Key Issues in Using the Freezing Method in Power Tunnel Rehabilitation Projects

Using COMSOL finite element software, a three-dimensional numerical transient model of an underground tunnel collapse vertical freezing repair project was established. The model was altered to change the head difference, allowing for analysis of the development of the permafrost curtain, the time of intersection circles, and the freezing temperature field cloud diagram. The results indicated that, without seepage, development of the upstream and downstream permafrost curtains was stable and uniform. However, under seepage conditions, development of the upstream and downstream permafrost curtains became increasingly uneven with increasing seepage velocity. The downstream side of the soil body began freezing earlier than the upstream side, and the final temperature was lower. The intersection time of the freezing wall was an important indicator of development of the permafrost curtain, and the freezing time of the freezing wall was the most critical indicator. A hydraulic head difference of 1 m was found to significantly impact the development of the freezing wall, with less influence from seepage velocity on the overall permafrost curtain intersection time. However, the intersection time of the isotherm increased significantly with increasing seepage flow rate. The findings from this project provide a theoretical reference for future restoration design.