Construction of extension shield tunnel using ground freezing method

This paper describes the construction of the extension tunnel for the major telephone artery (GL-44m, outer diameter of the segment 04450mm) owned by Nippon Telegram and Telephone Corporation(NTT) using ground freezing method. The existing shield tunnel links Kasumigaseki to Higashi-Ginza in Tokyo, Japan. The extension tunnel was constructed to connect the existing tunnel with the NTT Ginza office which was located on the route of the existing tunnel. At the junction, pneumatic caisson (04750mm) was buried to a depth 2πı right above the existing tunnel. The ground around the caisson and the shield tunnel (very fine sand, water pressure 0.294MPa) was in order to increase cohesion between the caisson structure and the surrounding ground, preventing the caisson from settling. Under the frozen condition, the shield tunnel segment (1200ton) at the junction was demolished, from where a rectangular widening was excavated upward in order for the caisson structure to be extended to the shield tunnel. After completion of the extension tunnel construction, the ground was forced to thaw and the chemical grout was injected.