Explain the methods of transferring reduced levels from surface to underground in a tunnel setting out work.

Explain the methods of transferring reduced levels from surface to underground in a tunnel setting out work. 

i) Setting out central line of tunnel

ii) Setting out inside tunnels

iii) Transferring of alignment through shafts

i) Setting out central line of tunnel:

The centre-line of tunnels are fixed on the surface along with shaft locations.

Generally the surface control points of the tunnels are not visible from each other. However, by the method of reciprocal ranging points on the summit can be established which can be joined to get the central line. The measurements should be made accurately. Linear measurements are made using invar substance bars with an accuracy of 1 in 10000. Angular measurements are made using 1 second theodolite with an accuracy 0f 15√ N where N is the number of angles. In case of N tunnels in hilly regions it is neither feasible to align the tunnel ends by direct ranging or reciprocal ranging. In such cases precise triangulation has to be used.

The figure shows a scheme of triangulation network with QR as base line for a tunnel

project. Here all the angles are measured accurately by one second theodolite. Usual corrections for length, temperature, terrain, sag and reduction of levels with respect to sea level are all followed in arriving at the values of the coordinates. The traverse is adjusted for angles and coordinates. The proposed tunnel axis is shown in figure as HR.

ii) Setting out Inside Tunnels:

After the coordinates of portals and shafts are finalized, setting out is started. Centre line of

tunnel is done as shown in figure from various portals and shafts.

Back sighting on the pillar, aligned and constructed as far as practicable on the extended

centre line such as pillar C and then by transiting. Reference points are constructed on the roof of tunnels or slightly below the invert for every 300 m.

iii) Transferring of alignment through shafts:

Transfer of alignment is done through shafts by adopting any one of the following methods:

i) By hanging two or more plumb lines down the shaft.

ii) By lighting directly from edge of shaft where shaft diameter to depth ratio is high.

Co-planning is done by hanging two or more plumb lines down the shaft and determining

the bearing of the plumb planes so formed which are connected to the surface. The plumb lines

should be well apart as for as possible. The plumb lines are of special type. The line shall be of

fine steel wire and carrying a symmetrical weight of 35 kg or more. The wire should be well

stretched to keep it tight. In order to keep the wires vertical, the bob should be contained in a

canister with a hood. This arrangement will shield the bob and will reduce oscillations set up by air currents or by water dropping down the shafts. The canister can be filled with water or oil to reduce the vibrations. The bearing of the plumb plane underground is assumed same as at the surface.

This forms the starting direction for the underground survey work.

The following procedure is adopted for transferring the centerline from top.

Ø A theodolite is set up on top of the hill at a suitable position to maintain the centre line of theshaft.

Ø The RLs of both the ends of the shaft are determined by a level. Knowing the bottom RLs ofthe ends the depth of the shaft is found.

Ø Excavation of the shaft is started and verticality is maintained with the help of the plumb-bob which is suspended from wires from top through pulleys.

Ø The excavation is continued until the required bottom level is reached. The depth of the shaft is measured by measuring the length of suspended wire.

Ø The centre line inside the tunnel is maintained by a precise theodolite. This type of

theodolites are provided with an artificial illumination system to enable work at night and in

the darkness of the tunnel.

Ø It should be properly taken care to see that the centre line is maintained from both ends and one transferred from top coincide.