An Artificial Freezing Technique to Facilitate Shield Tail Brush Replacement under High Pore-Water Pressure Using Liquid Nitrogen

During tunnelling, with the progression of the shield tunnelling machine, the shield tail brush, which is an important accessory required to seal the shield tail and prevent the tunnel from ground water or grouting fluid discharging, will gradually be worn-out/damaged and inevitably lead to safety problems. As a result, problematic tail brush needs to be replaced to prevent more serious problems from happening before any further tunnelling. The replacement of this shield tail brush can be very risky, especially when the shield tunnelling machine is located at a soil layer under high pore-water pressure. This study introduces an engineering practice to facilitate the shield tail brush replacement under high pore-water pressure through the development of a frozen soil wall with the help of an artificial freezing technique using liquid nitrogen. A case study is adopted to demonstrate the implementation of this technique. For the specific site condition, a numerical simulation is usually performed first to determine the design parameters (e.g., liquid nitrogen temperature, length and spacing of the freezing pipes) required for artificial freezing process. Several holes along the radial direction of the tunnel were then made to monitor the temperature of the soil around the tunnel. Subsequently, the artificial freezing process was performed according to the design. The replacement of the tail brush was initiated when the frozen soil wall developed during freezing met the requirements for tail brush replacement. The artificial freezing technique using liquid nitrogen proved to be a reliable and time-effective option to facilitate the tail brush replacement under high pore-water pressure.