Tunnel alignment

Tunnels can be aligned either straight or inclined. The check for accuracy of alignment and levels is achieved using vertical shafts. The determination of surface alignment and measurements are performed during tunnel surveying. This is called a setting out of a tunnel, which involves the main steps:

1. Surface Survey: In this method, a preliminary survey is performed by precise traversing using theodolite to join the two ends of the proposed tunnel. With the help of a standard plan and based on a detailed survey, a final alignment is selected.
2. Transfer of alignment from surface to underground: This step involves the transfer of centerline or alignment on the surface to the underground shafts.
3. Transferring the levels underground: The leveling of the underground surface is performed in the usual way. Nearest benchmarks are used to transfer the levels underground.