The influence of tunnel insulation measures on the temperature spatiotemporal variation of frozen soil during artificial ground freezing

Artificial ground freezing (AGF) has been widely used in the construction of subway connection channels to reinforce the soil around the tunnel, especially the water-rich soft soil, to ensure the stability of the tunnel and the overlying buildings. However, the direct contact between the frozen soil curtain and the tunnel segment would cause the loss of cooling capacity, which is not conducive to ensuring the freezing effect. The indoor freezing and thawing model of silty soft soil was established to explore the influence of tunnel insulation measures with different thermal conductivity on the temperature spatiotemporal variation of frozen soil curtain through indoor simulation tests. The effects of thermal insulation materials with different thermal conductivity on the stable temperature and development speed of frozen soil curtains were also compared. The results indicated that the interface of tunnel segments has a significant impact on the freezing speed and stable temperature of frozen soil during the freezing process, as well as the thawing time during the natural thawing process. Compared with steel segment tunnel, the use of thermal insulation coating with low thermal conductivity increased the temperature of the soil at the same distance by 5.88% to 42.5%, and prolonged the freezing time of the soil far from the tunnel by 38.8%. On the contrary, the insulation foam with higher thermal conductivity not only reduced the temperature of frozen soil by 13.8% at most, but also shortened the freezing time by 42.1% at most, which can significantly improve the construction efficiency. For the natural thawing process, the thermal insulation coating would reduce the thawing time of frozen soil by more than 30%, while the thermal insulation foam can increase the thawing time of frozen soil far away from the tunnel by at least 23%, which was more conducive to extending the allowable duration of frozen soil excavation. The results can provide a reference for the selection of segment insulation measures in the AGF construction of subway cross channels in silty soft soil.