12. Ground Freezing for Shield Tunneling, Japan

The ground freezing method is a ground improvement technique used as temporary work such as in shield tunnel construction. This chapter introduces two projects using the ground freezing method in Japan. One is a case adopted to remove the existing steel sheet piles under the subway tunnel structure; the other is a case adopted as shield arrival protection work of a discharge tunnel in the thermal power plant station. In the former case, it was necessary to excavate an adit under the existing subway tunnel structure in order to remove the existing steel sheet pile. The ground freezing method was adopted as a protection work for excavation of the adit. In the latter case, a newly developed CO₂ ground freezing method was adopted for the first time in the world, in order to cut off water at arrival point of the shield to build discharge tunnels. The ground freezing method of the CO₂ has advantages such as lower temperatures and smaller freezing facilities as compared with the conventional method, in addition to reducing environmental loads. In this case, the construction period was shortened utilizing the advantage of CO₂ system.