What Underground Utilities Change About the Dig

Why this matters

Getting the locate done is the part everyone knows, and a sibling card covers the ticket, the notice period and the colour code. This one starts after the paint is on the ground, because that is where the excavation actually changes. A located utility changes the dig twice. Before you reach it, it takes the machine out of a band of ground and replaces it with a method that cannot cut a pipe. After you expose it, 29 CFR 1926.651(b)(4) makes it something your excavation has to hold up, which is a duty most crews have never read and almost nobody plans for. And the marks themselves answer only one of the two questions you need answered, because a locate gives horizontal position and says nothing whatsoever about depth.

If you strike one

Response is by utility type and there is no generic version. Nobody re-enters the excavation for any of these.

• Gas. Everybody leaves on foot, upwind and away from the release and from any low ground it could collect in. Do not start or stop any engine, do not move any vehicle, do not operate a switch, a phone or a radio in the area, and leave a running machine running exactly where it is. Call 911 and the gas utility from a safe distance. Keep people out of buildings downwind. Do not try to stop the flow, crimp, or plug the line, and do not go back for tools.
• Electric. Treat the ground around the machine as energised. If the machine is in contact and you are on it, stay on it and wait for the utility to de-energise, unless fire forces you off. If you must leave, jump clear so you are never touching the machine and the ground at the same moment, land with feet together, and shuffle or hop away with your feet together for a good distance before taking a normal step. Nobody approaches the machine on foot. Call 911 and the utility.
• Water. Get everyone out of the cut first. A broken main does not just flood, it washes soil out from behind the wall, and the cave-in follows the water rather than the impact.
• Sewer. Everyone out, and treat the cut as an atmospheric hazard from that point until it is tested from the surface.
• Telecom or fibre. No immediate physical hazard, but do not attempt any repair, and do not look into the end of a cut fibre.

What the marks are, and the one thing they are not

The locate answers a horizontal question with a stated tolerance, and it does not answer a vertical one at all. There is no depth in a locate, no depth on the paint, and any depth a locator mentions verbally is an observation, not a location.

That matters because cover changes over a facility's life and nothing in the marking process notices. A street reconstruction, a mill-and-overlay, a regrade, a ditch cleaned out, a driveway cut in, erosion on a slope - all of them move the surface relative to a pipe that has not moved at all. Records held by the utility describe the cover at installation, which may have been decades ago.

The four duties, in the order 1926.651(b) puts them

1. (b)(1) Determine the estimated location of sewer, telephone, fuel, electric, water and any other underground installations that reasonably may be expected to be encountered, before opening the excavation.
2. (b)(2) Contact the utility owners within established or customary local response times, advise them of the work, and ask them to establish the location before excavation starts. If an owner cannot respond within 24 hours - unless a longer period is required by your state or local law, which is a real and common variation - or cannot establish the exact location, the employer may proceed with caution and using detection equipment or other acceptable means to locate the installation. Read that proviso carefully: it is permission to use another method, not permission to dig blind.
3. (b)(3) Determine the exact location by safe and acceptable means as the excavation approaches the estimated location. This is the clause that takes the machine out of the ground.
4. (b)(4) Protect, support or remove underground installations as necessary to safeguard employees, while the excavation is open. This one is a continuing duty for the whole time the cut exists, not a task.

The tolerance zone, and the line the machine actually stops at

The tolerance zone is the band either side of the marks inside which mechanical excavation is prohibited and a non-destructive method is required. Its width is set by your state's damage prevention statute, and it is commonly 18 to 24 inches measured from the outside edge of the marked facility, so check yours rather than carrying a number across a state line. Where your state has not fixed one, the Common Ground Alliance Best Practices use 18 inches from the outside edge of the facility, and those practices bind you only where your state adopts them by reference or your contract incorporates them.

Inside that zone the methods are hand digging or vacuum excavation, and both have conditions:

• Hand digging with a blunt or round-nose shovel. A sharp spade, a pick, or a probing bar driven into the ground is a strike waiting to happen on a plastic gas service or a direct-buried cable, and neither of those resists a point load.
• Vacuum excavation is better where you have it, but some utilities specify air rather than water near their facilities, and some specify a maximum pressure and a standoff, because a jet at pressure can cut a cable jacket or a pipe coating. The facility owner owns that answer; ask before you mobilise rather than at the hole.
• No mechanical equipment inside the zone. Not a small bucket, not carefully, not with a good operator.

Mark the stop line separately from the utility. The legal boundary is a distance from paint that is on the ground for a different purpose, and an operator working a bucket to a paint band on a broken surface is estimating. Paint or lath a distinct stop line at the outer edge of the tolerance zone plus one bucket width, in a different colour from the locate marks, and let the operator work to a line instead of to a judgement. That extra bucket width is the cheapest margin on the job and it runs in the only direction worth running in.

Cover is not located: one crossing, worked

The job. A 48-inch-deep service trench crossing a marked gas main in a residential street.

• Cover at installation, from the utility's own records: 36 in
• Street reconstruction since, which lowered the finished surface by 8 in
• Actual cover now: 36 - 8 = 28 in
• Trench depth: 48 in
• Where the main sits relative to the trench bottom: 48 - 28 = 20 in above it

So a bucket taking a full-depth pass at that station passes straight through the main's elevation with 20 inches still to go before it reaches design depth. There was no error in the locate, no error in the utility's records, and no carelessness in the plan. The horizontal position was correct and the vertical assumption was never located by anybody.

The correction is a sequencing one, not an arithmetic one. The crossing gets exposed by hand or vacuum before the machine reaches that station, not when it gets near design depth. Expose it, confirm the actual top of pipe by measurement, record the elevation, and only then does the machine work the stations either side. A crew that plans to "watch for it" is planning to find it with a bucket.

What would change the numbers. Any of the following makes the cover unknowable from records alone and moves the exposure earlier: a resurfacing history the utility's records do not carry, a regraded lot, a facility installed by directional drilling, which does not run at a constant depth between pits, or a service that was raised or lowered at a tie-in. The only sources that answer it are the utility's own records, an as-built, or a test hole, and of the three only the test hole is measured on the day.

Once it is exposed, it is a structure you have to hold up

1926.651(b)(4) says protected, supported or removed as necessary to safeguard employees, while the excavation is open. In practice that means a pipe crossing your trench has become a beam spanning a span nobody designed for.

• Joints have almost no bending capacity. Bell-and-spigot cast iron, clay and asbestos-cement pipe carry essentially nothing across a joint. A joint left hanging over an open span will part, and it will do it after you leave as often as while you are there.
• Support means bearing, not suspension by rope. Cribbing under the pipe, or a strongback or beam across the trench with proper slings, sized by somebody rather than assembled from what was on the truck. Rope over a plank is the standard field improvisation and it is the one that fails when the trench walls it bears on move.
• The support is also an obstruction. A supported line crossing the trench partitions it for egress purposes and can put a person under a live facility. Both of those need a plan before the crossing is exposed, not after.
• Backfill is part of the duty. Bedding and compaction under and around a re-supported line is what stops it settling into the void you created, and settlement after backfill is a strike you will not be there to see.
• Removal is never your call. Nobody cuts, moves, or relocates a live facility except its owner. If it has to move, that is a utility work order and a schedule item.

How to verify

1. Confirm the ticket is live and the marks are current before each day's dig, not just at the start of the job. Old paint is not a locate.
2. Reconcile the marks against every record you have - the utility's as-built, the street's reconstruction history, the plans for the parallel systems. Any disagreement is resolved by a test hole, not by picking the more convenient record.
3. Mark the machine stop line on the ground as its own line, in a colour that is not a locate colour, at the tolerance-zone edge plus a bucket width.
4. Expose every crossing by hand or vacuum before the machine reaches its station, working from grade or from inside the protective system and never from an unprotected cut, and with the machine stopped and out of swing while anyone is in the zone. Write the measured top-of-pipe elevation on the plan as you go, because the next crew will otherwise inherit the same unknown you just resolved.
5. Decide the support method for each crossing before it is exposed, and have the material on site. A crossing found at 15:30 with no cribbing on the truck is a crossing that gets supported with whatever is nearby.
6. Walk every supported crossing at the end of each shift, from grade, and check for movement at the bearing points and for material loss underneath. The failure mode that catches people is not the strike, it is the line that was fine on Tuesday, sagged slowly through Wednesday, and parted overnight into an open trench nobody was standing in yet.

References

• 29 CFR 1926.651(b), underground installations, including the estimated-location duty at (b)(1), the owner-contact and 24-hour proviso at (b)(2), exact location by safe and acceptable means at (b)(3), and the duty to protect, support or remove while the excavation is open at (b)(4)
• Your state's underground damage prevention statute, which sets the tolerance zone width and the notice period that apply to your work
• Common Ground Alliance Best Practices, in the edition your state adopts by reference or your contract incorporates, for tolerance-zone and non-destructive excavation practice
• See related: The Utility Locate: Before You Dig for the ticket, notice period and colour code; What Access and Egress Has to Be in Place Before Anyone Enters for what a supported crossing does to travel; universal-why-an-excavation-can-become-a-hazardous-atmosphere