Survey and Setting Out
Survey and setting out of the tunnel alignment at the time of construction constitutes a critical phase in the construction of a tunnel. The alignment of the tunnel is marked on the topographical sheet and the same is transferred to the ground with the aid of a theodolite, starting at one end, which point is identified with reference some identifiable benchmark and direction. Permanent benchmarks have to be established to carry the levels into the tunnels and for checking during construction. In the case of tunnels driven from portals, to start with one work point at the portal and a backsight point on the working line are adequate for extending the alignment into the tunnel. The tunnel axis is aligned on the surface directly or with reference to the intermediate triangulation stations. Transition curves can be set out with sufficient accuracy by perpendicular offsets; this is feasible, however, only if the width of the tunnel is adequate for the purpose.