Out-of-Square Setting Out for a Clinic Annex
A 15 m × 9 m single-storey clinic annex is being set out on a site that slopes about 1:20. The four corner pegs were set yesterday using a 30 m fibreglass tape and a 3-4-5 check on one corner. This morning the diagonals measure 17.55 m and 17.49 m. A truck has knocked the north-east peg, and the chain boy reset it by eye. The nearest benchmark is 250 m away, and the dumpy level has been set up but no closed level run has been recorded. The tape's zero end is worn but still readable. Excavation for the strip footing is booked for tomorrow. The client wants the setting-out certificate signed today. You are asked to decide whether the layout is acceptable.
1. Calculate the theoretical diagonal and assess the measured diagonals. What does the result tell you?
Read the worked answer
For a true rectangle, diagonal = √(15² + 9²) = √(225 + 81) = √306 = 17.493 m, say 17.49 m.
The measured diagonals are 17.55 m and 17.49 m. One is about 57 mm longer than the theoretical value, and the two diagonals differ by about 60 mm. In a rectangle the diagonals must be equal, so a 60 mm difference means the corners are not square; the layout is racked or one peg has moved. A typical acceptable tolerance for this size of building is about ±10 mm, so 60 mm is well outside tolerance. The north-east peg reset by eye is the likely culprit, but the worn tape end and the unclosed level run also need checking before the layout is accepted.
2. Describe how you would re-establish the rectangular layout and transfer the level from the benchmark.
Read the worked answer
Re-establish the layout from a clean baseline:
- Clear the area and identify two reliable corners on the longest side. Measure the 15 m side with a verified tape and set stable pegs.
- At one corner, establish a true right angle using the 3-4-5 method, for example 6 m along one direction, 8 m along the other and 10 m on the diagonal. Mark the perpendicular line.
- Measure 9 m along the perpendicular and 15 m along the baseline; set the fourth corner. Check all four sides: 15 m, 9 m, 15 m, 9 m.
- Measure both diagonals. If they differ, adjust the two movable pegs by about half the error in the direction that equalises the diagonals, then recheck sides and diagonals. Repeat until both diagonals agree within tolerance.
- For levels, run a closed level run from the 250 m benchmark to a temporary bench mark on site. Take foresight and backsight readings, close the run, and calculate the misclosure. If the misclosure is acceptable, transfer the required footing level to the pegs. Record all readings.
3. Should you sign the setting-out certificate today? Justify your decision and state what you would tell the client.
Read the worked answer
No. The layout is outside tolerance, one peg was reset by eye, and the level transfer has not been closed. Signing would certify work that could lead to an out-of-square building, inaccurate footings and expensive remedial work.
I would tell the client: the setting out is not yet acceptable because the diagonals differ by about 60 mm; the cause is being checked, including the disturbed peg and the tape condition; the layout will be re-established and the level run closed from the benchmark; the certificate will be signed only after the sides and diagonals agree within tolerance and the level readings are recorded. I would give a realistic time for the recheck and explain that correcting pegs now is far cheaper than correcting foundations later. Excavation should not start until the recheck is complete.