Numerical Analysis of the Critical Height of Railway Embankment in Permafrost Regions of the Tibetan Plateau

In permafrost regions, construction of a railway embankment may alter the original thermal equilibrium on the natural ground surface, resulting in the change of permafrost table under the embankment. In order to make sure that the permafrost table will not be service failure in the following 50 years after the embankment is built, the critical embankment height for coarse-grained soil in the permafrost regions on the Tibetan Plateau is calculated in this paper, considering the effect of globe warming. The mean annual air temperatures applied to the numerical analysis are (-6.5 ℃), (-6.0 ℃), (-5.5 ℃), (-5.0 ℃), (-4.5 ℃) and (-4.0 ℃), respectively, and the value of temperature rising is taken as 1'10 ℃ for the coming 50 years. The minimum embankment heights obtained from the analysis are 0'85 m, 0'92 m, 1'01 m, 1'18 m, 1'60 m and 2'66 m for different mean annual air temperatures mentioned above and the maximum embankment heights are 7'68 m, 7'55 m, 7'34 m, 7'00 m, 6'45 m and 5'85 m, accordingly. By summarizing the results of calculation and analysis, the following conclusions can be drawn: (1) Although mean annual air temperature is a key factor influencing the critical embankment height in permafrost regions on the plateau, the service life of the railway embankment must be taken into account; especially under the condition of globe warming, the lifespan of the railway is much more important to determine the critical height of the embankment. (2) On the condition that the service life of the railway embankment is 50 years, in the areas where mean annual air temperature is higher than (-3.5 ℃), the critical embankment height does not exist. Namely, however high the embankment is, the permafrost table under the embankment will fall anyway. If the lifespan of the railway embankment is lengthened or shortened, the critical mean annual air temperature will vary correspondingly. (3) No matter what the high or low embankment is constructed, the ground temperature under the embankment will rise and permafrost will be in a degradation situation. In other words, the construction of higher embankment may cause the permafrost table under the embankment to rise, but it is at the cost of consuming the cold preservation of permafrost underlain and cannot take the effect of permafrost protection actually, which has been confirmed by the field observation at the Qinghai-Tibetan Railway test sites.