Study On Mechanical Properties Of Supporting Materials And Constitutive Model Of Structural Interface In High Geothermal Tunnels

In recent years,with the expansion of scope of human activities and the rapid development of tunnel construction technology in China,tunneling engineering are increasingly characterized with deep bury,long length and crossing complex geological mountains.In areas with strong geological effects,the impact of adverse geological phenomena such as high geothermal on the safety of tunneling engineering has become more and more significant.When the tunnel is buried deeper or crosses the thermal conductivity fault,the initial temperature of the surrounding rock will rise significantly.This kind of tunnel is called high geothermal tunnel.In order to keep the comfort of the working environment,it is necessary to take several cooling measures.At this time,the condensation and hardening of support materials and structure interface have to go through the temperature change process from high temperature to normal temperature,coupled with the influence of the relative humidity,causing the degradation of mechanical properties and reduction of bearing capacity.However,there are still some problems in the current research of high geothermal tunnels at home and abroad.On the one hand,the constant temperature maintenance method is used to study the mechanical properties of the support materials,which is not consistent with the real change law of temperature in the high geothermal construction,and the coupling effect of temperature and humidity is also less considered.On the other hand,the changes of environment during excavation and their influence mechanisms are not clear enough.Moreover,there are few systematic basic research results on the nature of support materials and interfaces of support structures used in high geothermal tunnels.Therefore,literature research,field test,indoor experiment,theoretical analysis and numerical simulation were obtained to study the mechanical properties of supporting materials and constitutive model of structural interface in high geothermal tunnel.The paper conducted a large number of research tests and finished the theoretical analysis on the change of environment and the evolution of temperature field in high geothermal tunnels,and then proposed a variable temperature maintenance model for the support materials used in high geothermal tunnel.In the scope of this study,the mechanical and microscopic properties of cementitious materials,shotcrete-rock interface,rock-mortar-anchor interface and steelconcrete interface were studied.The following results were achieved in the study.(1)Based on the characteristics of environment and variation law of temperature field in high geothermal tunnels,the main influencing factors of high geothermal environment were identified.Based on the unsteady thermal conduction differential theory and the finite difference method,the theoretical calculation model of temperature field in high geothermal tunnels was established.The temperature variation law of surrounding rock and supporting structure was obtained,and the variable temperature curing(VTC)method for supporting structure material applied to high geothermal tunnels was put forward.(2)Through laboratory mechanical strength test,SEM,XRD diffraction,TGA and other microscopic tests,the mechanical properties and influence mechanism of supporting materials under VTC condition in high geothermal environment were studied.The calculation formula of strength of supporting material considering the coupling effect of temperature and humidity in high geothermal environment was proposed.(3)The shear test and SEM test on the interface of shotcrete-surrounding rock under VTC condition were conducted to study the mechanism of bonding damage and shear failure characteristics of the interface of shotcrete-surrounding rock composite support structure under high geothermal environment.The variation law of shear mechanical properties and characteristic parameters of shotcrete-surrounding rock interface with normal stress and temperature was obtained.Based on the principle of random damage mechanics of concrete,the interface shear constitutive model of shotcrete-surrounding rock in high geothermal tunnel was obtained.(4)The pull-out test and CT scan test on the interface of anchorage system(surrounding rock-mortar-anchor)under VTC condition were conducted to study the failure mechanism and failure mode of anchorage structure in high geothermal environment.The variation of mechanical properties of anchoring system under the coupling effect of temperature and humidity was investigated.Based on the mechanical deformation characteristics of the grouting material and the differential equation of elastomer equilibrium of the anchorage structure,the shear slip constitutive model of the full-length bond bolt interface considering the residual strength in high geothermal tunnel was established.(5)The push-out test on the interface of steel-concrete under VTC condition was carried out to study the interfacial bond failure mechanism and crack evolution law of steel-concrete under high geothermal environment.The change of mechanical properties of steel-concrete under the coupling effect of temperature and humidity was obtained.Based on the mechanism of interface bond slip and the mechanical deformation characteristics of shotcrete,a parallel mechanical model of elasto-friction element based on random damage mechanics was proposed,and a bond slip constitutive model of steel-concrete interface considering residual bond strength in high geothermal tunnel was established.