Abstract:In block-stone embankment construction of Qinghai—Tibet railway,different thicknesses of upper sand-and-gravel layer and same thickness of block-stone layer were used in different railway segment due to the uneven landform. By numerical stimulation,the temperature distribution were obtained in common embankment and block-stone embankment with upper sand-and-gravel layer in different thicknesses. The comparative analyses show that block-stone embankment has well active cooling effect on the railway foundation by raising the permafrost table and decreasing the permafrost temperature,and the variation of upper sand-and-gravel layer thickness has also great influence on the active cooling effect. The thicker the upper sand-and-gravel layer is,the smaller the temperature variation magnitude on the top of block-stone layer conducted from the embankment surface is,which will slow down the convection speed of the air in block stone and weaken the cooling ability of block stone. Although the permafrost table under the center of the embankment lifts up with thicker upper sand-and-gravel layer,the cost is the frozen intensity decreasing of the permafrost and the dissipation of cold generated by the block stone near the embankment foot. On the conditions given by the model,the cooling ability of block stone is stronger than that of heat disturbance caused by embankment construction when the thickness of upper sand-and-gravel layer is less than 6 m or larger than 9.5 m. The cooling of block stone near the embankment foot and the cold supplying of around permafrost can not counterweigh the heat effect of embankment anymore and the permafrost table under the center of the embankment will go down gradually. In sum,the increase in the thickness of upper sand-and-gravel layer will weaken the active cooling ability of block stone and lower the permafrost table beneath the railway embankment,and while the thickness exceeds a certain number the middle zone of block stone will almost loss the cooling capability and thus lead to an increase of permafrost temperature,which is disadvantageous to the stability of railway embankment and the protection of cold ground. Considering the warm tendency of global climate and the heavy load of high embankment,we suggest the discussion of thick sand-and-gravel layer up the block-stone embankment and seeking an optimal thickness of upper sand-and-gravel layer and block stone layer in construction.