Existing lines are not always where the map shows them
Before digging, existing-utility records are requested from each authority: power, water, sewer, telecoms, gas. Those drawings do not always reflect changes made over the years, and in older districts the real position can differ from the record.
So at critical crossings a hand-dug trial pit is faster than pushing on with the machine. A cut cable or a punctured water main takes back a day's gain many times over.
Traffic and pedestrian safety get planned like the work itself
Excavation inside built-up areas needs a traffic plan as much as a route plan: which lane closes when, where pedestrians cross, how night lighting and signage work. These are settled before work starts and agreed with the municipality.
An unplanned lane closure can lead to the work being stopped. A halted excavation is both a hazard and a cost for as long as it stays open.
Backfill and surface are the only visible part
However well the network is laid, the user only sees the surface. Backfill placed without layering and compaction shows up months later as a depression in the road — and that means reopening finished work.
Recording the compaction settles the argument before it starts, for client and contractor alike. Without a record the discussion stops being a technical one.
Levels and gradients cannot be corrected later
On gravity lines — foul and stormwater — the gradient is the job. A line laid with a reverse fall will not work however sound the pipe is: blockages keep coming back and the cause stays out of sight.
So levels are checked as the excavation advances, not after the trench is closed. Measuring the fall on a buried line means digging it up again.