Setting Out an 18 m by 12 m Building
An 18 m by 12 m single-storey panchayat building is to be set out on a site that slopes about 1 in 40. The mason sets the four corners with a steel tape and the 3-4-5 method, driving 12 mm bars and stretching nylon string between them. He reports the two diagonals as 21.60 m and 21.67 m and says the difference is small enough. The plinth is to be finished at 100.400 m. A site benchmark of 100.000 m exists on a culvert about 30 m away, and a dumpy level is available; the staff reading on the benchmark is 1.245 m. Footing excavation is due to start in the afternoon and the mason proposes to set the plinth level by eye from the string. The junior engineer has one hour to check the layout and the levels before the excavator arrives.
1. Check the setting out: compute the correct diagonal, interpret the two measured values, and describe how the mason should correct the layout without dismantling the whole grid.
Read the worked answer
Correct diagonal. For an 18 m by 12 m rectangle, diagonal = square root of (18 squared + 12 squared) = square root of (324 + 144) = square root of 468 = 21.633 m.
Interpretation. The readings are 21.60 m, which is 33 mm short, and 21.67 m, which is 37 mm long - a spread of 70 mm. If the four sides are genuinely 18 m and 12 m, this is a parallelogram with two opposite corners out of square, not a rectangle. Setting out to a 70 mm diagonal spread means the wall lines, the lintel bearings and the roof slab will all inherit the error.
But check the measurement before moving pegs. A 70 mm error over 21 m is about 1 in 300, which is within combined tape error. On a site sloping at 1 in 40, a tape held along the slope instead of horizontally reads long; sagging nylon string and reading to the outside of a 12 mm bar instead of its centre each add error.
Correction procedure:
- Re-measure both diagonals with the tape held horizontal and tensioned, reading to the bar centres. If they now agree within 10 mm, nothing more is needed.
- If the spread persists, fix the baseline (one 18 m side) on correct pegs, set 90 degrees at each end using the level's horizontal circle or a 9-12-15 m triangle, and drive the two far corners.
- Measure both diagonals again and shift the free corner about half the remaining difference, roughly 35 mm, perpendicular to the baseline. Repeat until both diagonals agree within 10 mm and all four sides are correct.
- Offset each corner peg by 1 m outside the excavation and record the offsets, so the bars survive the digging.
2. Compute the height of instrument and the staff reading at which the plinth should be set, and give two independent checks you would apply to the levelling.
Read the worked answer
Height of instrument. HI = benchmark reduced level + backsight = 100.000 + 1.245 = 101.245 m.
Required staff reading at plinth. Foresight = HI - required level = 101.245 - 100.400 = 0.845 m. The mason sets the plinth when the staff reads 0.845 m; every plinth corner is set to that same reading, and the reading is transferred to a peg or a paint mark on each corner bar so the string can be fixed to it.
Do not set levels by eye from the string. String levels are wrong for three reasons: it sags over an 18 m pull, it is set to the peg top rather than to the finished level, and the ground slopes at 1 in 40, so over 18 m the fall is about 450 mm - far more than any eye can judge.
Two independent checks:
- Close the level run. After taking the foresights, take a fresh backsight on the same benchmark or a second benchmark. HI must return to 101.245 m within the permitted misclosure, typically within 5 mm times the square root of the number of setups, in millimetres.
- Balance sight lengths. Keep the backsight and foresight distances roughly equal by pacing, so collimation error cancels; keep the staff vertical, check the bubble before and after each reading, and repeat the plinth foresight at each corner so the plinth is proved level in both directions, not just at one point.
3. What tolerances, records and protection would you insist on before excavation starts?
Read the worked answer
Tolerances. For a building of this size, accept a maximum deviation of 10 mm on any side length and a maximum difference of 10 mm between the two diagonals; corner squareness within about 1 in 1,000. Plinth levels at all corners within plus or minus 5 mm of 100.400 m, with no corner low enough to pond water. Any footing set out to a wider tolerance because of a ground beam or raft must be stated on the drawing, not agreed orally on site.
Records to complete before the excavator arrives:
- Field book entries showing the benchmark, the backsight, the calculated HI, and every foresight with the reduced level.
- A setting-out sketch with corner coordinates or offsets, the diagonal checks written on it, and the date and signature of the engineer and the mason.
- Levels of the existing ground at each corner, so earthwork quantities can be measured later.
- Photographs of the pegs and the benchmarks.
Protection. Corner bars offset at least 1 m beyond the excavation line, painted and flagged, with the offset distance recorded. A benchmark protected by a concrete collar or transferred to a permanent structure wall with a paint triangle and a written description. Hand over the level marks to the mason in writing, and re-check the diagonals and one corner level after the excavation is complete - pegs get knocked, and a re-check costs ten minutes against a rebuild that costs weeks.