Page 144 - Civil Engineering Project Management, Fourth Edition

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Site surveys, investigations and layout
12.2 Levelling
For most site survey work tilting or automatic levels provide for quick and
accurate results. As with all instruments, they should be checked regularly
for accuracy and returned to the manufacturer for overhaul and re-setting
at intervals. The levelling staff must be kept clean and its markings clear to
reduce reading errors and it is worth spending time to ensure the chainman
is clear as to where the foot of the staff should be placed and that it is held
vertically. The tripod must be in good condition, and set up so as to avoid
movement during operations. Once fixed to the tripod the instrument must
be levelled using the centre bubble to avoid excessive tilt of the telescope. For
the tilting level the split view of the tubular level bubble must be adjusted
to coincide for each reading while the compensation mechanism in the auto-
matic level ensures that the sighting is in the horizontal plane. On-site checks
on accuracy can be made using the two-peg test.
Even with a good instrument and set up, care must be taken to ensure the
levelling staff is correctly extended and that sighting lengths are not so great
that readings are indistinct. Good visibility is necessary and may be reduced
by heat haze or vibration in the wind. Accuracy of reading and of booking the
readings are also sources of error. Some errors can be reduced by keeping
sight lengths approximately the same and it is a good practice to close the run
of levels back onto a known benchmark.

12.3 Plane surveying

In the past, most survey work depended on triangulation from known fixed
points using a theodolite and this may still be a suitable method for smaller
sites. Again it is necessary to ensure the instrument is in good condition and
that its base is truly horizontal. Readings taken on both faces of the instru-
ment may reduce residual errors. Setting out by taping along a line given
by the theodolite may also still be the clearest way of providing centre lines or
points, particularly for regular structure layouts such as building columns.
The appropriate time for this is when blinding concrete has been placed to col-
umn and wall foundations. The base line, which is either the centre line of the
building, or a line parallel to it but clear of the building, should have been set
out previously by end pegs sited well clear of the work. It is usual to work from
co-ordinates along this base line from some fixed zero point, and measuring
right angle distances out from them. In this way lines of walls and column
centres can be marked on the blinding concrete.
Distances may be measured by steel or fibreglass tape pulled horizontally,
so it is a great convenience if the site is level. If not a plumb bob has to be used
to transfer distances. Distance co-ordinates along the base line from the zero peg
are set out, using the steel tape and marking a pencil line across the peg. The
theodolite is set out over the pencil line, and its position is adjusted laterally so