Operator Trench Work and Buried Utilities

The person in the cab shapes a trench more than anyone else on the job. You choose where the machine sits, where the spoil lands, how far you dig ahead of the box and how close the teeth get to a gas line. A crew member can do everything right and still get hurt by a choice you made from the seat.

You already know the basics: 811, soil types, protective systems, the competent person. This guide is about doing that work from the cab. It covers planning the dig, holding grade, handling a trench box and exposing buried lines. In US construction the rules are in 29 CFR 1926 Subpart P, and state plans can be stricter. You need hands-on training and your employer's evaluation before you do any of this. The competent person makes the protection decisions, and you take direction from them.

What You Will Be Able to Do

• Plan a dig around machine position, spoil, pipe and access.
• Work out trench depth and hold grade from outside the trench.
• Handle a trench box within its limits.
• Expose a marked line safely and protect it.
• Respond to water, soft walls and a struck line.

The Rules You Work Inside

Those are minimums. A machine parked 2 feet from the edge can still be too close. The competent person sets how far back heavy equipment stays.

Planning the Dig

Walk the route with the foreman before you start, and settle each of these.

1. The ticket. It is valid, every utility has responded, and the marks match what you see.
2. The machine. Set up on firm ground, with the sprockets away from the trench, so the tracks can pull you back, and so the drive parts are not hanging near the edge.
3. The spoil side. Use one side only if you can. Keep it away from where pipe, the box and the crew will be.
4. The pipe side. Stage pipe where you can reach it without swinging over people.
5. Access. Decide where the ladders go, and how trucks and people will cross.
6. Ground and weather. Look for old backfill, water, vibration from traffic, and rain in the forecast.

Sloping back takes a lot of room. A trench 7 feet deep in Type C soil, with 1-1/2 to 1 slopes, reaches back 10-1/2 feet on each side. On a street there is no room for that, which is why boxes are so common.

Digging to Grade

Working Out the Depth

Plans give the invert, the inside bottom of the pipe. You dig to the bottom of the bedding, which is lower.

Worked example. Ground at the manhole is 103.20. The invert is 96.50, and the plan calls for 6 inches of bedding, which is 0.50 foot. Ignore the thin pipe wall for now.

• Trench bottom: 96.50 - 0.50 = 96.00.
• Depth: 103.20 - 96.00 = 7.20 feet.

That is over 5 feet, so a protective system is required. It is over 4 feet, so a ladder must be within 25 feet of lateral travel for every worker.

Holding Grade From Outside

Nobody should stand in an unprotected trench just to read a rod. Use a laser and a rod or a grade pole from a safe spot, a machine with grade control, or a person inside the protected box.

Worked example. A laser is set at elevation 105.00. Trench bottom is 96.00. The rod reading at grade is 105.00 - 96.00 = 9.00 feet.

Pipe slopes, so the target moves. At 0.40 percent, the invert rises 0.40 foot per 100 feet. At 100 feet up the line, the bottom is 96.40, and a level laser reads 8.60. A laser set to the pipe's grade keeps reading 9.00, which is why pipe crews use one.

Dig technique:

• Take the trench down in passes, and keep the bottom flat and clean with the bucket's cutting edge. Trim the last few inches gently.
• Do not overdig. Bedding can fix a small low spot, but a deep overdig under a pipe needs to be filled and compacted the way the plan requires.
• Keep walls as straight as you can, because rough walls leave gaps behind a box.
• Stop if the soil changes, water shows up, or the wall starts to slough. The competent person needs to see it.

Working With a Trench Box

A box protects the people inside. It does not hold the walls up. Handle it by the rules for that box, which come from the manufacturer's tabulated data, plus OSHA's rules for shields.

• Limits. Do not load a box past its rating. The data states the depth and soil it is rated for.
• No one inside while the box is being installed, removed or moved vertically.
• Movement. Set it so it cannot shift or kick sideways if a wall lets go. Large gaps behind a box let it move.
• Entering and leaving. Workers must be protected on the way in and out too.
• Rigging. Use the lift points on the box, and use rated rigging. Do not drag it with a chain around a spreader.
• Digging below the box. You may dig up to 2 feet below the shield only if it is designed for the full depth of the trench and there is no sign of soil being lost from behind or below it.

Worked example. You have an 8-foot-tall box rated for 14 feet.

• Trench 10 feet deep, with the bottom of the box at 8 feet: that leaves 2 feet of wall below the box. This is allowed, because the box is rated past 10 feet. Keep watching for soil loss at the bottom.
• Trench 12 feet deep: that is 4 feet below the box, which is not allowed. You need a taller box, a stacked setup in the tabulated data, or a different system.
• The same box in a 16-foot trench: over its rating. Stop.

Dig ahead only as far as you can cover. Keep the crew inside the box, and do not let them work at the open end.

Finding and Protecting Buried Lines

OSHA requires you to find the exact location when digging gets close to the estimated location, using safe means. Lines left exposed must be protected, supported or removed. Paint on the ground is an estimate.

The Common Ground Alliance recommends a tolerance zone of the width of the line plus 18 inches on each side. Most states use 18 or 24 inches, and some use more. Inside the zone, states may limit you to hand tools or vacuum excavation, and some forbid power digging. Know your state's rule.

Exposing a Line

1. Stop outside the zone and get off the machine. Find the marks again.
2. Pothole. Have the crew hand dig or vacuum excavate a test hole until the line is visible. Marks give no depth.
3. Measure it. Note its depth and direction. Mark it so you can see it from the seat.
4. Check for more. Lines are often bundled, and a joint trench may hold several.
5. Dig past it carefully, with a spotter watching the bucket. Stay out of the zone with the teeth. Clear it by hand.
6. Support it. A span of exposed pipe or conduit must not hang on its own. Support it the way the owner directs.

Signs you are near something unmarked: warning tape, tracer wire, clean sand or gravel in a band, a concrete cap, or a change in soil color. Stop and look. Private lines are usually not marked through 811, and old lines often are not on any map.

Warning: Never use a machine to find a line. Do not probe with teeth to feel for it, and do not trust depth from a plan. A damaged coating can fail months later, so report every scrape, even if nothing leaks.

If You Hit a Line

• Gas. Stop. If you can do so safely, shut off the engine. Move people away and upwind. Do not try to crimp the line or pull the bucket out, because that can make a spark. Call 911 and the utility.
• Electric. Stay on the machine unless it is on fire. Warn everyone away. If you must leave, jump clear with your feet together, and shuffle away without lifting your feet. Call 911 and the utility.
• Water or sewer. Stop, get people out of the trench, and watch for washed-out walls.
• Every strike. Tell your foreman, and leave the area as it is until the owner has looked at it.

Pipe, Water and Backfill

Lowering pipe. Use rated rigging and a signal person. Nobody stands under the load or in the path of it. Workers in the box stay clear until it is low and under control. Your machine's lift limits still apply.

Water. Do not let anyone work in standing water unless the competent person has put protection in place. A pump needs someone qualified watching it. Seepage from a wall can change the soil's class, so tell the competent person when you see it.

Nearby structures. If digging puts a wall, pole, sidewalk or foundation at risk, it has to be shored, braced or underpinned first. Do not undercut it because it looks solid.

Backfill. Place bedding and backfill as the spec shows, in thin lifts, and do not drop rock on the pipe. Where shoring is used, take it out from the bottom up and backfill as it comes out. Keep compactors clear of the pipe until the cover is deep enough.

Common Mistakes

Check Your Understanding

1. Ground is 214.60, invert is 208.10, and bedding is 4 inches. How deep is the trench, and does it need protection?

4 inches is 0.33 foot. Bottom = 208.10 - 0.33 = 207.77. Depth = 214.60 - 207.77 = 6.83 feet. Yes, it needs a protective system.

2. A laser sits at 216.00 and you want to read trench bottom at 207.77. What is the rod reading?

216.00 - 207.77 = 8.23 feet.

3. A pipe layer wants to step into the box while you are raising it to the next section. What do you do?

Stop. Nobody can be in a box while it is moved vertically.

4. You see orange warning tape in the bucket, and there is no orange mark on the ground. What now?

Stop digging. Tape usually means a line is near. Find out what it is and where it runs before you dig further.

5. You nick a gas line, and the crew wants you to pull the bucket away to stop the leak. What is your answer?

No. Moving it can cause a spark or more damage. Get people away and upwind, call 911 and the utility, and stay clear.

Key Takeaways

• You shape the hazards in a trench. Plan the machine, spoil, pipe and access first.
• Work out depth from the invert and bedding, and hold grade from outside the trench.
• Nobody rides or stands in a box while it is moved vertically.
• Digging 2 feet below a shield is only allowed if the shield is designed for full depth and no soil is being lost.
• Paint is an estimate. Expose lines by hand or vacuum inside the tolerance zone.
• Report every strike, even a scrape.