Thawing Index and Freezing Index on the Embankment Surface in Permafrost Regions

Because thermodynamic characteristics are changeable on the interface between atmosphere and ground and then influence thermodynamic stability of permafrost under the interface while a highway or railway built above tundra, embankment of a certain height is used as a usual measure to keep the permafrost stable. After the embankment built, the heating effect that changes with direction of side slopes is a question, which must be taken into account in the engineering measure to keep the embankment frozen. In this paper, based on mathematical physical analyses and numerical simulation, methods for calculating the freezing index and thawing index of embankment surface and some thermodynamic parameters from the maximum and minimum monthly mean air temperature are developed and applied in the Beiluhe passage of the Qinghai-Tibet Railway in 2002 as an example. The quantitative analysis shows that to protect the permafrost along the Qinghai-Tibet railway is necessary and scientific. A case analysis shows that the permafrost in the Beiluhe region is in a critical status, even if no railway built. Slope exposure can cause strong asymmetric temperature distribution. The maximum difference of thermal status is between the south or near south direction and the north or near north direction of the embankment slopes, so it is necessary to take different measures for the two opposite embankment slopes, in order to mitigate the direct negative effect of locally taking soil to build embankment on the underlaying permafrost, to minish the asymmetric temperature distribution on the embankment surface and to avoid lengthways fissure taking place.