Tunnel alignment is simply the position or layout of the tunnel on the ground; the alignment can be either horizontal or vertical. While a horizontal alignment runs along the tunnel center, a vertical alignment defines the changes in elevations of the tunnel. Tunnel alignment is an essential design consideration one should perform before undertaking tunnel construction.
A detailed tunnel surveying with precise measurements is performed to set the alignment of a newly constructed tunnel. If the setting of alignment goes wrong, the ends of the tunnel won't meet. When the work starts, the centerline is marked on the ground with high precision, so that it gives the correct length to the tunnel. After this, an accurate survey is performed to ensure that the correct centerline or horizontal alignment of the tunnel and the other levels are transferred to the underground positions.
Alignment Monitoring with Total Station
A total station (TS) or total station theodolite (TST) is an electronic/optical instrument used for surveying and building construction. It is an electronic transit theodolite integrated with electronic distance measurement (EDM) to measure both vertical and horizontal angles and the slope distance from the instrument to a particular point, and an on-board computer to collect data and perform triangulation calculations.
Robotic or motorized total stations allow the operator to control the instrument from a distance via remote control. This eliminates the need for an assistant staff member as the operator holds the retroreflector and controls the total station from the observed point. These motorized total stations can also be used in automated setups known as Automated Motorized Total Station (AMTS).