Tunnelling for Metro Rail in the Ground Conditions of the Kathmandu Valley-Patan Line (Bhrikutimandap to Satdobato Section)

Growing traffic calls for a shift from private modes of transport to a sustainable Mass Rapid Transit (MRT) system in the core area of the Kathmandu Valley. A study on tunnelling for the urban rail of the Kathmandu Metro-Patan Line-South section (Bhrikuti Mandap to Satdobato Section) in the ground conditions of the Kathmandu Valley is presented. Underground twin tunnels with in internal diameter of 5.4 m at a tunnel axis depth ranging from 11 to 35 m below ground level are proposed adopting major road alignments wherever feasible. The geometric parameters of the 6.2 km proposed alignment were adopted in accordance with the Standard Urban Railway System for Asia (STRASYA). Based on the archived ground investigation reports, the Earth Pressure Balance (EPB) type of Tunnel Boring Machine (TBM) is found to be most appropriate for the tunnel passing predominantly within the Kalimati Formation of the Valley. Ground movement assessment at the surface due to tunnelling and station box construction is followed by the establishment of 5 and 10 mm settlement contours along the route. Tunnel lining design is performed using a closed-form solution such as the Curtis-Muir Wood Method. A six-segmented precast concrete lining (M40 Concrete grade) of 300 mm thickness is proposed. Such a lining is found adequate for the seismic condition anticipated in the Valley. Major heritage sites along the routes are tabulated to highlight the need for conservation of such sites during metro construction. Spoil generated from the tunnelling activity is estimated to highlight a need for spoil management.