Behavior of existing tunnel beneath braced excavation

In the construction of Nakanoshima Subway Line, one section constructed by cut and cover method crossed over an existing telecommunications tunnel, in alluvial soft clay. It was predicted that this work would influence the existing tunnel in both transverse and longitudinal directions, mainly due to the rebound of ground induced by braced excavation. Some predictions were conducted to investigate these influences, and the behaviors of the existing tunnel were monitored on site during excavation. By comparing the prediction values with measurement results, the influence of soil improvement is shown remarkably that cause the prediction values differ from the measurement results. Then, the prediction of the next construction stage was re-calculated based on the present observed data. The construction was finally accomplished smoothly and safely, while the most dangerous process, ground rebound in the final construction stage, was carefully constructed.