Effect of Artificial Ground Freezing on Deep Excavation for Shield Tunnel Restoration: Field Application and Numerical Simulation

Massive ground subsidence occurred during the construction of the cross passage of two main railway tunnels through a geologic formation bearing unexpectedly high gas pressure near the Nakdong River in South Korea. This subsidence caused structural damage to shield tunnel segments. To replace the damaged segments with box structures, a broad and deep excavation employing diaphragm walls was carried out, and artificial ground freezing was applied to connect the box structures with the intact tunnels outside the diaphragm walls. In this study, a three-dimensional thermo-mechanically coupled numerical analysis was conducted to assess the impact of artificial ground freezing on deep excavation. In order to reduce computational time, a one-way coupled approach was adopted in the numerical model, where the thermal analysis was initially performed considering the geometry and construction process, followed by mechanical analysis that incorporated changes in the ground properties based on the freezing temperature. The proposed numerical model was validated by comparing the results with the in situ measurements. Furthermore, the behavior of tunnel segments located outside the diaphragm walls was numerically investigated in relation to the freezing and thawing process. During the freezing process, the tunnel segments experienced heaving due to frost expansion of the ground, while they subsided during the thawing process.