ABSTRACT
Since tunnels in urban areas are, in many cases, driven through soft ground and in locations close to various utilities and structures, maintaining the stability of not only the tunnels but also such nearby existing structures is of utmost importance. Besides, the presence of groundwater could greatly affect the stability of the cutting face during excavation. To solve these problems, Mito Tunnel, constructed by NATM, adopted a new auxiliary system for minimizing displacements as well as that for controlling groundwater. Several new approaches, including ground improvement which stabilize the foot of top heading by a special jet grouting (hereafter called the “X-Jet method”), forepiling with long steel pipes, and drainage system specially designed for unconsolidated ground with high groundwater level, were adopted to ensure the safety of tunnel construction.
