The Utility Window Test: Check This Before You Fix a Pile Line

Drilling next to live gas and water mains is routine work, until a shaft that looked fine on paper won't fit in the field. The usual cause is checking clearance to each utility separately instead of checking the space between them.

The mistake

It is natural to check one main at a time: the bore clears the gas line by 8 inches, and separately it clears the water line by 4 inches. Both checks can pass at different positions, while no single position satisfies both. Moving away from one main moves you toward the other.

The test

1. Find the two utilities that bound the shaft.
2. Measure the clear distance between them.
3. Compare it with the widest hole the drilling will make. Ask for the whole sequence: the pilot hole is often wider than the casing or the rock socket.

If the window is only a few inches wider than the hole, the line doesn't work. As a working rule, we look for a window at least about a foot wider than the hole for a cased shaft between two live mains. Below that, the answer is relocating or temporarily removing a utility, or choosing a different line, decided before the rig mobilizes.

Know what the measurement means

• Pipe wall or centerline? A distance to the center of a pipe is not a clearance to the pipe.
• Which face? The near face and far face differ by a full pipe diameter.
• Both ends of the run. A single measurement assumes the utility runs parallel to the line. Pothole at both ends.
• Do the numbers agree? When two field measurements close on a surveyed value, record it. That is the strongest confirmation you can get.

Check the rest of the system too

• Do any rakers, struts or tiebacks cross a main in plan?
• Does a waler trench sit over a shallow main?
• If one utility is relocated, does that solve the whole conflict, or only one side of it?

How potholing fits in

Hydrovac potholing exposes the utilities safely so their real location, depth and size can be recorded. Done before the line is fixed, it turns assumptions into facts and keeps surprises out of the excavation. See what a shoring submittal should include for how these findings carry into the package.

How we help

We pothole and locate utilities with our own hydrovac truck, run the window check against your shoring or caisson layout, and flag conflicts to your engineer and the utility owner before drilling starts. Call (800) 606-3880.