A post-tension slab is a stressed structure. Cutting a cable is not a drilling mistake, it is a structural event: the tendon loses prestress, the slab can deflect, and the repair is an engineered solution, not a patch.

Non-negotiable sequence

1. Get the drawings. Tendon layout, banded and distributed zones, and any as-built changes. If drawings are missing, assume tendons everywhere.
2. Scan. GPR plus a cover meter, on a grid, marking every indication on the slab.
3. Confirm. Where an indication sits within the clearance zone, use a small 12-20 mm pilot to verify depth and position, or move the hole.
4. Clearance rule. Keep the core edge at least 50 mm from any tendon indication, and further where the engineer specifies. Some specifications require 75-100 mm in banded zones.
5. Record. Photograph the marked slab and the scan printout before coring.

What the scan does not see

Item	Detected by GPR / cover meter?
Tendon	Usually, as a linear indication
Grout duct	Sometimes; duct may read as void
Sheared or displaced tendon	Poorly; layout may not match drawings
Edge stressing anchor	Only if within scan depth

That last row is the one that causes incidents: a tendon displaced during a previous pour is not where the drawing says. Treat every scan as advisory and keep the clearance generous.

Cutting method near tendons

• Wet coring, low feed, no impact tools.
• No chipping hammers within the clearance zone; impact can de-bond a tendon.
• If a cable is struck, stop, evacuate the area, and call the structural engineer. Do not attempt to cut the strand out.

If the hole must sit in a banded zone

Scanning before you mark: what each tool actually shows

The scan is the only step that makes drilling near tendons safe, and different tools answer different questions. Using a rebar locator to find tendons is the single most common mistake on post-tensioned slabs.

Tool	What it finds	What it misses	Use it for
Cover meter	Rebar depth and spacing	Tendons, plastic ducts	Establishing cover before a GPR pass
GPR (ground-penetrating radar)	Ducts, tendons, conduits, voids	Small diameter, deep targets	Locating tendon lines before marking
X-ray / radiography	Dense steel and tendon profile	Requires access both sides, safety controls	Confirmation where GPR is ambiguous

GPR on a slab with a mesh and a duct grid produces a picture, not a coordinate. Mark the tendon lines on the surface in chalk, then treat every line as a no-drill band rather than a line: the duct has width, and the strand can sit off-centre inside it.

Rules that keep a working margin near tendons

• Keep the hole edge at least 50 mm clear of the marked tendon line, and 75 mm where the slab is thin or the tendon is near the surface. Chalk lines are approximate; the margin absorbs the error.
• Never drill through a duct to "see what is inside". A cut strand loses its anchorage and the slab loses capacity at that point.
• Where the layout cannot avoid a tendon band, relocate the hole and get the change approved in writing before drilling, not after.
• Confirm the stressing state with the structural drawings. Tendons are most sensitive before grouting and during stressing, and the same hole can be acceptable one week and not the next.
• Stop immediately on any change in resistance, a metallic sound, or water returning at a different rate. All three can indicate a duct has been breached.

If a duct is hit, stop, do not backfill, and call the engineer. A grouted duct that has been breached can sometimes be injected and re-tensioned; a duct filled with concrete from a hurried repair usually cannot.

What to put on the record when the hole runs close

Holes near tendons are the ones an engineer will be asked about years later, so the file has to answer three questions without a site visit: where exactly the hole went, what was found during the scan, and who approved the proximity.

• Scan images filed against the hole number, not against the floor area.
• Measured clearance from the tendon line to the hole edge, taken with a tape before drilling and written on the mark-up.
• The name and date of the structural approval where clearance is under 75 mm.
• A note of the tendon state at the time: stressed, grouted, or both.

That record takes ten minutes at the rig and settles a dispute that otherwise costs a day of drilling and a structural review.

If a cable is hit: the first ten minutes

Striking a tendon is rare but it is not a situation to improvise. What happens in the first ten minutes decides whether it stays a repair or becomes an incident report.

• Stop drilling and stop all work on the slab. Vibration and further holes near the strike make a local failure into a wider one.
• Do not pull the bit out with force or hammer it free. A jammed bit can drag the tendon; withdraw slowly while reversing, and if it will not move, cut the core body away.
• Mark the location, photograph the exposed strand and note the drilling depth and the drawing reference used. This is the evidence the engineer will ask for first.
• Call the structural engineer and the building owner before anyone fills the hole. A filled hole changes what the engineer can inspect.
• Barricade the area, keep loads off the slab in the affected bay and stop other trades working overhead until cleared.

The commercial point: the cost of a struck tendon is almost never the repair. It is the delay, the engineer's report and the loss of confidence, all of which are reduced by a clean record and an immediate stop.

Options in order of preference: relocate the penetration, reduce the core diameter, use a sleeved penetration cast during a later pour, or get a designed opening with supplementary reinforcement. Coring through a band is a decision for the engineer, not the driller.