In-situ study on cooling effect of the two-phase closed thermosyphon and insulation combinational embankment of the Qinghai–Tibet Railway

Based on the observation data of the two-phase closed thermosyphon and insulation combinational test embankment of the Qinghai–Tibet Railway, variations of permafrost table, thermal regime changes, heat budget and heat exchange of the combinational embankment were analyzed. As control groups, natural ground and a traditional soil embankment were also analyzed. Considering thermal asymmetry in the north and the south slopes, comparisons of ground temperature and heat budget in the north and the south slopes were made. The results showed that permafrost table dropped slowly but continuously under the natural ground, rose at first then dropped beneath the traditional soil embankment, and kept a longer period of rising underneath the combinational embankment. Also warming with small magnitude occurred under the natural ground. Under the traditional soil embankment (northern slope), cooling happened in the upper part while warming existed in the lower part. Universal significant cooling was resulted in under the combinational embankment after the application of the thermosyphon. An asymmetric thermal regime was demonstrated under both of the traditional soil embankment and the combinational embankment. The former presented an asymmetric degradation in the permafrost, and the latter presented an asymmetric evolvement. For the natural ground, the ground surface absorbed heat, and heat from ambient layers partly accounted for permafrost degradation. For the traditional soil embankment (northern slope), the embankment surface released heat, and heat from ambient layers mastered the permafrost degradation. Controlled by the cooling process caused by the thermosyphon, heat exchange of the combinational embankment was different from the former ones. The thermosyphon absorbed heat from ambient layers, cooled embankment, and its underlying layers. At the same time, it altered the embankment surface from heat releasing way to heat absorption way. And, for the combinational embankment, cooling effect in the northern slope was better than in the southern slope. The consequence was partly attributed to lower air temperature in the northern slope, and the thermosyphon cooled the ground to a lower temperature and its working duration was longer, correspondingly. And it was partly attributed to the application of insulation board.