As a building rises the vertical
alignment must also be controlled. This can be done by extending
building centre lines at right angles to each other out to fixed
points clear of the structure.
These lines can then be projected up
the building and marked, allowing accurate measurements from these
marks at 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 reference points at each level.
The standard of setting out for tunnels
must be high using carefully calibrated 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 distortion 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 baseline. 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 construction 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 positioned 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 excavation or filling with minimum input other than by the
machine operator.
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