Degradation of warm permafrost and talik formation on the Qinghai-Tibet Plateau in 2006-2021

Permafrost is degrading globally,particularly those with low thermal stability on the Qinghai-Tibet Plateau,owing to climate change.However,the inadequacy of direct research on permafrost degradation based on in-situ monitoring limits the prediction of permafrost degradation and engineering practices.This study explored the processes and modes of permafrost degradation into talik by analyzing ground temperature data from five points in the hinterland of the Qinghai-Tibet Plateau from 2006 to 2021.The results showed that the degradation of the warm permafrost layer with a geothermal gradient of zero occurred simultaneously in the top and bottom directions.The rate of permafrost degradation from the top down and bottom up increase during the degradation process,but the increase of the former is more drastic after the formation of thawed interlayer.Additionally,the construction of the Qinghai-Tibet Railway changed the degradation modes of the permafrost in adjacent natural sites through horizontal heat transfer,particularly after through talik formation under the embankment.The findings suggest that taking countermeasures before or immediately after forming thawed interlayer is more effective.When evaluating the thermal impact of projects in warm permafrost regions,special attention should be given to the horizontal heat transfer process that may result from the formation of a through talik.

Field observations of the thermal stability of permafrost under buildings with an underfloor open ventilation space and pile foundations in warm permafrost at high altitudes

Pile foundations combined with ventilation spaces under floors are the most common method in buildings over permafrost.The safety and stability of buildings are closely related to the temperature of permafrost.However,there are limitations of understanding on this method in the high-altitude,warm(>−1℃)permafrost areas on the Qinghai–Tibet Plateau.In this study,the thermal stability of permafrost foundation soils under buildings with an underfloor open ventilation space and pile foundations in warm permafrost at high altitudes was studied through field observations of ground and air temperatures,wind speed,net radiation from 2017 to 2021.The results indicated that the open ventilation space exerted an effective cooling effect on the underlying permafrost and pile foundations from March to October,while a thermal insulation effect was observed from November to February of the following year,but overall,the cooling effect dominated;the cooling effect of open ventilation spaces differed spatially.The permafrost temperature on the south-facing side was higher than that on the north-facing side,and those on the east and west sides were higher than that directly under the open ventilation space of the building.This study also demonstrated that radiation shielded by the building was a main factor of the cooling effect of open ventilation spaces,and the cooling effect of open ventilation spaces could accelerate the back-freezing of the cast-in-place(CIP)pile foundations.This structure could effectively maintain the frozen state of the underlying warm permafrost at high elevations on the interior Qinghai–Tibet Plateau.