Influence of the ballast layer of qinghai-tibet railway on the thermal regime of the frozen soil roadbed

In order to study the influence of railway ballast layer on the thermal regime of frozen soil roadbed, a test section was built at Beiluhe test site along Qinghai-Tibet Railway. Analyses of the monitoring results of ground temperature and heat flux show that the ballast layer can play an insulating role in protecting the permafrost of Qinghai-Tibet Railway, which can prevent outside heat accessing the frozen soil roadbed in warm season and keep heat from diffusing into outside in cold season. The insulating effect of the ballast layer is well in warm season, but the cooling effect of it is not obvious in cold season. Compared with the gravel surface, the cooling effect of the ballast layer is very evident. The annual mean ground temperature is about 2 °C lower than that at the same depth beneath the gravel surface. The main reason for the cooling effect of the ballast layer is that most of the heat is shielded in warm season, and the soil under the ballast layer has a negative heat budget, which plays a very well cooling effect to the frozen soil roadbed. The ballast layer is very helpful to protect the permafrost under the roadbed as well as can ensure the stability of frozen soil roadbed to some extent.