Ground Improvement by the Freezing Method for Underground Space Utilization

In underground spaces under high water pressure, the need to seal the periphery and face of the working environment and establish a stable front is a challenging issue. One method of creating a stable working front and sealing it against underground water pressure is ground freezing. In this study, application of a freezing method for underground space utilization is investigated. A case study related to part of a comprehensive underground expansion project in a Tokyo suburb is considered. Construction includes a junction of a main tunnel with a rampway that connects the underground tunnel route to the ground surface. At this construction site, the segment connecting the main route and rampway needs to be widened. Because of this enlargement and sequence of tasks below the water table, a temporary stable working front and hydraulically waterproof zone are necessary. As an effective method of ground improvement, freezing is used. Ground behavior around the underground expanded periphery and on the greenfield level due to freezing and thawing action is predicted, and a parametric study of the effect of the surrounding soil stiffness on the freezing behavior is conducted. Additionally, the closing time between freezing pipes is also predicted.