Mechanical Properties And Design Method Of Supporting Structures Under Heterothermal Curing Conditions In High Geothermal Tunnel

When the tunnel is deep buried or passes through geothermal conductivity fault,the surrounding rock temperature will increase significantly.As a result,the air temperature during tunnel construction will exceed 28?.Generally,such tunnels are defined as high geothermal tunnel.The construction of high geothermal tunnel encountered some difficulties,for example,the change of temperature field in surrounding rock is difficult to predict,the high-temperature resistance characteristics of construction materials is unknown,and the stress of support structure is complex.With the emergence of more and more high geothermal tunnels at home and abroad,the requirements of design standards become more urgent.However,at present,with regard to high geothermal tunnels,systematic research results are few,the understanding in the mechanism of high-temperature effect is not clear,also uniform design method and control standard have not been formed.Therefore,in this paper,we study the mechanical properties of support materials under the heterothermal conditions and structural design method in high geothermal tunnels,and this research is significance to the design,construction and operation for high geothermal tunnels.Based on the project of sangzhuling tunnel in Sichuan-Tibet railway,a typical high geothermal tunnel,adopting various means such as literature research,field measurement,theoretical analysis,laboratory test and numerical simulation,prediction model of temperature field and heterothermal curing method of support materials are established on the basis of temperature field's varying characteristics in high geothermal tunnel,by the method of field tests and theoretical deduction.By means of laboratory test,mechanical properties and strength calculating formula of concrete in primary support and secondary lining under the heterothermal condition were obtained.According to the stress characteristics of support structure under different initial geothermal condition via thermal-stress coupling calculation,and based on the ultimate strength of support materials,and heat insulation materials characteristics,further research on safety characteristics of support structure in high geothermal tunnel was developed,and also reasonable support structure pattern was proposed.Finally,being verified by massive field tests,research results were obtained as follows:(1)Based on the field tests on temperature of surrounding rock-support structure in Sangzhuling Tunnel and unsteady thermodynamics thermal conductivity theory,by discretizing spatio-temporal regions of surrounding rock-support structure and adopting finite difference method,the analytical method of spatio-temporal evolutionary theory in temperature field was deducted,the spatio-temporal evolutionary prediction model was established,the spatio-temporal evolutionary law was researched,and the high heterothermal curing methods of support materials were proposed.(2)According to the spatio-temporal evolutionary law in temperature field of tunnel support structure and site situation of Sanzhuling tunnel,by adopting the heterothermal curing methods,the experiments including 140 sets of shotcrete and formworking concrete were carried out in laboratory.Under the condition of varying temperature,the influences of temperature and humidity on concrete mechanical properties were studied,and then the varying laws of mechanical properties were figured out,finally the calculating formulas of compressive strength,tensile strength and elastic modulus,taking into account of temperature and humidity coupling effects,were established.(3)By establishing the calculating models of formation-structure and loadstructure,numerical simulations of thermal stress coupling of support structure under different geothermal conditions were conducted,and the varying laws of mechanical and safety characteristics about support structure in high geothermal tunnels were figured out.Stress field tests of support structure in high geothermal tunnels were conducted to verify its mechanical characteristics and calculating models.(4)By establishing finite element heat-transfer models of surrounding rock-heat insulation layer-supporting structure,and researching on the numerical simulations of different heat-insulation materials and supporting structure system,the optimal heatinsulation supporting system were determined.Finally,reasonable support structure patterns under different initial temperatures in high geothermal tunnels were proposed,mainly considering the control of structure safety and crack resistance.