Checking a 12.6 m by 8.4 m Setting Out Before Excavation
A single-storey house 12.6 m by 8.4 m is to be set out on a sloping block in Kandy. The front wall must be 1.5 m from the boundary and parallel to the road. A labourer has set four pegs using only a tape and a spirit level. The measured diagonals are 15.12 m and 15.18 m. The rear-left peg is 80 mm inside the required boundary setback, and the ground falls 600 mm from front to rear. A dumpy level and a benchmark on the road kerb are available but have not been used. Excavation for strip footings is booked for tomorrow and a concrete truck is ordered. The owner wants the veranda steps to align with the road. You are asked to check the setting out before the excavator arrives.
1. Diagnose the diagonal discrepancy and state whether the layout is acceptable. What checks would you make before deciding?
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The two diagonals should be equal in a true rectangle. A 60 mm difference over roughly 15 m is too large for a building set-out; it means the corners are not square, even if the 12.6 m and 8.4 m sides were measured correctly. Do not split the difference or average the diagonals.
Checks: confirm the tape is not sagging, is held at the same point on each peg and is read consistently; check that the sides are actually 12.6 m and 8.4 m; check that the front line is parallel to the road, not just 1.5 m from the boundary at one point; use the dumpy level to transfer the benchmark and to check that the pegs are set at the same level or that slope corrections are applied. Also check the rear-left peg against the boundary with a perpendicular offset, not a diagonal measurement. If the sides and one diagonal are correct, the other diagonal is wrong and the layout must be re-established from the reference line. The 80 mm encroachment is a separate failure and cannot be corrected by rotating the rectangle.
2. Describe the step-by-step correction, including the boundary encroachment and the sloping ground.
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Step 1 - establish a reliable reference. Set a string line parallel to the road at exactly 1.5 m from the boundary, using offsets measured perpendicular to the boundary at both ends. Step 2 - set the front corners on that line at 12.6 m apart. Step 3 - square the corners using a 3-4-5 triangle scaled up (for example 6 m, 8 m, 10 m) and check with equal diagonals. Step 4 - set the rear corners from the front corners at 8.4 m, then measure both diagonals; adjust only the peg that is out, then recheck all four sides and both diagonals. Step 5 - correct the encroachment by moving the whole rear line forward, not by pulling one corner; the building must keep the required setback along its full length. Step 6 - transfer levels from the road kerb benchmark with the dumpy level, and mark the footing depth from that level, not from the sloping ground. Step 7 - protect and record the pegs with offset stakes and a sketch. Get the owner or engineer to approve the corrected corners before excavation.
3. What will you record and communicate before the excavator starts?
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Give the excavator operator a simple, signed set-out sketch showing the building corners, the 12.6 m and 8.4 m dimensions, both diagonals, the 1.5 m boundary offset, the road line, the benchmark and the required footing depth. Mark each corner with a peg and two offset stakes so the pegs can be re-established if the machine disturbs them.
Record the actual measurements, the diagonal check and the level readings in the site diary, and note that the original layout was rejected because of the 60 mm diagonal error and the 80 mm encroachment. Hold a short toolbox talk with the operator: no excavation outside the marked lines, stop if a peg is moved, and keep the spoil away from the boundary. Inform the owner and the concrete supplier of the corrected start time if the check causes a delay. The key point is that the excavator must work to a checked layout, not to the labourer's original pegs.