Minimum ground pre-freezing time before excavation of Guangzhou subway tunnel

Artificial Ground Freezing (AGF) has been in use for more than a century. However, there are few published papers concerning about thermal and mechanical calculation and analysis of AGF. Especially, in the practical project, when other factors such as freeze temperature, quantity and spacing of freeze pipes, are same, the ground pre-freezing time affects the project cost as well as the strength of frozen wall. So, in this paper, a subway double-line tunnel in Guangzhou constructed by AGF is taken as an example. At first, a numerical model for thermal and mechanical analysis is proposed. Then, based on this model, the surface settlements are calculated and compared by using finite element method (FEM) when the ground pre-freezing time are 60, 50, 40 and 30 days, respectively. According to the criteria of code that the maximum surface settlement is less than 30 mm, a minimum pre-freezing time could be selected. At last, under the condition of this minimum pre-freezing time, the temperature fields, stress fields and displacement fields at every excavation step are calculated and analyzed. The safety of frozen earth wall is also evaluated by applying long-term strength equation of frozen soil.