Sang-baik Woo, Jong-ho Park, Hong-Sung Lee, Yong-ki Choi
NATM (New Austrian Tunneling Method) has been widely used in Korea for tunnel construction. The main feature of NATM is to utilize all available means to develop the maximum self-supporting capacity of the surrounding rock or soil itself, and to undertake investigation and monitoring during construction to provide the stability of the tunnel. If undetected worse ground condition is encountered, the strengthening works will be carried out to ensure safety. In this case study, the ground at the entrance of tunnel was revealed to be silty sand contained a lot of core stones during portal excavation, not just as the weathered and soft rocks as originally predicted before design. Thus the tunnel face collapsed with water spouting, and the upper ground surface was also sunk at the same time. The remediation measures including ground grouting, tunnel reinforcing and invert lining were then implemented and completed successfully. It could be a useful reference to the design and construction of tunnels with soil entrance."