrete was studied,and the self-healing mechanism of vibrated concrete was obtained.Through the study of the pore size distribution of full size,the corresponding relationship between pore size and concrete performance was obtained.Meanwhile,combined with the change of macroscopic properties,the micro-level reasons why the dynamic load changes the macroscopic properties of concrete were found.(4)Based on the theory of fiber-reinforced,the macroscopic properties and microscopic experiments of fiber-reinforced concrete were designed.The mechanics,permeability,and pore structure changes of different types of fiber-reinforced concrete after vibration were studied.The vibration reduction effect of fiber reinforced concrete was discussed,and reinforcement and vibration absorption mechanisms of various types of fiber were further obtained.Combined with the comparison of various properties and pore structures,the advantages and disadvantages of vibration absorption of fibers were evaluated.Based on this,it is proposed to use fiber-reinforced concrete as a material countermeasure to solve the change of concrete vibration durability,and the comprehensive effect is best with basalt fiber reinforced concrete.