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.
