Project managment

Responsibility

The responsibility for setting out the works usually lies entirely with the contractor 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. Aseries 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.

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