A railway tunnel was designed parallel to an existing 275m long 147 years old tunnel between Jamalpur and Ratanpur section in the state of Bihar in India. The planned horse-shoe shaped tunnel is 7.30m high and wide. The tunnel passes through the jointed quartzites and slates of Precambrian age. Slates are present for 126 m length, whereas quartzites are present for 150 m length of proposed tunnel. Maximum rock cover above the tunnel is about 60m at RD 3495m. The wider section having nitch for trolley refuse is also designed in the tunnel at two locations, one in Quartzite and other in Slates formations. The empirical approach based on the Q-system was used to estimate the support pressures and to design the tunnel supports. Numerical analysis using FLAC3D code has also been carried out. The safe distance of 17m is worked out between the walls of two tunnels. The 20m length of tunnel in portal zone from both ends has been designed with steel rib supports and the rest of the tunnel designed with rock bolt and shotcrete supports.
The existing Sahibganj Loop on the Kiul- Bhagalpur section of the East Central Railway crosses the Jamalpur hills through a tunnel between Jamalpur and Ratanpur Stations. The entire section has double broad gauge (BG) track but through the existing tunnel only single BG track is available. In order to have double track for operating up and down traffic on separate lines through the Jamalpur hills, another tunnel in the close proximity to existing tunnel is envisaged. The existing tunnel of the Ratanpur - Jamalpur section is 275m long aligned in East- West direction. The tunnel was constructed in 1861 and working satisfactorily since then [1]. No major problem and repair/maintenance works have been reported during the use of the tunnel. The D-shaped tunnel has arched roof of radius 3.963m. The height of the tunnel above the rail level is 6.502m whereas the overall height is 7.225m. The tunnel is partially lined with brick masonry. The tunnel passed through two different categories of rocks, viz. quartzites and slates. The quartzites are hard and strong in the tunnel stretches towards the Jamalpur portal whereas the stretches of tunnel towards the Ratanpur portal consist of slates which are comparatively weak and also charged with water. The 113.39m portion of existing tunnel in slates from Ratanpur end, therefore, has been fully lined by brick masonry to protect the tunnel roof and sidewalls. The tunnel in the quartzites, on the other hand, is partly lined at the western side from Jamalpur portal to 32.30m inside the tunnel and the balance 129.61m is unlined. Apart from the brick masonry lining no other primary support was apparently used in the existing tunnel. Slope above portals have been naturally stabilized and the brick masonry lining has been used in the portal area.
