Setting Out of Works — Buildings, Culverts, Bridges, Canals & Alignments

Principles of setting out

Setting out (staking out) is the transfer of positions and levels from design drawings to the ground so that construction is carried out in the correct place, orientation and elevation. It is the reverse of surveying.

1. Work from the whole to the part — establish a control framework (main reference points and baselines) accurately, then set out details from it.
2. Use independent checks — measure diagonals, check distances between set-out points, repeat from different control points.
3. Protect and reference control — place reference (offset) pegs outside the working area so that points can be re-established after excavation disturbs them.
4. Accuracy consistent with the work — higher for structural elements (columns, bolts) than for earthworks.
5. Coordinate with the construction sequence — marks must be available when needed and not obstruct work.
6. Keep records — field notes, sketches and check measurements.

Equipment

• Steel tapes, pegs, nails, string lines, marking paint, pins.
• Theodolite / total station — angles, distances, coordinates (stake-out program).
• Level / automatic level / laser level — levels and gradients.
• GNSS (RTK) — setting out large sites, roads and embankments.
• Laser equipment — rotating lasers (levels over sites), pipe lasers (sewer gradients), line lasers.
• Profile boards, sight rails, travellers (boning rods), batter boards, plumb bobs, spirit levels.

Setting out buildings

Horizontal control

1. Establish a baseline — usually a line parallel to the road boundary or an existing building, as shown on the site plan (building line).
2. Centre line method — the centre lines of all walls are set out and trench widths marked on either side; common for load-bearing walls where the wall thickness is uniform.
3. Face line method — the outer faces of walls are set out (useful where wall thicknesses vary or for framed structures).
4. Right angles are set out by:
   • 3-4-5 method — a triangle with sides in the ratio 3 : 4 : 5 (e.g. 3 m, 4 m, 5 m) gives a right angle.
   • Theodolite/total station.
   • Optical square/cross staff (rough work).
5. Check diagonals — for a rectangle, the two diagonals must be equal: $d=\sqrt{L^2+B^2}$.

Profile boards

Profile boards (horizontal boards nailed to pegs) are fixed about 1–2 m outside the excavation at the ends of each wall; nails or saw-cuts on the boards mark the centre line and trench edges. Strings stretched between opposite boards re-establish the lines at any stage after excavation.

Framed structures

• A grid of column centre lines (axes) is set out from the baseline, preferably by total station using coordinates.
• Column positions are marked by offsets from grid lines; anchor bolts and base plates are set with templates to close tolerances.
• Grid lines are transferred to upper floors by plumbing (optical/laser plumb) or total station.

Levels

• A temporary benchmark (TBM) is established near the site from the nearest benchmark.
• Excavation depths, plinth level, floor levels, lintel and roof levels are set with a level or laser; level pegs are driven to the required RL or marked with offsets.
• Sight rails and boning rods control trench bottom levels for foundations on sloping ground.

Setting out culverts

1. Establish the centre line of the road and the centre line of the stream/drain; their intersection gives the culvert centre.
2. Measure the skew angle (if the culvert is not at right angles to the road).
3. Set out the abutments, wing walls, and headwalls by offsets from these two centre lines.
4. Establish reference pegs outside the excavation on both lines.
5. Transfer bed level, invert level and foundation levels from a TBM; check the waterway gradient.

Setting out bridges

Centre line and span

• The bridge centre line is established on both banks and marked by permanent reference pillars well away from the construction area.
• The span length across a river cannot be taped directly; it is determined by:
  • Triangulation — a base line measured along one bank, angles observed to a point on the other bank (well-conditioned triangles; base lines on both banks for checks).
  • EDM/total station or GNSS.

Location of piers

By intersection: from two or more stations on a base line of known length, angles computed from the design positions of piers are set out with theodolites; the lines of sight intersect at the pier centre (on water, a boat/pontoon or staging is guided to the intersection). With total stations, polar coordinates (angle and distance) from a control station are used directly.

Levels across the river

Levels are carried across wide rivers by reciprocal levelling to eliminate curvature, refraction and collimation errors.

During construction

Pier and well foundation centres are checked regularly during sinking (tilts and shifts), and the positions of bearings and deck levels are set out accurately.

Setting out canals, roads and railways

Centre line

• The centre line is set out from the approved alignment by pegs at regular intervals (e.g. 20–30 m, closer on curves), including tangent points and curves (see Curves).
• Reference pegs are placed outside the construction width.

Slope stakes

Slope stakes mark the points where the side slopes of cuttings (top edges) or embankments (toes) meet the natural ground.

• Grade stakes mark formation levels at intervals; batter boards (sloping profiles) guide the slopes of cuttings and embankments.
• For canals: centre line, bed level, full supply level, banks, berms and spoil banks are set out; bed gradient controlled by levels at each station.

Setting out sewers and pipelines

Sight rails and travellers (boning rods)

1. Sight rails — horizontal rails fixed across the trench (on posts) at manholes/changes of gradient, set at a known height above the invert level of the sewer.
2. The top edges of successive sight rails define a line parallel to the designed gradient of the sewer.
3. A traveller (boning rod) — a T-shaped rod of length equal to the height of the sight rails above invert — is placed on the pipe invert/bedding; when its top is in line with the sight rails, the invert is at the correct level.

• Pipe lasers placed in the pipe or manhole project a beam along the designed gradient — widely used today.
• Horizontal alignment is maintained by offsets from a line or a laser/total station.