| Based on 33 operating heat-supplying tunnels in Beijing,the mechanical response of the heat-supplying tunnel structures under the unique effect of multi-field coupling has been studied by combining three-dimensional numerical simulation with field monitoring.Then through a large number of field investigations and element composition analysis of lining concrete on micro scale,the structural status and the main types of erosion degradation of Beijing heat-supplying tunnels were analyzed.Afterwards,the subset simulation method was improved to calculate and analyze the safety reliability of the existing tunnel structure.Subsequently,on the premise of the reliable current structure,experiments and researches on the mechanism of the carbonization erosion were carried out by means of macro accelerated deterioration test and micro numerical simulation.In the aspect of the rust expansion cracking,the macro and micro analysis of the rust expansion cracking mechanism would be discussed through theoretical deduction and micro simulation.Finally,on the basis of the damage deterioration research results,the prediction method for the early-warning life for maintenance of the heat-supplying tunnel structure can be proposed.The main research contents and achievements are summarized as follows:· Study on the Coupling Effects of Huge Thrust and Thermal Effect on theHeat-Supplying Tunnel StructureFirstly,the principle and basic assumption of coupled analysis have been established through the working characteristics during operation.According to the mechanical parameters obtained from laboratory tests,a three-dimensional numerical model can be established.The results show the rule that the multi-field coupling effects attenuate with the increase of the distance between the structure section and the fixed brackets.Furthermore,the coupling effects apply the greatest influence on the floor,the crown and on the side wall in the respective order.The tunnel structure in the range of the fixed bracket at 6 m is greatly influenced by the coupling.At the distance of 12 m from the bracket,the change of longitudinal strain has been reduced to 5.5με and lateral strain to 3με.For the lining structure far from the fixed brackets,the influence of coupling effect is mainly embodied in the form of circumferential stress and strain of tunnel lining structure caused by the single temperature field.·Investigation of Heat-Supplying Tunnel Structure and Microscopic Judgment of theType of Erosion DeteriorationAfter making a large number of systematic field detections and disease investigations on 33 heat-supplying tunnels throughout Beijing within two years,the characteristics of heat-supplying tunnel during operation in Beijing have been discussed.The results of the water leakage testing show that existing tunnel structures are mainly under good condition except for a few tunnels.The number of cracks within 6-meter fixed trestle accounts for about 50%of the total cracks,half of which are appeared on the floor.It can be concluded that under the numerical simulation,the tunnel floor near the fixed trestle should be the weakest part.The detection of environmental conditions shows that the temperature is in the range from 40℃ to 60℃ in the heat-supplying tunnels,and the average CO2 concentration generally attains 1200ppm.The internal environment with high temperature and high carbon dioxide concentration could be formed under such a condition.Besides,the erosion of the external surrounding rock and groundwater environment is generally weak.Therefore,carbonization erosion is preliminarily judged as the typical erosion type of Beijing heat-supplying tunnel.Combined with the ESEM-EDS analysis on the microscopic structure and elements of the lining concrete,the thesis further proves that the main erosion type of Beijing heat-supplying tunnel is carbide erosion.Carbonization and the cover cracking caused by steelbar corrosion become the main research objects of the heat-supplying tunnel structures.· Evaluation and Analysis on the Safety Reliability of the Current Heat-SupplyingTunnel StructureCorresponding to the mechanical response of heat-supplying tunnel lining structure under the condition of multi-field coupling in operation period,the general subset simulation method was perfected by introducing Hua-Wang point set which was an attempt to improve the accuracy and efficiency of reliability calculation of lining structure.The results show that the present structural reliability of Chegongzhuang Heat-supplying Tunnel meets the requirements of engineering reliability.As calculation error is concerned,both the slope and the correlation coefficient of fitting curve reach 0.98,which show that the improved method has higher accuracy and lower dispersion.In terms of calculation efficiency,the maximum time-effect ratio of the improved subset simulation method can attain 158.8,which proves its effectiveness.· Multi-Scale Study on Carbonization Mechanism of the Heat-Supplying TunnelStructureOn the macro scale,considering the unique stress-temperature effect of the heat-supplying tunnels,through a large number of indoor accelerated carbonization tests,the correction coefficients of K7(temperature)and K8(stress)were drawn up,and the carbonization erosion fitting model with coupling conditions was completed.The engineering practice demonstrates that the prediction model can reflect the distribution of carbonation depth of Beijing heat-supplying tunnel lining structures.On the meso-scale,the time and space distribution of carbide erosion numerical method was concluded by analyzing the carbonation mechanism,using the finite difference method and combining with random circular aggregate and polygonal aggregate mesoscopic model.Besides,the impact factors like the position of the aggregate,content and aggregate shape on the carbonation of concrete erosion were also studied.The results show that the random polygonal aggregate model of erosion carbide in meso analysis has less error than that of circular aggregate model;the increase of aggregate content can promote the carbonation resistance of concrete.The distribution of aggregate location has great influence on concrete carbonation,which prevents from concrete carbonation erosion and distorts the originally straight carbonation front.The average carbonization depth would be lower 4mm to 6mm than the frontier face.Finally,the rationality of meso-analysis could be verified by macroscopic acceleration test.· Macroscopic and Mesoscopic Research on the Rust Expansion and CrackingMechanism of the Heat-Supplying Tunnel StructureReinforcement bars begin to rust when the carbonization depth of concrete reaches the cover.On the basis of elastoplastic damage mechanical theory,the lining structure of non-uniform corrosion damage crack model was established.Compared with the existing general reaming theoretical model,this paper derived the analytical formulas of macro-scale corrosive cracking process.By reference to other scholars’testing results,it was proved that the non-uniform corrosion damage crack model was rational.In view of the corrosion mechanism and the corrosion current density condition of the heat-supplying tunnel,the meso-analysis model was also established.The distribution and development of the rust swelling and cracking area in the protective layer within 20 years were also studied.It is found that the effect of non-uniform corrosion on reinforced concrete lining is more serious than that of uniform corrosion.The minimum protective layer thickness can be recommended as 1.5 times of the diameter of reinforcing bar for the structure design.Under the premise of reasonable thickness of the cover,the corrosion life of the heat-supplying tunnel lining structure can be generally maintained from 15 to 20 years.By comparing the results of macroscopic and mesoscopic analysis,it is found that the prediction results by macroscopic corrosion model has a deviation of 15.3%when the spacing distance among rebars exceeds 150mm.While w=60mm,the deviation reaches 42.7%.It is shown that the theoretical corrosion model proposed in this thesis is suitable for predicting critical corrosion depth of the heat-supplying structures.· Study on the Early Warning Life for Maintenance of the Heat-Supplying TunnelStructure based on the State Transition MethodConsidering the problems of complex parameters and weak practicability of the natural life prediction model,the early-warning life prediction method for the heat-supplying tunnels was proposed and the Average State Index(ASI)which can reflect the performance of tunnels in service was also defined.The results show that when the structure of Huaneng Tunnel provides service for 47 years(until the year of 2043),the structure will reach its warning life.Due to the design reference period of this tunnel is 50 years,the maintenance preparations need to be made slightly in advance. |
