| As the source of Yangtze River, Yellow River and Lancang River, Qinghai-Tibet Plateau has heavy rainfall and is rich in surface water. Residual water exists at the foot of subgrade slope in some sections along Qinghai-Tibet railway. Persistent residual water may cause seepage through subgrade and break the balance of water and heat. That has direct impact on thermal stability and causes potential damage to slope stability of subgrade.In this paper, a programe for stability assessment of subgrades unpon a slope in permafrost is developed. In this programe, seepage is taken into account and temperature field is gained by seepage-heat coupled computing. The assessment of slope stability of the subgrade is based on the interface of frozen and thawy soil. The differences in temperature fields and safety factors between subgrades with seepage and ones without seepage are studied. The results show that seepage has great impact on subgrade upon a slope in permafrost. With seepage, there is significant thermal erosion at the lower foot of the subgrade slope, upper limit of frozen soil goes down, and the safety factor of subgrade stability is reduced.Comparing temperature fields and safety factors of subgrades on slopes with different gradients, it is found that the gradient of the slope on which subgrade is built has impact on subgrade too. If the gradient increases, the decline of the upper limit of frozen soil will be more significant, the core of thawy soil in subgrade will exist for a longer time during refreezing in winter. If seepage exists, greater gradient strengthens the impact of seepage. The greater the gradient is, the poorer the stability of the subgrade goes. Safety factors of subgrade stability at different times from construction of subgrade to the end of the next year are figured out. It is in summer that subgrade on a slope in permafrost has the poorest stability. When surface temperature goes below 0℃, safety factor of subgrade stability goes up significantly.
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