Study On The Reliability Of Permafrost Roadbed In The Qinghai-tibet Railway

China is one of the biggest countries which have large area of permafrost, accounts for10% of world’s permafrost, and 21.5% of whole country’s territory. The main problem of the permafrost regions construction is how to ensure the thermal and mechanical stability. As the climate warming and increasing engineering activities on the Tibet Plateau, the temperature of the Tibet Plateau has raised, it also change the exchange relation of water-heat within the permafrost, which deepen the permafrost table and accelerated the degradation of permafrost,also influence the railway roadbed’s long-term stability and the train operation safety. In the20 th century, the construction of the Qinghai-Tibet railway roadbed on permafrost regions of the Qinghai-Tibet plateau mainly use passive insulation measures to reduce engineering damage under the frost and thaw condition, and then use the active cooling measures such as insertion thermal conductive rods in the roadbed to reduce the frost damage, but always couldn’t completely solve the frost heave and thawing settlement which bring about engineering destruction in the cold region, caused by climate warming and engineering activities. Due to the special nature of the permafrost, the stability of roadbed is not only influenced by its structure and material, but also heavily influenced by outside factors. So the reliability of permafrost roadbed includes many uncertain factors, which can’t be completely accurate in the short term to predict the consequences of these uncertainties. So some methods must be taken to solve the uncertainty in engineering.In order to control and governance the permafrost roadbed diseases better, as the background of global temperatures will rise by 2.6℃ in the next 50 years, this paper based on the thermal-hydro-mechanical coupling of permafrost roadbed, using the COMSOL Multiphysics simulation software to simulate the roadbed temperature, stress and displacement change in the next 50 years. And on this basis, we evaluate the reliability of permafrost roadbed settlement, and it has achieved the following results:(1) We found the temperature is lowest on roadbed surface in January, while the highest temperature on roadbed surface is in July. The top ballast temperature is-10.42℃ after50 years,which raise the temperature of 2.02℃ than one year ago when the roadbed completed.And the temperature is 19.60℃ after 50 years, which is about 3.31℃ higher than the temperature when the roadbed was finished. And the temperature of permafrost roadbed decreases gradually from surface to inside, as times go on, the temperature of roadbed rising.(2) The change of temperature field causes redistribution of stress field. The soil frost heave in January and the maximum stress occur at the top of the roadbed, while in July, it occurs at the slope toe of the roadbed.(3) The vertical displacement shows a trend of decrease in January. Following the temperature rising, the frost heave is abating. While in July, the maximum vertical subsidence occur in the center of the roadbed surface. As times go by, the permafrost thawing settlement will become evident.(4) The maximum horizontal displacement is limited at the range of slope angle in January. While in July, the maximum horizontal displacement is in the slope toe. Due to the thawing settlement, the horizontal displacement near the slope toe and natural surface has a significant development. In the long run, the displacement descend rapidly first, then decreased and keep steady.(5) The reliability index of permafrost roadbed settlement is changing over time. This is significantly reducing reliability at the settlement prime; this change tends to be slowly with time, and finally tends to be stable. And the failure rate in 50 years has increasing tendency,but the change trend was opposite to the reliability.