Potholing (also called daylighting or test holing) means digging a small hole to expose an underground utility and confirm its exact location, depth and direction before excavating near it. Today most potholing is done by vacuum excavation, a "soft dig" method that loosens soil with water or air and vacuums it into a truck.
Hydro vs. air vacuum excavation
| Method | How it works | Pros | Cons |
|---|---|---|---|
| Hydro excavation | Pressurized water cuts the soil; the slurry is vacuumed into a debris tank | Fast in most soils, including clay and frozen ground | Produces slurry to dispose of; wet spoil can't be reused as backfill right away |
| Air excavation | Compressed air loosens soil; dry spoil is vacuumed up | Spoil can be reused as backfill; no water needed | Slower in hard clay; dust |
Both are far less likely to damage a line than a bucket, though operators must still follow pressure and nozzle-distance limits to protect coatings and cables.
When to pothole
- Inside the tolerance zone around marked lines, typically 18 to 24 inches on each side, set by state law.
- Before boring or directional drilling across or near existing lines.
- Where marks are unclear, conflicting or missing, or records are old.
- At crossings where your excavation will pass over or under a line.
- Before driving piles, posts or anchors near utilities.
Doing it right
- Confirm the 811 ticket is current and all owners have responded. See utility marking colors.
- Center the pothole on the mark and dig in small, controlled passes.
- When the line appears, expose enough to confirm direction, size and material.
- Record the location (station, offset, GPS), depth to top, size, material and owner, and photograph it.
- Protect and support the exposed line while it's open. OSHA requires underground installations to be protected, supported or removed as needed while an excavation is open (1926.651(b)(4)).
- Backfill per the owner's and agency's requirements, and restore the surface.
Why it pays
Utility strikes cause injuries, outages, fines and delays. A few potholes at crossings and inside tolerance zones cost far less than repairing a gas or fiber line.
Sources and official references
Last reviewed: October 25, 2026 · Free general education based on OSHA 29 CFR 1926 Subpart P. It does not replace the site-specific training and competent person your employer must provide, and it does not issue a certificate. Nothing you enter on this page is saved or sent.