Study On Spatial Temperature Fields And Deformation Evolution Characteristics Of Railway Roadbed-bridge Transition Section In Permafrost Region

The Qinghai-Tibet railway is the highest and the longest plateau railway line which goes across the continuous permafrost zone. Small changes in temperature will lead to the mechanical properties of frozen soil change greatly, thus inducing the diseases of the track. Roadbed-bridge transition section as the key position of a combination of road and bridge, its stability and strength will have direct impacts on smooth and unobstructed of the whole line.Because the heat sensitivity of permafrost is remarkable, plus the disturbance of construction to permafrost, the differential settlement problems of roadbed-bridge transition section in continuous permafrost zone are more complicated than that in noncontinuous permafrost zone. Also the settlement problems of roadbed bridge transition becomes more pronounced as the service time of the Qinghai-Tibet railway increases. In view of the present study of roadbed-bridge transition section in the continuous permafrost zone, few studies have involved its thermal and mechanical stability and evolution rule. So it is necessary to carry out the research about the stability and other aspects of roadbed-bridge transition section in continuous permafrost zone. The results of this study both have great practical significance and application value to the operation and maintenance of Qinghai-Tibet railway.In this paper, the research focused on roadbed-bridge transition section in the continuous permafrost zone, comprehensively using indoor test, model test, numerical simulation and other research methods, got the degraded layer thawing settlement characteristics and after-melting compression characteristics. Through the in-depth analysis of the uneven settlement deformation mechanism of roadbed-bridge transition section, we revealed the distribution and its evolution law of the space temperature field and deformation field of roadbed-bridge transition section. Main achievements and progress are as follows:(1) Through the soil double direction freezing-one direction thawing freeze-thaw cycle test device developed by our own, indoor tests about thawing settlement characteristics and after-melting compression characteristics of the silty clay from Qinghai-Tibet plateau have been done. Systematically analyzed effect law of the freezing temperature, moisture content, dry density and freeze-thaw cycles on thaw settlement coefficient and the relative coefficient of compressibility of the permafrost melts.(2) Designed and developed the space temperature field simulation test system by our own, conducted indoor model tests about roadbed-bridge transition section in the continuous permafrost zone. Through contrasting and analysing the time-dependent temperature characteristics from different location temperature measuring point, the distribution and variation of roadbed-bridge transition section temperature field have been obtained.(3) Established the calculation model of space temperature field on roadbed-bridge transition section in continuous permafrost zone, through analyzing the thermal stability of the model, we reveals the distribution and evolution law of space temperature field of roadbed-bridge transition section. Many researches have been done about the relationship between the upper limit of permafrost, melting rate and backdistance.We further analysed the influence of transition section height, stone horizontal thickness, frozen soil types and the sunny-shady slopes on the distribution of roadbed-bridge transition section space temperature field.(4) Combining the thawing settlement characteristics of transition section in permafrost region, we analysed space deformation field of roadbed-bridge transition section in continuous permafrost zone, revealed the distribution and evolution law of space deformation field of the roadbed-bridge transition section, and analysed the influence of transition section stiffness, transition section height, stone horizontal thickness, the sunny-shady slopes on distribution of roadbed-bridge transition section space deformation field. The results show that the ratio D( the distance from the maximum settlement to the back and transition section surface width) increased over the operation time, and obey the exponential decay function.(5) The U-shaped crushed-rock embankment is applied to the roadbed-bridge transition section, through analyzing the indoor model test and the numerical analysis results of roadbed-bridge transition section. It shows that: the U-shaped crushed-rock embankment’s cooling effect is better than that of the gravel interlayer structure on the ground, U-shaped crushed-rock embankment can become the new structure of roadbed-bridge transition section.