How do you avoid hitting buried services when digging?
HSE guidance HSG47 describes a safe system of work in three parts: plan the work and get service plans, detect, identify and mark the services, then dig safely. Treat every service as live until the owner has disconnected it and it is proven safe.
Plan first and get the plans
Before breaking ground, get plans or other suitable information on all underground services in the area, from every owner. Most electricity cables belong to the regional network operator, but highway authorities, Network Rail, the Ministry of Defence and others own some, and gas can be owned by independent transporters or private LPG owners. One-call services can help find who to ask.
HSE says the client, with the principal designer where there is one, must give designers and contractors the pre-construction information about underground services. If you cannot get information, as in an emergency, work as though services are there.
Plans are only a guide. They do not normally show gas service connections, which you should assume exist, and having no services on a plan or no signs on the surface does not mean the ground is clear.
Scan with a CAT and Genny
A cable avoidance tool (CAT) is a cable locator. A signal generator (the Genny) puts a signal onto a cable or pipe so the locator can follow it. HSE's guidance says signal generators greatly improve accuracy and should be used whenever possible.
Locators are for trained people. HSE says anyone who uses one needs thorough training in its use and limits, must follow the maker's instructions, and should have it checked, calibrated and maintained. Scan repeatedly during the job, because readings improve as cover is removed.
A clear reading is not clearance. Some locators miss cables carrying little or no current, may show two close services as one, and do not detect plastic pipes without a tracer wire. Services also kink and loop, so trace them across the whole work area.
Mark up what you find
Record the services on the working drawings and mark them on the ground, using a system everyone on site understands. HSE suggests waterproof crayon, chalk or biodegradable paint on paved surfaces and wooden pegs on grass, preferably to one side of the service. Do not use steel pins or spikes that could damage a shallow service.
Dig safely
Give the people digging a written plan showing where the services are. Once the locator has fixed the line, dig trial holes by hand tool or vacuum excavation to confirm position, depth and type. HSE also advises:
- Dig alongside the service rather than above it, and expose it in the end by digging horizontally.
- Use insulated tools near electric cables, and ease spades in with foot pressure instead of spiking or throwing them. Keep picks out of soft clay near services.
- Where practicable, use hand-held power tools to break a hard surface only 500 mm or more from the marked line, then expose the service by hand.
- Have a second person watch the excavator driver from outside the arm's reach, and keep mechanical excavators at least 500 mm from a gas pipe.
- Wear proper clothing: ordinary work clothes reduce burns, bare-chested work makes them worse, and man-made fibres can melt onto skin.
Assume it is live
HSE says to treat all services as live until disconnected and proven safe at the point of work, and to get written confirmation from the owner before removing a redundant one. Until you have identified an exposed service, assume black plastic is a live electricity cable, treat iron or steel pipe as gas, and if in doubt treat it as electricity or gas. Warning tiles and tape can be moved, so do not rely on them.
If you hit one
Always report damage to the owner, however slight. For a cable strike, HSE says the driver should stay in the cab, the area should be isolated and secured, and nobody should approach the machine, cable or excavation until the owner has made it safe. For gas, do not attempt a repair. Evacuate, ring the National Gas Emergency number, 0800 111 999, and put out naked flames and ban smoking within at least 5 m of the leak. An electrical strike can also be a reportable dangerous occurrence under RIDDOR if the resulting fire or explosion stops the plant for more than 24 hours or causes a significant risk of death.