ABSTRACT
A freezing method has been adopted to achieve the safe and accurate launching of large shield machines (12.14m in diameter) at 45m below ground surface. In this work, a huge frost heave pressure of 1.10 MPa was expected to occur because of the freezing of the hard silt layers, therefore raising concern about damage to the diaphragm wall and shaft. In order to avoid such damage, new countermeasures were employed: a sand buffer trench placed between the diaphragm wall and the frozen wall, rejection of the sand, and a heat fence placed in front of the frozen wall. In addition, a special system to measure the frost expansion and deformation levels of the frozen wall was created, to observe and evaluate the countermeasures1 effects. After consideration of the various measurement data, it was proved that frost heave pressure was controlled from 0.15 to 0.29MPa. Therefore the pressure was reduced to approximately one-fifth of that prior to countermeasures. Furthermore, the increase of reinforcement stress in the shaft was confirmed to be small, and there was no problem in structure. Concerning deformation of the frozen wall, the maximum deflection of the wall due to elastic deformation and creep was measured to be 11mm at the center of the opening part, and such deformation kept to a safe level. Through these measurement results and considerations, we confirmed that the countermeasures designed for frost heave and execution was adequate.
