The Core Drill That Stopped at the Stamp

Why this matters

A two-person plumbing crew had a 12 in core bit on the stand, the rig anchored, the water hooked up and the center mark on the garage floor. The core never got drilled, because the helper swept the floor near the garage door and read the words pressed into the concrete. The scan that followed put a live tendon 2 in from that center mark. This is the record of how the crew got from the stamp to a relocated hole, and of the one number along the way that looked like a safe answer and was not.

The safety rule first, because it decides everything after it. Once a slab is known or suspected to be post-tensioned, nothing cuts, cores or drills into it past a shallow fastener until the tendons are located and the engineer has answered in writing. A cut tendon releases its stored tension at once and can burst out of the slab edge or up through the floor, and it costs the slab its squeeze along the whole tendon's length. The crew stopped the drill, unplugged it and left the rig standing so the setup could be photographed. See related: Post-Tensioned Slabs and the Tendon You Must Not Cut, which owns what a tendon is and why the cut has no field repair.

The job as it was planned

The customer wanted a utility sink on the west wall of a two-car garage in a slab-on-ground house. A camera and locator run, done the week before, had found the building drain running north and south under the garage, 6 ft 0 in from the west wall, its crown about 14 in below the slab surface. The plan was the one this shop used often on slabs: core a 12 in hole over the drain, vacuum-excavate the soil down through it, cut a wye into the drain with a pair of shielded couplings, bring a 2 in branch up through the hole, and patch around it.

The center mark sat on the drain line, 9 ft 0 in out from the back wall of the garage, the wall the house is on. That spot was chosen because it put the new branch about 3 ft from where the sink would hang.

Nobody had asked whether the slab was post-tensioned. The house was a 1990s tract build in a region with expansive clay, which on its own makes a slab a candidate, but the crew's habit was to look for pocket patches outside and they had arrived in the rain and gone straight in through the garage door.

The stamp, and what the crew did in the next ten minutes

The helper found it a foot inside the garage door, under a strip of dirt: a stamped warning reading roughly "post-tensioned slab, do not cut or core". The lead did four things before anyone discussed the schedule.

• Photographed the stamp with a tape in frame, and the rig in place with its center mark, so the job record would show exactly where the hole was going to be.
• Walked the outside of the slab with the customer's permission and found grout patches along the west edge of the garage, the signature of tendon anchors.
• Told the customer in plain words that the hole was paused, why, and that the next steps were a scan and an engineer. The customer had the closing papers; they did not include a tendon plan.
• Opened the shop's structural-cut approval process, which names who may approve a cut in a post-tensioned slab: the engineer of record if one can be found, otherwise a registered design professional the customer engages. See related: The Structural Cut Approval SOP.

The crew then made the mistake this card exists to warn about. They tried to solve it with a tape measure.

The number that looked like an answer

The west-edge pockets ran at regular intervals out from the back wall: 0 ft 6 in, 4 ft 6 in, 8 ft 6 in, 12 ft 6 in and on toward the door, a 4 ft pattern. Each pocket starts a tendon running east and west across the garage. Drawing a line from the 8 ft 6 in pocket straight across the slab put that tendon at the south edge of the planned core: a 12 in core centered at 9 ft 0 in runs from 8 ft 6 in to 9 ft 6 in.

The lead's first reading was that the tendon was right at the edge and the fix was easy: slide the core 6 in north, so it ran from 9 ft 0 in to 10 ft 0 in and cleared the pocket line by 6 in. The drain was still under it. The sink was still 3 ft away.

That reading treats a pocket line as a location, and it is only an estimate. Tendons are placed by hand on chairs before the pour, drift around plumbing and openings, and sag between their high and low points, so a straight line between two pockets tells you where to start scanning, not where the steel is. The lead knew this in the abstract. What stopped the shift was the approval process, which does not accept a pocket line as a site condition, and the customer, who asked the obvious question: how do you know it is there?

What the scan found

A concrete scanning firm came out two days later and ran ground-penetrating radar, which sends a pulse into the slab and reads the reflections off embedded steel, over a square of about 10 ft around the planned spot. The operator marked each tendon on the slab with paint and gave depth readings in the report, and the report stated its own tolerance for the marks.

The marks did not match the pocket lines:

• Tendon A, the one started at the 8 ft 6 in pocket, crossed the drain line at 9 ft 2 in from the back wall. It had drifted 8 in north of its pocket line. That put it 2 in north of the original center mark, inside the core.
• Tendon B, from the 4 ft 6 in pocket, crossed at 4 ft 4 in, 2 in south of its line.
• A north-south tendon, from pockets on the front and back edges, ran 7 ft 9 in from the west wall at that point, 21 in east of the drain line.
• The scan also showed the drain's cleanout riser near the back wall and nothing else embedded in the area.

Run the lead's proposed shift against those marks. The shifted core, 9 ft 0 in to 10 ft 0 in, would have contained tendon A at 9 ft 2 in, 2 in inside its south edge. So would the original. The move meant to clear the tendon would have cored straight into it. The 8 in drift is not unusual, and it is the whole reason a pocket line cannot be cut against: the error ran in the one direction the estimate said was safe.

 Plan view near the drain line, north up
 (distances from the back wall of the garage)

 10'-0" .......[ shifted core ].........
  9'-2" ===========X==================== tendon A
  9'-0" .......[ original core ]........
  8'-6" - - - - - - - - - - - - - - - - - pocket line A
               |                |
  6'-9"   [relocated core]      |
               |                |
  4'-6" - - - - - - - - - - - - - - - - - pocket line B
  4'-4" ================================= tendon B
               |                |
             drain        N-S tendon
          6'-0" from     7'-9" from
          west wall      west wall

Moving the hole, and who decided

With the tendons painted on the floor, the crew could see where a core might fit, but deciding was not theirs. The customer engaged a structural engineer, since the builder's engineer of record could not be traced. The lead sent the stamp photo, the scan report, the drain locate, the planned hole size and a sketch, and asked one specific question: is there a location along the drain line where a 12 in core is acceptable, and with what clearance to the marked tendons?

The engineer's written answer set the condition for this slab: no core edge closer than 12 in to any marked tendon, the scan's stated tolerance already allowed for in that figure, and the hole patched with a nonshrink grout around the new sleeve. That 12 in belongs to this engineer and this slab; it is not a rule to carry to the next garage.

The crew then found the spot. Along the drain line, between tendon A at 9 ft 2 in and tendon B at 4 ft 4 in, the gap is 9 ft 2 in minus 4 ft 4 in, which is 4 ft 10 in, or 58 in. Centering the core in that gap puts its center 29 in north of tendon B, at 6 ft 9 in from the back wall. Its edges run from 6 ft 3 in to 7 ft 3 in.

• North clearance: 9 ft 2 in minus 7 ft 3 in leaves 1 ft 11 in, or 23 in, to tendon A.
• South clearance: 6 ft 3 in minus 4 ft 4 in leaves 1 ft 11 in, or 23 in, to tendon B.
• East clearance: the north-south tendon sits 21 in east of the core's center, and the core's edge is 6 in east of center, so 21 minus 6 leaves 15 in.

Every edge clears the engineer's 12 in, the smallest margin being the 15 in to the east. The lead sent the relocated point back with those three figures and the engineer approved it in writing, naming the hole size and location. The new position is 9 ft 0 in minus 6 ft 9 in, which is 2 ft 3 in, south of the original mark, so the sink's branch arm grew by 27 in of pipe. The customer signed the change.

The core, done the way the approval said

On the coring day the lead checked the painted marks against the scan report before setting the rig, because a mark that has been walked on for a week can be confused with the cleanout layout paint. The rig was anchored with a vacuum base rather than a drilled anchor, since an anchor hole is a hole too. Coring went wet, which keeps the silica down at the bit, and the slurry was vacuumed rather than washed toward the garage door drain; the cord ran through a GFCI because the work is water plus electricity at floor level. See related: How to Follow Table 1 When You Core or Grind Concrete, which owns the silica controls.

The crew watched the cut for the signs that would have stopped it: a bit that suddenly bound or chattered, a change in the slurry color, or a gray plastic fleck in the slurry, which is tendon sheathing. Any of those meant stop, back the bit out without forcing it, and call the engineer before anything else moved. The core came out clean at the slab's full depth with no steel in it. The excavation, the wye and the patch followed the original plan.

What would have taken the crew to a different answer

Three conditions would have flipped this, and each sends the job somewhere other than a moved hole.

• A tendon running along the drain instead of across it. Had the north-south tendon sat within 6 in plus 12 in, or 18 in, of the drain line, no point along that line would clear it, and moving the core north or south would not help. The answer then is to stay out of the slab: route the sink's drain through the wall to an existing above-slab connection, or pump it.
• An elevated post-tensioned slab. A garage deck or a multi-story floor carries its own weight on the tendons. The sibling card is plain that nothing in this kind of process makes drilling one acceptable, and a crew standing on one walks away from coring altogether.
• No stamp. The stamp is what stopped this crew, and its absence would not have meant the slab was conventional. The pockets outside, the house's age and the region all pointed to post-tensioning; checking for them before setting the rig, rather than after, is the habit that does not depend on a helper with a broom.

What the job record holds

The record is what protects the shop if the slab ever cracks near that sink and someone asks whether the core caused it. This one holds the stamp photo with a tape in frame, the photo of the rig at the original mark, the scan report with its marks and tolerance, the engineer's two written answers, the relocated point with its three measured clearances, the customer's signed change, and a photo of the finished core showing clean concrete walls. See related: The Penetration Record SOP. Whether that record settles any later dispute is a question for the shop's attorney and insurer; this is not legal advice. What it does show, in the shop's own documents, is that the crew asked before cutting and cut where the engineer said.

References

• See related: Post-Tensioned Slabs and the Tendon You Must Not Cut
• See related: The Structural Cut Approval SOP
• See related: How to Follow Table 1 When You Core or Grind Concrete
• See related: The Penetration Record SOP
• Ground-penetrating radar scan report from a concrete scanning firm for the specific slab