Page 146 - Civil Engineering Project Management, Fourth Edition

Site surveys, investigations and layout
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each floor. Alternatively an optical plumb can be used to project a fixed point
up through openings in the floors of the building so as to provide a set of ref-
erence points at each level.
The standard of setting out for tunnels must be high using carefully cali-
brated equipment, precise application and double checking everything. An
accurate tunnel baseline is first set out on the surface using the methods
described above. Transference of this below ground can be done by direct
sighting down a shaft if the shaft is sufficiently large to allow this without dis-
tortion of sight-lines on the theodolite. With smaller shafts, plumbing down
may be used. A frame is needed either side of the shaft to hold the top ends of
the plumb-lines and to allow adjustment to bring them exactly on the base-
line. The plumb-line used should be of stainless steel wire, straight and
unkinked, and the bob of a special type is held in a bath of oil to damp out any
motion. By this means the tunnel line is reproduced at the bottom of the shaft
and can be rechecked as the tunnel proceeds.
Many tunnels are nowadays controlled by lasers, the laser gun being set up
on a known line parallel to the centre line for the tunnel and aimed at a target.
Where a tunnelling machine is used, the operator can adjust the direction of
movement of the machine to keep it on target so that the tunnel is driven in
the right direction. For other methods of tunnelling, target marks can be set on
the soffit of rings, the tunnel direction being kept on line by adjusting the
excavation and packing out any tunnel rings to keep on the proper line.
Lasers are also used in many other situations, usually for controlling con-
struction rather than for original setting out since their accuracy for this may
not be good enough. The laser beam gives a straight line at whatever slope or
level is required, and so can be used for aligning forms for road pavements
or even laying large pipes to a given gradient. For the latter, the laser is pos-
itioned at the start of a line of pipes and focused on the required base line. As
each new pipe is fitted into the pipeline a target is placed in the invert of the
open end of the pipe, using a spirit level to find the bottom point, and the pipe
is adjusted in line and level until the target falls on the laser beam. Bedding
and surround to the pipe are then placed to fix the pipe in position.
Rotating lasers are also widely used and once set up give a constant reference
plane at a known level. Use of a staff fitted with a reflector allows spot levels to
be obtained anywhere in the area covered by the laser. Earthmoving equipment
fitted with appropriate sensors can also be operated to control the level of excav-
ation or filling with minimum input other than by the machine operator.

12.5 Setting out floor levels

Acarpenter’s spirit level should not be used for setting out the level of anything
more than incidental work. It is not sufficient, in conjunction with a straight
edge for instance, for getting a floor screed uniformly level. It is difficult to get
concrete floors uniformly level to an accuracy better than 5mm, and a contractor