How are underground services located and relocated?

Striking an unidentified service is one of the few construction incidents that can injure someone, shut down a site and cut supply to a whole street at the same time.

It is also entirely preventable, and the prevention is procedural rather than technical.

Plans are the starting point, not the answer

Authority plans are obtained for every service before any ground is broken: electricity, gas, water, sewer, telecommunications and the client's own private services.

Those plans establish what should be there. They do not establish where it is.

Depths vary. Alignments were changed during original construction and never recorded. Private services, the ones installed by the site owner over decades, frequently appear on no plan at all.

On school and council sites, which have been added to repeatedly over long periods, the private services are the ones that cause the problems.

Locating on the ground

A qualified locator scans the work area and marks what is found, using electromagnetic location and ground penetrating radar as the conditions require.

The marks go on the ground and the findings go on the drawings, so the information survives the first rain.

Where a service is critical or its position is uncertain, it is proven by non-destructive excavation before machine work starts. Hydro excavation exposes the service without touching it, and the position is then a measurement rather than an assumption.

Excavation method follows what was found

Machine excavation stays clear of located services by the required margin. Inside that margin, the work is done by hand or by hydro excavation.

Permits, where they are required, are issued against the located information rather than against the plans.

Relocations sit with the authority

Where a service has to move, the asset owner controls it.

The design has to be approved by them. The method has to be approved by them. In many cases the work itself can only be carried out by the authority or its accredited contractor, on their programme.

Which means a relocation is not an item the contractor can compress. It is an external dependency with its own lead time, and the only real mitigation is starting the application early.

On projects with a fixed handover date, an unidentified relocation discovered mid-programme is one of the more serious risks available. Identifying it at tender is worth considerably more than resolving it quickly later.

Keeping the site supplied

On an occupied site, a relocation frequently means an interruption to supply.

That interruption is planned the same way an electrical isolation is planned: agreed date, agreed duration, temporary supply where the service cannot be lost, and confirmation before it happens.

The record afterwards

The located and as-built service positions are recorded and handed over.

For a client who will hold the asset for decades and dig it up again, that record is one of the more valuable parts of the handover package, and it is the reason the next project does not start from the same uncertainty.

Discuss a project with service constraints.