Thermodynamic Characteristics Of Lightweight Aggregate Concrete And Thermal Insulation Optimization Of Lining For High Rock Temperature Tunnel

The construction of tunnels in rock strata often faces various complex geological conditions,and high rock temperature is one of the typical bad geological problems.With the increase of tunnel length and buried depth,the problem of high rock temperature heat damage has become increasingly serious.In the face of high rock temperature tunnel construction and operation process,there is no effective and unified treatment method for the two heat damage problems of high temperature construction environment and structural thermal damage,which seriously affects the construction progress and operation safety.Therefore,based on the concept of active cooling,guided by the problem of heat damage during tunnel construction and operation,this paper uses lightweight aggregate concrete as both initial support and thermal insulation structure,and carries out field measurement,indoor test and numerical simulation respectively.The mechanical properties and thermal insulation properties of lightweight aggregate concrete are explored,and the following conclusions are mainly obtained :(1)Based on the background of high rock temperature tunnel,the quasi-horizontal orthogonal test of lightweight aggregate concrete L934 was carried out.The results show that the content of shale ceramsite is a significant factor affecting the physical and mechanical properties of lightweight aggregate concrete,followed by vitrified microsphere.The apparent density,compressive strength,tensile strength,flexural strength and thermal conductivity decrease with the increase of shale ceramsite and vitrified microsphere content.With the increase of curing temperature,the compressive strength,tensile strength,flexural strength and thermal conductivity decreased first and then increased.The horizontal weight of shale ceramsite and vitrified microsphere content is the largest with low content,and the influence weight decreases with the increase of content.The prediction model of physical and mechanical properties of lightweight aggregate concrete is obtained by multiple linear regression analysis of the test data.The results show that the fitting accuracy is high.(2)The thermal conductivity of lightweight aggregate concrete was studied by establishing a two-dimensional three-phase random aggregate meso-model composed of aggregate,mortar and ITZ.The results show that the steady-state temperature field of lightweight aggregate concrete basically presents a layered distribution,and the existence of aggregate makes the layered distribution of temperature field fluctuate.The aggregate exists in the shadow area formed in the heat flow field cloud map,and the position of the shadow area corresponds to the position of the aggregate,which proves the mechanism that the lightweight aggregate has good thermal insulation performance.With the increase of aggregate content,the thermal conductivity shows a linear decreasing trend.Under the same aggregate content,as the aggregate becomes larger,the thermal conductivity shows a decreasing trend;as the thickness of the interfacial transition layer increases,the thermal conductivity shows a linear increase trend.The prediction formula of interface thermal resistance coefficient is obtained by multivariate linear fitting of the simulation data.The optimized Maxwell model considering interface thermal resistance is proposed.The maximum error between the calculated value and the simulated value is 0.79 %.(3)The numerical model of temperature field of high rock temperature tunnel is established by ABAQUS to study the thermal insulation performance of lightweight aggregate concrete primary support.The results show that the thermal conductivity of different lightweight aggregate concrete has a significant influence on the distribution of surrounding rock temperature field.With the increase of radial depth of surrounding rock,the temperature of surrounding rock under different thermal conductivity shows a power exponential growth trend,and tends to be stable with the increase of radial distance.The smaller the thermal conductivity,the higher the surrounding rock temperature at each radial depth.With the increase of ventilation time,the temperature difference of surrounding rock at different radial depths under different thermal conductivity decreases,and the curve under different thermal conductivity gradually shrinks.The distribution law of surrounding rock temperature field and thermal conductivity under different spray layer thickness are basically the same.The relationship between the change of wall temperature and the change of thermal conductivity shows a positive correlation.The larger the thermal conductivity,the wall temperature gradually increases and tends to be stable.The greater the thickness of the spray layer of lightweight aggregate concrete,the lower the temperature of the secondary lining wall,and the effect of increasing the thickness of the spray layer to increase the thermal insulation performance of lightweight aggregate concrete is limited.(4)The numerical simulation of the stress of the lining support structure under the action of thermal coupling is carried out.The results show that the internal force of the lining structure under the action of temperature stress is significantly larger than that without temperature stress.The maximum principal stress of the lining under different thermal conductivity is mainly located in the inverted arch,the arch waist and the vault also have no small tensile stress,and the minimum principal stress is located at the arch foot.With the increase of thermal conductivity,the minimum principal stress of the vault begins to decrease gradually and bears more and more pressure.The maximum tensile and compressive stress values are also increasing,but the rate gradually slows down.The maximum principal stress of lining structure under different spray layer thickness is mainly located in the inverted arch and haunch.With the increase of spray layer thickness,the maximum tensile and compressive stress values show a decreasing trend.Increasing the thickness of the spray layer can play a certain thermal insulation effect,but the increase is not as large as the thermal conductivity.