Cable strikes are still one of the fastest ways to turn a routine dig into a life‑changing incident, a public outage and a wrecked programme. Electricity, gas, fibre, water, and district heating often sit closer to the surface than people expect, and older records don’t always match what’s in the ground. The two controls that consistently keep people out of hospital and projects off the news are a meaningful permit‑to‑dig and competent, layered service locating. Both only work if they’re planned, briefed and policed on the day.
TL;DR
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– Layer service locating: current plans, visual checks, CAT/Genny, GPR if needed, then trial holes before machines.
– Make the permit‑to‑dig specific: exact dig limits, depth envelope, hold points, stop‑work triggers, and named supervision.
– Keep machines away from suspected lines until services are proven; hand dig with insulated tools and expose fully.
– Re‑scan and re‑brief as layouts change; record what you find and update the permit before carrying on.
Ground risks and the controls that actually reduce them
/> Underground services rarely run straight or at the same depth for long. Repairs, diversions and legacy installs mean routes meander, and non‑metallic services can sit directly above or below metallic ones that mask them on tracing equipment. The control principle to hold onto is layering: start with current utility records and site reconnaissance, then trace with CAT and Genny, supplement with GPR where the picture is messy or high‑risk, and only then put a spade in. Trial holes prove or disprove what the instruments suggest, and they must be done carefully with insulated tools and a competent observer.
A permit‑to‑dig is not a rubber stamp; it is the site’s control document that sets where, how and by whom the ground is disturbed. It should link to temporary works requirements for support, specify exclusion zones and approach distances to live services, and list escalation points if you hit uncertainty. Finally, remember that detection is not perfect. Wet ground, rebar, parallel cables and broken tracer wires can all confuse readings. When the picture is unclear, step back and reassess rather than “just taking a nibble with the bucket”.
How it plays out on real jobs
/> A civils gang on an urban infill housing site needs a 7 m trench for a new water service across a footpath to the boundary. The plans show multiple services: an old street lighting cable, a telecoms duct and a plastic gas main near the kerb. The supervisor has a permit drafted for the morning with CAT/Genny scanning and trial holes at 2 m intervals. A late delivery shunts the programme and the excavator arrives while the supervisor is tied up relocating barriers. The groundworker sets the bucket in, assuming the earlier spray paint lines are enough. He clips a small duct that turns out to be live fibre, hidden directly above the old lighting cable which masked the trace. Footpath closed, angry neighbours, and a two‑day recovery just to get back to where they started.
This kind of near‑miss is common: schedule pressure, partial information and light‑touch supervision at the wrong moment. The fix isn’t complicated but it is disciplined—no dig until the permit is briefed, locate again if anything changes, and keep machines off until services are proved in daylight and in the exact spot you’re about to cut.
Making the permit‑to‑dig do real work
/> A useful permit starts with scope clarity: the exact trench line or pit, depth envelope, and any plant that will be used. It attaches current utility records and marked‑up prints from a competent locator, and it defines the sequence—scan, mark, trial hole, expose, protect, then machine dig where allowed. It names the permit issuer and the working supervisor, sets hold points (for example, before crossing under a suspected HV route), and explains the stop‑work triggers (uncertain readings, unexpected ducts, or loss of the lookout). It also links to temporary works where trench support, edge protection and access are needed, and sets housekeeping expectations so spray lines, barriers and spoil management don’t get buried under the day’s progress.
Finally, the permit should control interfaces: who else is working nearby, where deliveries will pass, how pedestrians are kept away, and how the dig will be secured if it carries over to another shift. Close‑out matters too—photos, sketches and as‑found notes reduce tomorrow’s surprises and help the next phase go faster.
# Permit‑to‑dig essentials – quick supervisor checklist
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– Confirm utility records are current and match the work area; mark up a working copy for the briefing.
– Arrange CAT and Genny tracing by a competent person and capture what could not be traced or was inconclusive.
– Specify where GPR or a specialist locator is required (congested corridors, suspected non‑metallic services, critical crossings).
– Set out trial hole positions and method: insulated tools, gentle technique, expose the full diameter and run, and maintain approach distances.
– Define exclusion zones, banksman/lookout roles, and plant travel routes to keep tracks/tyres off suspected corridors.
– Record as‑found locations with photos and sketches; update the permit and re‑brief before moving beyond the proven section.
Service locating that stands up to scrutiny
/> Electromagnetic locating with a CAT reads current or a signal from a Genny on metallic conductors; it will not see plastic pipes unless there’s a tracer wire or inserted sonde. GPR can help reveal non‑metallic utilities, but its clarity drops in very wet clays and in rebar‑heavy slabs, and interpreting outputs takes training. Good practice is to plan locating in calm conditions with site noise minimised, uncover inspection chambers and cabinets to find access points, and use combined methods so one tool’s weakness is covered by another.
Mark‑ups should be unambiguous: lines, arrows for direction, depth estimates where reliable, and clear “no dig” zones. Re‑scan immediately before you start a fresh cut or new section—overnight changes, repositioned barriers or spoil heaps can alter the set‑up and readings. Where the route is mission‑critical, bring in a specialist and don’t let cost pressure push you to “good enough” tracing. The hour spent getting certainty is far cheaper than a street being blacked out.
Common mistakes underground
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Copying yesterday’s permit for today’s dig
Conditions, routes and teams change. A permit tied to yesterday’s line, weather and interfaces won’t control today’s risk.
# Treating a CAT sweep as a clearance certificate
/> A single pass isn’t proof. Use induced signal and power modes, sweep at different angles, and accept that inconclusive areas need trial holes or GPR.
# Hand digging like machine digging
/> Aggressive picks and spades can cut as reliably as a bucket. Use insulated tools and expose gently until you can see the full service.
# Leaving exposed services unsupported
/> Once a line is uncovered, protect it. Props, padding and clear spans prevent sagging, abrasion and accidental knocks as the dig proceeds.
Frequent pitfalls and practical fixes
/> Pitfall: assuming services run parallel to kerbs or fences. Fix: verify with multiple sweeps and potholes at corners and junctions, and expect crossings at odd angles where historic diversions were made.
Pitfall: scanning once, then digging in stages over several days. Fix: re‑scan each new section and whenever plant, barriers or spoil positions change; wet weather or new metal on the surface can alter the picture.
Pitfall: confusing survey data quality with certainty. Fix: treat drawings and even good GPR plots as informed guesses until a trial hole proves the line and depth in the exact place you intend to dig.
# Seven‑shift actions to de‑risk excavations
/> In the next week, pick one congested area and organise a fresh locate using both CAT/Genny and, where useful, GPR, then update the marked‑up plan. Re‑brief all gangs on permit hold points and stop‑work triggers, and insist the issuer attends the first trial hole of the day. Walk the plant routes with the foreman and set physical barriers to keep tracks off suspected corridors. Inspect insulated tools, sondes and batteries, and replace anything suspect so nobody is tempted to improvise. Finally, capture as‑found positions from current digs and feed them into tomorrow’s permits so the learning sticks.
Good sites win this risk through planning and insistence, not luck. Expect closer scrutiny on permits and competence where live corridors are present; the basics done well will keep you off the incident log and on programme.
FAQ
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Do we always need a specialist survey before we dig?
Not always. On simple, low‑risk digs with clear tracing and good records, competent in‑house locating and trial holes can be enough. Bring in specialists where routes are congested, the services are critical, or the readings are unclear. If you’re hesitating, it’s usually a sign to escalate.
# What depth is it safe to dig with a machine after scanning?
/> There isn’t a universally “safe” depth. Machines should only be used once services are proven by trial holes and the permit allows it for that specific section. Maintain the approach distances and protection measures set in the permit, and keep a lookout focused on the bucket edge. If anything changes, stop and reassess.
# Can we rely on utility drawings from the undertakers?
/> Treat them as indicative, not definitive. They help you plan, but they may be out of date, incomplete, or inaccurate in depth and position. Always combine them with on‑site locating and trial holes before you commit to machine digging.
# Who should issue and close a permit‑to‑dig?
/> A competent supervisor or manager with authority to stop the work and independence from the digging gang should issue it. They should review the locating evidence, brief the team, and set hold points and controls. Close‑out should include as‑found records and any changes noted, ready for the next phase.
# What should we do if we find an uncharted cable?
/> Stop work and make the area safe—no prodding or pulling. Inform the supervisor and permit issuer, set an exclusion zone, and contact the likely asset owner through site channels. Update the permit and only continue when the service is identified, controlled, and the method adjusted.
