Estimates on thermal impact of gas-line and alleviating effect of countermeasures in patchy permafrost of northwestern China

Linear engineering across cold regions has a negative effect on the stability of underlying permafrost. The permafrost degradation affected by engineering activities would induce ground thaw settlement; hence, the feedback from ground alternation would damage the engineering structure conversely, which is detrimental to both engineering safety and the surrounding environment there, especially differential thaw settlement in patchy permafrost. To detect the engineering influence of gas pipeline on permafrost environment, a series of field monitoring was conducted and a numerical model considering the phase changing was introduced to simulate the thermal dynamic of underlying frozen soil and estimate the effect of several countermeasures chosen to alleviate the thermal disturbance from the pipeline. The results show that frozen soil was unstable and easy to degrade because of thermal disturbance from the pipeline—the active layer thickness is 1.5~2.0 m while the maximum thawing depth under the pipe goes over 7.0 m. Both cooling gas temperature and applying insulation layer would alleviate the permafrost degradation, but in different degrees of effect. The former did not have a significant alleviation for permafrost degrading, especially in the short term, while the latter was more effective during the whole life.