A precise and detailed survey is necessary before setting the alignment of a tunnel on the ground. A small error in setting the alignment would result in the two ends never meeting at all. When starting work, both the ends of the tunnel as well as the centre line are marked with precision on the ground so that the correct length of the tunnel can be determined. An accurate survey is then carried out to ensure that the centre line of the alignment and the levels are transferred properly to their underground positions.
The following points require special attention when deciding the alignment and gradient of a tunnel.
(a) The alignment should be straight as far as possible since normally such a route would be the shortest and most economical.
(b) The minimum possible gradient should be provided for a tunnel and its approaches.
(c) Proper ventilation and adequate lighting should be provided inside the tunnel.
(d) The side drains in a tunnel should be given a minimum gradient of 1 in 500 for effective drainage. In longer tunnels, the gradient should be provided from the centre towards the ends for effective and efficient drainage.