Utility Tolerance Zone: The Hand-Dig Strip Around Services

What is a utility tolerance zone?

It is a protective band around a located utility. Its width is the diameter of the pipe or cable plus a set distance each side of the outside edge. Within that band, excavation must be by careful hand digging or vacuum methods until the service is exposed; outside it, normal machine digging may continue with care.

Where do the common widths come from?

Mostly from US state damage-prevention laws. Many states define the tolerance zone as the facility's width plus 18 inches (about 0.46 m) on either side; some use 24 inches (about 0.61 m). The figure exists because no locating method is exact and because a marked line shows the centre of a service, not its edges. Ground penetrating radar (GPR) and cable locators both carry positional error, since a geophysical survey infers a service from signals instead of seeing it.

Utility detection comes first, and it is charged per metre scanned; see more about the GPR survey cost calculator.

How is it applied in India?

Through the asset owner's permission conditions, the project specification and good site practice. The Department of Telecommunications' Call Before u Dig app connects excavating agencies with asset owners. The safe distance for a gas main, a high-voltage cable or a fibre route is set by its owner. Where nothing is specified, we advise the client to keep machines at least half a metre clear of the outside edge, and further where our mark carries a lower confidence.

A worked example

A 200 mm water main is marked along a footpath. With 18 inches each side, the hand-dig strip is 0.2 + 2 × 0.46 = about 1.12 m wide. The crew digs by hand or vacuum inside that strip to expose the main, measures it, then works the excavator outside it. The method is described in our guide; read Vacuum Excavation (Potholing).

A second example with a less certain mark

When the mark itself carries a stated tolerance, that tolerance is added to the zone. A 110 mm HDPE fibre duct is located by sonde with a horizontal confidence of ±0.3 m. Our planning default of 0.5 m each side plus the 0.3 m uncertainty gives 0.8 m each side. The hand-dig strip is 0.11 + 2 × 0.8, about 1.71 m wide. A pit for a new pole planned 1.2 m from the mark therefore falls inside the strip and is hand-dug until the duct is exposed and its true position recorded.

Mistakes that defeat the tolerance zone

The zone protects a service only if it is measured and maintained properly.

• Measuring from the paint line. The mark is the centre; the zone runs from the outside edge.
• Assuming a straight line between marks. Utility lines bend between paint marks spaced several metres apart.
• Exposing one point and machining the rest. One test hole fixes one point, not the whole run.
• Working from faded marks. Rain, traffic and sweeping remove paint within days on a busy road.

How the zone appears on Mezux drawings

On request, each located service carries a hatched hand-dig band in its own CAD layer. The band uses the asset owner's figure where one is given, or our 0.5 m default widened by the mark's stated tolerance where it is not. Site teams then see the strip, drawn from the data collected in the field, on the plan as well as on the ground. In the United States the Common Ground Alliance sets out the idea in its Best Practices guide under the excavation tolerance zone practice. In India, the Department of Telecommunications launched Call Before u Dig in March 2023 to connect excavators with asset owners before work begins.

What the zone does not cover

• Depth: it is a horizontal band. Horizontal drilling under a service needs its own vertical clearance.
• Unmarked services: a zone protects only the underground utilities that have been found. A sweep with a locator comes first; see the Mezux guide: Active vs Passive Mode in Cable Locators.
• Changed sites: ground conditions shift as marks wash away and fill gets moved, so old marks should be checked again.

Read our note on SUE survey to detect and map every service in a full investigation, or send your site details for your project.

Frequently asked questions

Is the tolerance zone measured from the paint mark?

From the outside edge of the utility. Because the mark shows the centre, add half the pipe or cable width.

Can a mini excavator work inside the zone?

Not until the service is exposed and its position confirmed. Inside the zone, hand tools or vacuum excavation only.

Who marks property boundaries beside a utility corridor?

A land surveyor; for boundary work see who to call for land survey services.

How long do paint marks last on a road?

Often only days under heavy traffic or rain. On a longer construction project, marks are refreshed or backed up with pegs, flags and coordinates on the drawing.

What if two tolerance zones overlap?

Treat the combined area as one hand-dig strip. Closely spaced cables and water pipes are common in footpaths, and the whole width is dug by hand or vacuum.

Does the zone apply to trenchless boring?

The horizontal zone governs the entry and exit pits; the bore itself also needs a vertical clearance from each service it crosses.

Is there a legal tolerance zone in India?

There is no single national figure for all utilities. The distance is set by each asset owner's conditions, the contract, or the permission granted for the works.

Who sets the zone for a fibre route?

The telecom operator that owns it, in its permission or protection conditions. Where none is given we work to our default and record that assumption on the drawing.

Written by Saniya Parveen · Technically reviewed by Rahul Rai · Updated 8 November 2026