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Every linear project crosses existing networks: water, gas, electricity, telecoms, drainage, sometimes oil and district heating. Most of them are known only from the owners' records, drawn at different times, to different standards, often from memory of where the trench went. Hitting one can kill someone, cut a town's supply or stop the job for weeks.
| Level | How the utility was found | Typical reliability |
|---|---|---|
| D | Records only (desktop search) | Metres, sometimes worse; may be missing |
| C | Records plus site reconnaissance (covers, valves, markers) | Better near visible features |
| B | Detected on site (electromagnetic location, ground-penetrating radar) | Decimetres in plan; depth less certain |
| A | Exposed and measured (trial hole, vacuum excavation) | Centimetres, position and depth |
The UK specification PAS 128 and the US standard ASCE 38 both grade utility information this way, with sub-levels for detection accuracy. Read the edition your contract names for the exact definitions; the principle is the same: say how each line was found.
Carry the level into the model
A utility in the model should carry at least:
- its quality level (and the survey that gave it);
- its owner and type (gas, water, HV cable…);
- its depth and how depth was obtained;
- its date of survey.
Then design rules can follow it: a wider clearance zone around level D and C lines, trial holes required before any work near them, and only level A positions used for detailed design at crossings.
The model shows a water main from the owner's record and the piling layout keeps 1.5 m clear of it. Nobody asked how the line was found. During piling the main is struck: it was 2 m from where the record showed it.
The utility model carries quality levels. The main is level D, so the rules give it a 3 m zone and require trial holes before piling. The trial holes find it 2 m from the record; the pile is moved on paper, and the main is upgraded to level A in the model.
Give the quality level for each, and say what you would do before excavating within 2 m of it.
- A telecoms duct shown on the owner's record plan, nothing on site.
- A water main traced with electromagnetic location along its full length.
- A gas main exposed in three trial holes, with position and depth measured.
- Drainage manholes seen on site and lifted, with the pipe directions noted.
Key takeaways
- Utility records show where networks were believed to be; their accuracy varies widely.
- Survey standards grade each utility by quality level: from records only (D) to exposed and measured (A).
- In the UK the reference is PAS 128; in the US, ASCE 38. Both use levels D, C, B and A.
- The quality level should travel with every utility in the model and decide safety zones and trial holes.