Design of Tunnels
A tunnel design has to cover the following elements: geometry of alignment, tunnel profile or cross-section, design of supports and lining and design of transition. In surface tunnels which pass through hilly regions, alignment is mainly governed by the topography. Placing a tunnel much below the groundwater table should be avoided. From the drainage consideration also, a certain minimum gradient may have to be provided in the tunnel portion for easy drainage of water entering the tunnel. Electrification was done in 1909 through the tunnel for increasing traction capacity. In 1929, by providing a single long tunnel across a spur, all the seven reversals were eliminated. The curvature resistance inside a tunnel is the same as that outside and hence the principles governing fixing of the curvature with respect to proposed speed of trains are applicable to the tunnel also. The material of the tunnel lining must also be taken into consideration when choosing the cross-section.