_edited.jpg)
Tunnel Surveying
The process of setting out the alignment of the tunnel on the ground and then transferring the same to inside of the tunnel through shaft is called tunnel surveying.
The survey work of a tunnel involves following operation
1. Locating center line on the ground
2. Constructing shaft over the center line
3. Transferring the center line inside the tunnel
1. Locating center line on the ground
After deciding the route for the tunnel, its center line is accurately set out on the hill or ground by means of a common theodolite on a calm and clear day when the length of tunnel is short. If tunnel is long and to be constructed under high mountain; the alignment is set out by triangulation preferably with the help of a micrometer transit theodolite. The alignment is then finally set out by permanent monuments of stone or concrete.
The position of such permanent monument should be located by taking offset from nearby permanent structure for controlling or checking the alignment till the tunnel project is completed.
2. Construction of shaft over the center line
After setting out the alignment on the ground shafts are constructed suitable intervals for transferring the center line to inside of the tunnel.
3. Transferring the center line inside the tunnel
After constructing the shafts, the center line of the tunnel is to be transferred down the shafts. For this purpose, generally two small pillars are constructed on opposite edges of shaft along the center line of the tunnel. On the top of these pillars, the points corresponding to the center line are correctly marked and the wire is then stretched between them. After this, two plumb-bob are suspended by a piano-wire inside the shaft as shown in fig (a). Two points are then marked by lowering the plumb-bob to the bottom of the shaft. The line joining these two points represents the center line of the tunnel marked on the ground. These lines are further extended into the tunnel heading as the work advances, by a theodolite placed in the shaft. Points along the center line are marked by a peg provided with a plumb-bob, fixed to the roof of the tunnel as shown in fig (b).
For preventing the oscillation or vibration of hanging wires having plumb-bob are used and kept immersed in a pot containing oil, which is placed on floor of the shaft.
