Page 143 - Civil Engineering Project Management, Fourth Edition

12
Site surveys, investigations

and layout

12.1 Responsibility

The responsibility for setting out the works usually lies entirely with the con-
tractor who will work to the dimensions shown on drawings and from levels
or reference points given on the contract drawings or notified to him.
Benchmarks for levels may be national marks, such as Ordinance Datum in
the UK, or they may be special marks set up locally as used by the original
surveyors for design of the works. Where the absolute level of the works is
not critical in relation to other structures a local level mark may be chosen,
preferably given high enough value to avoid negative levels in the deepest
expected excavations. From the given benchmarks, levels should be brought
to convenient benchmarks on the site itself in positions which are unlikely
to be disturbed. Existing structures, or cast concrete blocks should be used
with the exact point marked clearly and precisely levelled in.
Location of the works in plan is provided by reference to national grid points
or to a local reference line. A series of fixed points must be provided around the
site to allow accurate setting out relative to the reference. If these are to last any
time they will need to be set in concrete or on existing structures and marked
and protected. The exact position of these survey points can be established by
triangulation where suitable Ordinance or other points exist, or by traverse
using electromagnetic distance measurement (EDM) or total station equipment
(see Section 12.3). It must be emphasized that establishing the site level and
survey points accurately is essential. Many contractors will have professional
survey staff to carry this out, but otherwise the work may be subcontracted to
a specialist firm of surveyors.
Once the basic stations are established the contractor’s engineers will set
out detailed grid lines, levels and sight rails to allow construction to proceed.
The resident engineer’s staff should check critical lines and levels, although
they do not need to replicate all the setting out work. Although the contractor
remains responsible for setting out errors both he and the engineer have a
duty to see that the works are properly constructed to line and level.