Topic 1 of 6 · Form 5
Site investigation, surveying and setting out
Desk study (maps, records), site reconnaissance, trial pits and boreholes to establish soil type, bearing capacity, water table, services and access.
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Site investigation
Desk study (maps, records), site reconnaissance, trial pits and boreholes to establish soil type, bearing capacity, water table, services and access.
Levelling
Instruments: dumpy and automatic levels, staff, and benchmarks (BM). Readings: backsight (BS), intermediate sight (IS) and foresight (FS).
- Height of collimation method: HPC = RL + BS; RL = HPC − IS or FS.
- Rise and fall method: compare consecutive readings; arithmetic check ΣBS − ΣFS = Σrise − Σfall = last RL − first RL.
Worked example: A backsight of 1.250 m is taken on a benchmark of RL 100.000 m. A foresight of 2.100 m is taken on a peg. Find the reduced level of the peg.
HPC = 100.000 + 1.250 = 101.250; RL = 101.250 − 2.100 = 99.150 m
Setting out
Base line and building line; 3-4-5 triangle and theodolite for right angles; checking diagonals; profiles and corner pegs; transferring levels with a level or water level; excavation depths from reduced levels.
Modern methods
Total stations and GPS for accurate surveying; building information modelling (BIM) introduction.
Check yourself
- What is a benchmark?
- The HPC is 52.400 m and a staff reading is 1.850 m. Find the RL.
- Why are the diagonals of a rectangular building checked?
Show answers
- A fixed point of known reduced level used as a reference.
- 50.550 m
- Equal diagonals confirm the corners are square.
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