Abstract:Climate warming and engineering activities could cause changes in the thermal conditions of permafrost,mainly in two ways：warming of the permafrost and increasing the thickness of the active layer,which will lead to a marked change in the thermal stability of foundations in permafrost regions.Generally based on the IPCC SRES A2,A1B,and B1 climatic changes,this study uses numerical methods to research the freezing and thawing process,the changes in the active layer,the thaw depth and the soil temperature of fabricated foundations in permafrost regions along the ± 400 kV Qinghai-Tibet power transmission line.The results show that engineering activities and climate warming change the thermal conditions of permafrost and promote degradation; these are important factors affecting the long-term stability of foundations.Enhanced heat conduction from concrete pile foundations aggravates the freezing and thawing process,and climatic warming leads to a decrease in the thickness of the active layer and a rise in soil temperature,With increasing depth,the permafrost response decreases,and the rate of change of temperature becomes small.The permafrost response increases and the rate of change of temperature is greatly altered closer to the active layer.Under three warming patterns,after 50 years,the soil temperature at 1 m depth increases to 2.80 ℃,4.66 ℃,and 5.60 ℃ from 1.90 ℃,the soil temperature at 2 m depth increases to 1.22 ℃,3.66 ℃,and 4.82 ℃ from 0.50 ℃,and at 10 m depth,the permafrost temperature will rise from-0.70 ℃ to -0.42 ℃,-0.35 ℃,and-0.30 ℃.With the three warming patterns,IPCC SRES B1,A1B,and A2,the maximum thaw depths under the centers of foundations are 3.12 m,5.07 m,and 6.02 m after 50 years,while the thicknesses of the active layers of natural ground are 2.07 m,4.37 m,and 5.62 m.The maximum thaw depth of natural ground is much shallow than the maximum thaw depth under the centers of foundations.This shows that permafrost has different responses to different warming patterns,and the degradation of permafrost under the centers of pile foundations is faster under the twin effects of climatic warming and engineering activities.There will be hidden troubles with foundation bases after 50 years if steps to prevent the degradation of permafrost under warming patterns A2 and A1B are not taken.The thawing rates of the permafrost under the foundation centers are 1.5,6.2,and 8.6 with the three warming patterns B1,A1B,and A2 from the tenth year to the fiftieth year.The thaw depth increases with a rise in the rate of change of temperature,and the speed of degradation accelerates with increasing rate of change of temperature.The thaw depth of permafrost under the centers of foundations is three times deeper as that of the permafrost of natural ground.This is because engineering activities promote permafrost degradation with a low rate of temperature increase.At this point,permafrost mainly is changed by engineering activities,while permafrost degradation is mainly influenced by climate warming with a higher heating rate.
