SETTING OUT VERTICALITY, TUNNELS AND PIPELINES IN CIVIL ENGINEERING PROJECTS


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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