Emergency mitigation strategies for the settlement of in-service pile foundations in Permafrost Regions: Application of artificial ground freezing

Abstract

The warming and degradation of permafrost surrounding pile foundations frequently result in settlement problems. As a hidden deep foundation, there has been a lack of effective methods to address the settlement of pile foundations in permafrost regions. Conventional settlement control methods, such as thermosyphons and auxiliary piles, have been proven to be time-consuming and limited in effectively controlling settlement deformation, as demonstrated in the treatment of severe settlement damage at the K1401 dry bridge of the Qinghai-Tibet Railway. This study examines the application of artificial ground freezing as an emergency technique to mitigate settlement damage by restoring thermal stability and enhancing the ultimate bearing capacity of pile foundations. Laboratory experiments demonstrate that a 10-h freezing process reduces the average pile temperature from −0.5 °C to −3.5 °C and decreases the deformation rate from 0.0007 mm/h to 0.0002 mm/h under a 5 kN load condition. Artificial freezing for 96-h in engineering can lower the temperature of the pile side by 1.3 °C on average, leading to a 24 % increase in the ultimate bearing capacity through numerical simulation. The freezing efficiency is primarily affected by the coolant temperature, the arrangement of the freezing pipes and the ice content of the permafrost. As the ice content increases, the cooling effect of the same cooling pattern diminishes, the increment of ultimate bearing capacity decreases, with increases of 28 %, 23 %, and 17 % observed in ice-poor, ice-more, and ice-rich soils, respectively, and the time required for the ground temperature to return to its natural state also increases after artificial freezing. Based on the requirements of ultimate bearing capacity, it is recommended to freeze ice-poor and ice-more regions once every three years and ice-rich regions once every four years. To make up for the lack of freezing efficiency in the ice-rich regions, the freezing effect can be improved by further optimizing construction parameters, such as lowering the temperature of the coolant, shortening the distance between the freezing pipe and the pile side, and increasing the number of freezing pipes. This study offers a practical engineering solution and design guideline for addressing pile foundation settlement in permafrost regions.