Underground utilities and digging safely

Beneath most work sites lies a hidden network: electrical cables, natural gas lines, telecommunication and fiber-optic cables, water mains, sewers, and sometimes oil, steam or district energy pipes. Underground electrical cables can carry thousands of volts. Striking one with a shovel, an excavator bucket, an auger, a ground rod or a fence post can kill the operator or anyone touching the equipment, cause an arc flash, cut power to hospitals and homes, and ignite gas from a nearby line. This lesson covers how to find buried utilities before you dig and how to dig around them safely.

Who digs, and why strikes happen

Excavation is not just for construction crews. Landscapers, fence installers, sign installers, farmers, homeowners, irrigation contractors, telecom installers and maintenance staff installing posts or anchors all disturb the ground. Damage to buried utilities is common. In the United States, the Common Ground Alliance's damage information reporting tool analyzed 196,977 unique damage reports for 2024, and the single largest root cause, at about a quarter of damages, was failure to notify the one-call center before digging. The next largest causes were excavators not maintaining clearance after the lines were marked, and lines that were missed or marked inaccurately by locators. Canadian damage prevention organizations publish similar reports, and their findings point the same way.

Call or click before you dig

In the United States, the national number 811 connects excavators and homeowners to their state one-call center, which notifies the utility owners in the area so they can locate and mark their buried lines. Every state has a one-call law. Most require notice two or three business days before digging, excluding weekends and holidays, and set how long a locate ticket stays valid before it must be renewed. The service is free to the caller. Online requests are available in most states.

Utility owners locate and mark only the lines they own, typically up to the meter or the property line. Private lines beyond that point, such as power to a detached garage, parking lot lights, a sign, a gate operator or a well pump, belong to the property owner and are not marked by the one-call system. They must be located by a private locating company hired by the owner or contractor. Many strikes happen on these private lines because the crew assumed everything had been marked.

Reading the marks

Locators mark the approximate location of buried lines with paint, flags or stakes in a uniform color code used across North America.

Marks show the approximate horizontal position of a line, not its depth. Depth varies with grading, frost, erosion and how carefully the line was installed, and a line may have been moved since it was put in. Several lines may share a trench. Marks also fade, wash away or get scraped off by traffic, so keep them visible and request a re-mark if they are lost. The excavator can mark the proposed dig area in white before calling, which helps locators focus on the right place.

Tolerance zones and hand digging

Because marks are approximate, state laws define a tolerance zone on either side of the marks, most often 18 in or 24 in from the outside edge of the line, in which only hand tools or vacuum excavation may be used until the line is exposed and its position confirmed. Some owners, especially for high-pressure gas and transmission cables, set larger zones and require their own staff to be present.

Safe practices in and near the zone include:

• Expose the line by careful hand digging or vacuum excavation, also called potholing or daylighting, before using machines nearby.
• Use blunt or rounded hand tools and gentle prying; do not stab or jab with picks, bars or sharp spades.
• Once the line is exposed, protect and support it so it cannot sag, be struck or be damaged when the trench is backfilled.
• Keep machine buckets and augers outside the zone, and keep a spotter watching where marks are close to the work.
• Watch for warning tape, concrete caps, sand bedding or conduit, which often lie above cables.
• Remember that trenchless methods such as boring and drilling also strike lines; their path must be planned and the line exposed where the bore crosses it.

If you strike or expose a line

If equipment contacts an underground electrical cable, treat it like contact with an overhead line. The operator should stay on the machine if it is safe to do so, move the bucket away only if that can be done safely, warn others to keep away and call 911 and the utility. If the operator must leave, they should jump clear, land with feet together and shuffle away, as described in lesson 6. Never try to pull a cable out of the ground or push it aside with tools.

If you damage a gas line, leave the area immediately on foot, upwind, without operating switches, engines or phones near the leak, then call 911 and the gas utility. Even minor damage, such as a nick in the coating of a cable or pipe, must be reported to the owner so that it can be repaired before it fails later. Hiding damage is dangerous and, in many jurisdictions, illegal.

Ground rods, stakes and posts

Driving a ground rod, fence post, survey stake, tent peg or anchor is excavation for the purposes of locate rules in most jurisdictions, even though no hole is dug. Shallow cables, particularly for street lighting, parking lot lighting, sprinklers and landscape lighting, are often only a few centimeters or inches below the surface. Before driving anything into the ground, check that locates have been done and that the stake is well clear of any marked line.

Responsibilities

The excavator is responsible for requesting locates, waiting until they are complete, keeping the marks visible, digging carefully in the zone, and reporting damage. The utility owner is responsible for marking its lines accurately within the time allowed. The property owner is responsible for private lines. The employer must train workers and plan the work, including emergency procedures. Workers have the right and duty to stop if locates are missing, expired or unclear.

Before the locate arrives

Planning starts before the one-call request. Gather site drawings and as-built records, look for clues such as electrical pedestals, meters, manholes, valve boxes, light standards and signs, and ask the property owner about private lines. Mark the proposed excavation in white. Plan how the excavation will be done near likely lines, which equipment will be used, and how the crew will respond if a line is struck. On the day of excavation, check that all owners have responded and that marks are present for every utility listed on the ticket. If an owner has not responded, or marks are unclear, do not dig.

Vacuum excavation

Hydro-vacuum and air-vacuum excavation use pressurized water or air to loosen soil and a powerful vacuum to remove it into a tank. They expose buried lines with much less risk of damage than mechanical digging and are widely used for daylighting near critical utilities. They still need care: high-pressure water can damage cable jackets and pipe coatings if the nozzle is held too close, and the operator must follow the owner's minimum distances and pressure limits.

State laws typically require excavators to maintain marks, renew tickets before they expire, and notify the one-call center if marks are lost. Exposed facilities must be protected and supported, and owners may need to inspect them before backfilling.

Locate marks are also a communication tool between crews. When several contractors work on the same site, the excavator who requested the locate should share the ticket and the marks with others who will dig, rather than letting each crew assume someone else checked. A short meeting at the start of the day to walk the marks with the crew, point out the lines and agree on the tolerance zones prevents many strikes.