Vacuum Excavation (Potholing): Exposing Services Safely

What is vacuum excavation?

Vacuum excavation is a non-destructive digging method. A hand-held lance breaks up the soil with air or water, and a truck- or trailer-mounted vacuum unit pulls the spoil up a large hose into a debris tank. Because nothing sharp touches the ground near the service, it is the safest common way to expose underground utilities.

Air or water?

Holes are placed only where the scan shows a service; the scan itself can be priced with more about the GPR survey cost calculator.

How is a pothole used in SUE?

It provides quality level A, the highest of the four ASCE 38 levels. Ground penetrating radar (GPR) surveys and electromagnetic locating, both forms of geophysical survey, give QL-B: position and approximate depth. A pothole exposes the service so the crew can tape the depth to its top, record diameter, material, condition and number of ducts, and fix the position by survey. That is why potholes are placed at conflicts and tight crossings found during utility detection, not at random. More on that structure: a closer look at SUE survey.

A typical test hole

On a city road, the surface is cored or cut, the hole is cleared by lance and suction, and the service appears in well under an hour in normal soil. The measurement taken in the drawing above, 0.92 m to the top of the pipe, goes straight into the data collected for that site. The GPR depth at that point is logged beside it, so the scan's accuracy on that site is known. The hole is backfilled and reinstated the same day.

Mezux does not own vacuum trucks; we place the holes from our scan, attend when the client's contractor digs, and record each exposure.

Using a pothole to calibrate the whole survey

Each exposed service gives a measured depth that can correct every GPR depth nearby. Say the scan put a water main at 0.85 m using 0.10 m/ns, and the pothole tapes it at 0.92 m. The velocity is then about 0.10 × 0.92 / 0.85, roughly 0.108 m/ns, and other depths on that stretch are scaled up by the same ratio. Two or three holes at different points of a route show whether one velocity holds or whether the soil changes. The spoil is small: a 0.3 × 0.3 m hole 1.0 m deep removes about 0.09 m³, some 90 litres.

Mistakes on test-hole work

Small slips at the hole can undo the value of the whole exercise.

• Digging off the mark. A hole half a metre to one side may miss the service and give a false all-clear.
• Too much water pressure. High-pressure water can damage cable sheaths and pipe coatings.
• No survey of the hole. A depth with no coordinates cannot be placed on the drawing.
• Backfilling without a marker. Warning tape over the exposed service helps the next excavator.
• Over-reading one hole. One exposure proves one point; the line between holes is still QL-B.

What a Mezux test-hole record contains

One sheet per hole, linked to the drawing and the conflict matrix. It gives the hole ID, coordinates, surface type and depth to the top of the service, with its diameter, material, duct count and condition. A photograph with a scale follows, then the GPR-predicted depth at that spot and the difference between the two. ASCE 38-22 counts such an exposure as quality level A. PAS 128:2022, the UK specification for detection, verification and location, likewise treats exposure as the means of verifying what a survey detected.

Where do test holes earn their cost?

At the places where a wrong depth would hurt most. Vacuum excavation is slower and dearer per point than scanning, so it is used selectively. Holes go at every row in the conflict matrix where clearance is tight, and wherever a gas main, high-voltage cable or trunk fibre route is involved. They also go at HDD entry and exit pits, under any bore that passes close beneath a service, and wherever ground conditions change, to check the radar velocity. A few well-placed holes can confirm kilometres of scanned line, which is more cost-effective than a single repair to a damaged main.

Related locating methods

A pothole shows one point; locators follow utility lines between points. To trace a metallic service from the hole, see the Mezux guide: Active vs Passive Mode in Cable Locators. For plastic water pipes and ducts, a transmitter can be pushed in through an open end; read more on Sonde (Locating Transmitter).

To plan scan-and-pothole work for a construction project, send your site details for your project.

Frequently asked questions

Is potholing the same as a trial pit?

The purpose is the same: to expose a service. A trial pit is larger and hand- or machine-dug; a pothole is small and vacuum-dug.

Is vacuum excavation always required?

No. Careful hand digging is common in India. Vacuum methods are faster and safer where services are dense or fragile.

Can pothole results be tied to coordinates?

Yes. Each hole is fixed by DGPS or total station; for boundary and level work on the same site, see who to call for land survey services.

How big is a typical pothole?

About 0.3 m square or round, large enough to see the service and measure it. Duct banks may need a longer slot.

Can test holes be dug in a concrete road?

Yes, after the surface is cored or saw-cut. The core is kept or replaced and the hole reinstated the same day where traffic allows.

How many test holes does a route need?

Enough to cover every critical conflict and to check calibration in each soil type. On many routes that means a few per kilometre, more at junctions.

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