Setting Out, Levelling and Site Preparation
This domain covers the journey from cleared ground to a square, level, correctly positioned building. The exam tests the order of operations, the 3-4-5 method, profile boards, and level book arithmetic.
Site preparation sequence
Clear and grub → strip topsoil 150–200 mm and stockpile it → establish a datum and benchmark → set out the building → excavate → hardcore in 150 mm layers, each watered and compacted → anti-termite treatment → blinding → footing.
Setting out a right angle
Use the 3-4-5 (or 6-8-10) triangle. Set the first corner peg on the baseline, measure 3 m along one direction and 4 m along the other, and adjust until the diagonal reads exactly 5 m. Then set all corners, and check the rectangle by making the two diagonals equal. For a 6 m × 8 m building the diagonal must be 10 m. Transfer pegs about 1 m clear of the excavation onto profile boards, with a nail marking each line, and stretch lines between them. Protect pegs and profiles from plant and foot traffic.
Levelling
Set up the dumpy or automatic level, take a backsight to a known benchmark, then a foresight. Height of collimation HI = RL + backsight; reduced level RL = HI − foresight. Book readings immediately, in the field, and check the arithmetic.
Worked example
A benchmark is at RL 100.000 m on the site. Backsight on the staff held at the benchmark = 1.450, so HI = 100.000 + 1.450 = 101.450. A foresight on the trench bottom reads 2.150, so the trench bottom RL = 101.450 − 2.150 = 99.300. Check: backsight − foresight = 1.450 − 2.150 = −0.700, and 99.300 − 100.000 = −0.700, so the booking agrees. The trench is therefore 0.700 m below the benchmark; if the drawing requires RL 99.000, a further 0.300 m must be excavated.
Traps that cost marks
Measuring along the face of the wall instead of the setting-out line; skipping the diagonal check and accepting a “square-looking” corner; using a sagging tape; leaving pegs unprotected so the first dumper wipes them out; writing “2.150” without noting where the staff was held; forgetting that a level must be checked against a second benchmark or a closed run.