A Case Study of Cavity Formation and its Rectification Works in Small Sized Tunnels with Excessive Water Inflow

Abstract

Tunneling in Himalayan geological conditions can always present difficulties in underground excavations due to presence of active tectonic divisions, fault, thrust, complex geomorphology and ground water inflow. These factors can be pre-determined through various methods of geological investigations such as detailed Geological Survey, bore hole logging, ERT survey, Seismic survey, etc. and can be constructive in tunnel alignments design. As such investigations are rarely done in case of hydropower projects in Nepal, encounter of weak geological conditions and subsequent cavity formation is common. The particular case of penstock tunnel of MMKJA (14.3 MW) includes similar weak geological conditions, presence of clay bands, open joints and cavity zone formation due to high water ingress. This paper establishes a particular model and methodology for advancement in small sized cavity related tunnels with high water ingress. The model consists of Rib erections, forepolling, Shotcrete / Shotcrete backfill and channelization of water inflow. The model purposed in this paper can be helpful for advancement in similar conditions encountered elsewhere.