Out-of-Square Setting-Out on a Sloping Plot
A 12 m x 8 m rectangular community hall is being set out on a plot that falls about 600 mm across the 12 m frontage. The mason established the first corner with a peg and set the 12 m line using a steel tape, then used the 3-4-5 method to set the 8 m return. After fixing all four corner pegs, the site engineer measures both diagonals: AC = 14.39 m and BD = 14.47 m. The trench for the front footing has already been marked with lime powder. A dumpy level is available, with a temporary benchmark (TBM) 45 m away across a busy access road. The mason says the 80 mm diagonal difference is 'close enough for a village hall' and wants to start excavation. The drawing shows a 230 mm wall and a 1.0 m wide verandah on the front.
1. Interpret the diagonal readings. Is the setting out acceptable? Calculate the expected diagonal and describe how you would correct the setting out.
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For a 12 m by 8 m rectangle the expected diagonal is the square root of (12 squared plus 8 squared), which is the square root of 208, or 14.422 m. The measured diagonals are 14.39 m and 14.47 m. Their average is close to 14.42 m, but the 80 mm difference means the corners do not form a true rectangle; the shape is likely a parallelogram. An 80 mm error is too large for a building with a 230 mm wall and a 1.0 m verandah, because it will throw the wall lines and the verandah setting out out of parallel.
The correction should start from a reliable fixed line, preferably the 12 m frontage. Keep the two front corners fixed if they are correct, then reset the rear corners by intersecting a steel tape at 8 m from each front corner with the required diagonal of 14.422 m. Pull the tapes taut and measure at the same temperature and tension. Adjust the pegs until both diagonals agree within a few millimetres, then recheck all four sides. Only after this should the lime powder be re-marked. A theodolite or total station would be faster and less cumulative-error prone.
2. Explain how to transfer the TBM level to the four corners with the dumpy level, including the checks needed to avoid error.
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Set up the dumpy level midway between the TBM and the building where possible, so that the backsight and foresight distances are roughly equal; this cancels collimation error. Take a backsight on the TBM and calculate the height of instrument. Then take foresights to a staff held on each corner peg or on a stable transfer point. Keep the staff vertical and on firm ground, and avoid taking readings while traffic is passing if vibration is felt.
Because the TBM is across a busy road, use a safe crossing and a turning point rather than carrying the instrument across traffic repeatedly. Close the level run by returning to the TBM or to a second known point; the misclosure should be small and consistent with the accuracy needed for setting out. Check the bubble before every reading and protect the instrument from direct sun. Record the backsight, intermediate sights, foresights and calculated reduced levels so that the corner levels can be checked independently.
3. What setting-out records should be kept before excavation begins?
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Records should include a sketch of the building with the corner pegs, dimensions, diagonals and offsets from a fixed boundary or reference line. The TBM location, its reduced level, the instrument used, backsight and foresight readings, and the calculated corner levels should be recorded. The date, time, weather and names of the surveyor and checker should be entered.
The diagonal check should be recorded as measured values, not just 'OK', and any correction should be noted. The approved drawing revision, the position of the verandah and the wall thickness should be referenced. Photographs of the pegs and lime lines are useful before excavation disturbs them. These records allow the setting out to be rechecked after excavation and provide evidence if a later dispute arises about the building line.