Study On Long-term Reliability Of Cold Region Tunnel

With the increasing scale of railroad and highway construction in cold and high altitude areas in China,the number of cold area tunnel projects has been gradually increasing.For a long time,frost damage in cold zone tunneling has been a hot scientific issue in the field of permafrost engineering,and a key engineering technical problem that must be solved.Climate warming has a significant impact on the long-term stability and reliability of tunnels in cold regions,and the study of the changes in tunnel reliability and characteristics under different warming conditions is of great theoretical significance and engineering application value for cold region tunnel engineering design and operation management.In this paper,the research is based on the prediction results of the IPCC and domestic and foreign experts and scholars on the temperature changes of the Qinghai-Tibet Plateau in the next 50 years.The main research contents include:(1)The basic principles of structural reliability are summarized,and the calculation methods such as First Order Second Moment Method and Monte Carlo Method are summarized;the frost damage mechanism of tunnel lining structures in cold regions is summarized;the basic principles of hydro-thermal coupling and the coupling control equations are analyzed.(2)Taking Bahadaban tunnel as the research background,a hydro-thermal coupling numerical model is established to simulate and analyze the variation of temperature field,stress field and displacement field within 50 years after the completion of the tunnel.The results show that: the insulation layer can significantly reduce the thickness of the freezing-thawing circle of the tunnel in the cold regions;when the temperature increases by2.6℃ and 3.0℃,the thickness of the freezing-thawing circle of the surrounding rock on both sides of the tunnel at the intersection of the elevation arch and the side walls is 0 in 40 years,which can ensure the thermal stability of the tunnel surrounding rock in the first 40 years;when the temperature increases by 4.0℃,the surrounding rock at the intersection of tunnel arch and side wall appears 0.03 m melt layer in the 30 th year,which can only ensure the thermal stability of the tunnel surrounding rock in the first 30 years.This can only ensure the thermal stability of the tunnel envelope during the first 30 years.When the temperature increases by 2.6℃,3.0℃ and 4.0℃,the maximum compressive stresses occurred at the intersection of tunnel invert and side wall;the maximum vertical displacements occurred at the top of the arch;and with the passage of time,the values of the stresses and displacements of the tunnel increased continuously with time,and the changes are obvious in the first 30 years,and gradually tend to be gentle thereafter.(3)The Monte Carlo Method,Response Surface Method and First Order Second Moment Method were used to calculate and analyze the failure probability and reliability indexes of the tunnel within 50 years after completion,and the minimum values of the secondary lining and insulation thickness to ensure the tunnel reliability above 90% after 50 years were calculated in combination with sensitivity analysis.The results show that when the temperature rises 2.6℃,3.0℃ and 4.0℃ in the 50 th year,the failure probabilities calculated by Monte Carlo Method are 12.5%,14% and 17.8%,respectively;the failure probabilities calculated by Response Surface Method are 13%,14.6% and 18.3%,respectively;the failure probabilities calculated by First Order Second Moment Method are 17.5%,19.4% and 22.8%,respectively.If the tunnel reliability should be larger than 90% 50 years later,the thickness of secondary lining and insulation layer of increasing 2.6℃ at least reach 42.3cm and 7cm respectively,which increase 6% and 40% than the designing thicknesses.When the temperature rises by 4.0℃,the secondary lining and insulation layer reach 45.2cm and 9cm,respectively,which is 13% and 80% higher than the designing thicknesses.According to the IPCC report,if the temperature rise of the third pole can be controlled within 3.0℃ in next 50 years,the reliability tunnel is larger than 90% when the secondary lining is 42.7cm and the insulation layer is 8cm.