Investigating Influence of Metro Jet System Hydration Heat on Artificial Ground Freezing Using Numerical Analysis

Abstract

With the development of urban subway system, intersection is sometimes inevitable during the construction of underground tunnels. When a tunnel to be excavated is underneath an existing subway station, the support wall of the station that lies on the way of the tunnel is often required to be partially dismantled to facilitate the construction of the tunnel. To ensure safety of the existing station and the tunnel, both metro jet system (MJS) using cement paste and artificial ground freezing (AGF) techniques can be applied and sometimes combined to stabilize the area under the influence of tunnel construction. However, when two techniques are combined, the hydration heat from the cement paste will bring an increase in the active freezing time. By taking advantage of an existing project, a preliminary numerical simulation is performed to evaluate the influence of hydration heat on a 100-day ground freezing process with considering the effect of ambient temperature, thermal insulation, and the timing of the initiation of the freezing after MJS treatment. Simulation results show that the active freezing time can be reduced by 53 and 3 days with thermal insulation at the end sections in summer and winter, respectively. Hydration heat leads to an increase of the active freezing time from 14 to 15 days, and the effect of timing for the initiation of artificial ground freezing is insignificant on the active freezing time. Subsequently, consistent with the construction plan and site condition, using the ambient temperature from the previous year, a numerical simulation is also performed to simulate the soil temperature variation during the releasing of hydration heat and the freezing followed. The simulation results show that 52-day active freezing time is required for the formation of a frozen soil wall which met the requirement for further tunneling.