Analysis of cooling effect of block-stone expressway embankment in warm temperature permafrost region

Based on long-term and continuous ground temperature observation data, this paper analyzed the temperature, thermal state, freeze-thaw cycle, artificial permafrost table and rate of change of block-stone embankment along the Gongyu expressway located in a high-temperature permafrost region with an average altitude of 4 260 m. The influence of asphalt concrete pavement and cement concrete pavement over underlying permafrost is also studied in order to evaluate its applicability. It is found that the asphalt concrete pavement absorbed more heat and warmed up the underlying permafrost, which leaded to the rapid degradation of permafrost. During the observation period, the permafrost degradation rate in the centre of the block-stone embankment with asphalt concrete was 33.5 cm/a, almost five times that of the natural foundation. Moreover, the effect of shady-sunny slope was serious. The degradation rate of the permafrost of the sunny shoulder was 30.00 cm/a, which was significantly higher than that of shady shoulder (22.0 cm/a). Compared with asphalt concrete pavement, cement concrete pavement had higher thermal reflection and smaller thermal radiation absorption, which is conducive to raising the permafrost table or slowing down the degradation rate of the permafrost, and the cooling effect is better. However, during the observation period, it was found that the area growth rate of melting plate under asphalt concrete pavement in block-stone embankment in warm permafrost region was 12.24 m(2)/a, while that under cement concrete pavement was 9.28 m(2)/a, indicating that the area of melting plate always increases at different rates. Therefore, the existence of block-stone layer and the change of pavement type have not completely solved the heat balance problem of expressway embankment in warm temperature permafrost region. It is suggested that reinforcement measures or composite embankment structure should be adopted.