Trenching and Excavation Safety

A trench looks harmless until it fails. A trench wall can drop with no warning, in seconds, and a person caught in it is usually pinned too tightly to breathe. A cubic yard of soil weighs about 2,500 to 3,000 pounds, roughly the weight of a small car. That is why a trench only waist deep still deserves respect.

This matters to anyone who digs or works below grade: pipe layers, plumbers, irrigation installers, gas and water crews, electricians and telecom crews running conduit, landscapers, foundation crews and equipment operators. In US construction, OSHA's excavation rules are in 29 CFR 1926 Subpart P (sections 1926.650 to 1926.652). States that run their own OSHA plans may be stricter, and your employer's rules can be stricter still.

An excavation is any cut, cavity, trench or depression in the earth made by removing soil. A trench is a narrow excavation that is deeper than it is wide and no more than 15 feet wide at the bottom.

What You Will Be Able to Do

• Get underground utilities marked before digging and read the paint and flags.
• Explain when a trench needs a protective system and what the three main systems are.
• Check that safe access, spoil placement and traffic controls are in place before you climb in.
• Spot the warning signs of a failing trench wall and get people out.
• Respond correctly to a utility strike or a cave-in.

Why Trench Walls Fail

Soil stays in a vertical wall only because of its own strength and the pressure of the ground around it. When you dig, you take away half of that support. These things make failure more likely:

• Previously dug soil. Backfill around old utilities has lost its strength and acts nothing like undisturbed ground.
• Water. Rain, seepage and a leaking pipe all weaken soil. Drying can also open cracks.
• Vibration. Traffic, compactors, pile driving and running equipment shake the wall loose.
• Weight near the edge. Spoil piles, stacked pipe and parked machines push on the wall.
• Time. A wall that stood fine this morning can fail this afternoon as it dries, cracks or gets wet.

A cave-in is the best known hazard, but it is not the only one. People in and around excavations also get hurt by falling loads, equipment, struck utilities, bad air, water and falls into the opening.

Before Anyone Digs: Call 811

Every US state has a one-call center. Dial 811 or file a ticket online, and the center tells the utility owners to mark their lines. The service is free. Skipping it risks an electrocution, a gas explosion, a flooded trench and a large repair bill.

1. Mark the dig area in white if your state asks for it. White paint or flags show the locators where you plan to dig.
2. Call 811 or file online before the legal deadline. Most states require at least 2 to 3 working days of notice, and some require more. The exact number comes from your state's one-call law.
3. Wait for the positive response. Do not start until the required time has passed and every utility on the ticket has answered or marked.
4. Check the marks against the ticket. If a utility you can see, such as a meter or pedestal, has no mark, stop and ask.
5. Renew the ticket before it expires. Tickets last a set time that varies by state. If marks fade, wash out or get scraped away, call again.

Private lines are not covered. The one-call center marks the utility's lines, usually up to the meter. A line the owner installed after that, such as a feed to a shed, a sprinkler main or a private gas line, needs a private locator.

The American Public Works Association color code tells you what is under the marks:

Marks show the approximate location, not the exact one. Each state sets a tolerance zone, commonly 18 to 24 inches on either side of the outer edges of the line. Inside that zone, use hand tools or vacuum excavation. Dig carefully with a blunt or rounded spade, working from the side, until you have daylighted the line, meaning you can see it. Keep exposed lines supported and protected for as long as the trench is open.

Warning: Never use a machine bucket, a pick or a pointed bar inside the tolerance zone. A gas line or a buried power cable can be damaged by a hit that seems minor.

The Competent Person

OSHA requires a competent person on every excavation. That person can identify existing and predictable hazards and has the authority to fix them right away. The title is not about seniority. The person must be trained in soil analysis, protective systems and the requirements of the standard.

The competent person must inspect the excavation, the area around it and the protective system:

• Every day before work starts.
• As needed during the shift.
• After every rainstorm or other event that raises the risk.

If they see signs of a possible cave-in or a failed protective system, they must get everyone out until it is fixed. If you have not been told who your competent person is, ask before you get in.

How Soil Is Classified

The competent person classifies the soil using at least one visual test and at least one manual test. Common manual tests include rolling a thread of moist soil and pressing a thumb into a fresh sample. Reading this section does not qualify you to classify soil, but you should know what the words mean.

Soil cannot be Type A if it is cracked, has been disturbed, is exposed to vibration, or sits in layers that tilt toward the trench. When in doubt, the soil is treated as Type C. Most jobs in existing streets and yards are not in Type A soil.

Protective Systems

A trench 5 feet deep or more must have a protective system unless it is entirely in stable rock. A trench under 5 feet needs one too if the competent person sees any sign of a possible cave-in. The depth rule is a minimum, not a safe zone.

There are three main choices.

Sloping and Benching

Sloping cuts the walls back at an angle so they cannot fall in. Benching does the same thing with a series of steps. Benching is not allowed in Type C soil. Slopes are written as horizontal to vertical (H:V). These are OSHA's maximum allowable slopes for excavations under 20 feet deep:

Worked example. You have a trench 8 feet deep and 3 feet wide at the bottom, in Type B soil. A 1:1 slope means each side goes back 8 feet.

• Width at the top = 8 + 3 + 8 = 19 feet.

If the same trench is in Type C soil, each side goes back 8 x 1.5 = 12 feet.

• Width at the top = 12 + 3 + 12 = 27 feet.

Now add at least 2 feet on each side for spoil. This is why sloping often does not fit on a street or a tight lot, and why crews use shields instead.

Shoring

Shoring is a support system that pushes against the trench walls to hold them in place. Hydraulic aluminum shores are common, and timber shoring is still used. Install shoring from the top down and remove it from the bottom up, and backfill as you take it out. Workers install it from outside the trench or from a protected area wherever they can.

Shields

A shield, also called a trench box, is a steel or aluminum structure that sits in the trench. It does not hold the walls up. It protects the people inside if the walls fall. These rules apply:

• Stay inside the shield while you are in the trench. Do not stand at the end of an open box or in front of it.
• No one may be in the shield while it is being installed, removed or moved vertically.
• The trench may be dug no more than 2 feet below the bottom of the shield, and only if the shield is rated for the full depth and no soil can be lost from behind or below it.
• Where the upper part of the excavation is sloped, the shield must stand at least 18 inches above the top of the vertical side.
• Follow the tabulated data for that shield. A shield rated for 12 feet of depth does not protect you at 16.

For any excavation deeper than 20 feet, the sloping or benching must be designed by a registered professional engineer. Manufactured shoring and shields must follow the manufacturer's tabulated data. Anything used outside that data needs an engineer's approval.

Getting In and Out

In any trench 4 feet deep or more, a ladder, stairway or ramp must be within 25 feet of lateral travel for every worker. In a long trench, you need more than one. Ladders should extend at least 3 feet above the top of the trench and be secured. Never climb the shoring or a shield wall unless it was built for climbing, and never ride a bucket in or out.

Where people or equipment must cross a trench, provide a walkway. Walkways over a drop of 6 feet or more need guardrails. Pits, wells and other excavations 6 feet or deeper also need barricades, covers or guardrails at the edge so no one walks in.

Spoil, Loads, and Equipment

• Keep spoil at least 2 feet from the edge, or hold it back with a retaining device. The same applies to tools, pipe and other materials.
• Keep heavy machines back from the edge as far as the competent person directs. If an operator cannot see the edge clearly, use a spotter, a barricade or stop logs.
• Stay out from under loads. Never stand under a bucket or a pipe being lowered, and stand clear of trucks being loaded.
• Protect from traffic. Anyone exposed to vehicles needs a high-visibility vest or other suitable garment.
• Protect nearby structures. Digging next to a wall, pole or sidewalk can undermine it. It needs shoring, bracing or an engineer's approval first.

Water and Bad Air

Do not work in a trench with standing water unless the competent person has put protection in place. That can include extra supports, pumping or a harness and lifeline. Someone qualified must watch the pumps. Water that seeps in from a wall is a sign that the soil may be changing class.

If bad air is reasonably possible, test before anyone enters an excavation deeper than 4 feet. Likely places include near landfills, fuel tanks, sewers and gas lines, and anywhere engine exhaust can settle. OSHA treats an atmosphere as hazardous when oxygen is below 19.5 percent or flammable gas is above 20 percent of the lower flammable limit. Keep testing as often as conditions call for. Where bad air may develop, keep rescue equipment ready.

Warning Signs in the Trench

Get out and tell the competent person if you see any of these:

• Cracks in the ground running parallel to the trench.
• Soil crumbling or sloughing off the wall.
• A bulge in the wall or a bow in the bottom.
• Water seeping in, or a new wet spot.
• Shoring that has shifted, or a shield that has moved.
• The smell of gas, or feeling dizzy or short of breath.

When Something Goes Wrong

If you hit a utility:

1. Stop the machine. Do not try to fix the line or pinch it off.
2. For gas, move everyone away, upwind, and shut off nearby engines if you can do so safely. Do not use anything that can make a spark.
3. For electric, stay on the machine unless it is on fire, and keep everyone away from it.
4. Call 911 for gas, electric, or anything else that threatens people, then call the utility and your supervisor.
5. Report every hit, even a scrape. A damaged coating on a pipe can fail later.

If a trench caves in:

1. Call 911 at once and say it is a trench rescue, so the right team is sent.
2. Do not jump in. A second collapse often follows the first, and would-be rescuers have died this way.
3. Keep everyone away from the edge. Shut down nearby equipment to stop vibration.
4. Note where the buried person was last seen so you can tell the rescue team.

Common Mistakes

Check Your Understanding

1. A trench is 4 feet deep and the walls show fresh cracks. Does it need protection?

Yes. Under 5 feet, a protective system can be skipped only if the competent person finds no sign of a possible cave-in. Cracks are such a sign.

2. You are digging for an irrigation main and the paint shows a yellow line crossing your route. What do you do?

Yellow means gas, oil or steam. Stop the machine outside the tolerance zone and hand dig or vacuum excavate until you can see the line, then protect it.

3. A trench 10 feet deep is cut in Type C soil with sloped walls and a 4-foot bottom. How wide is the top?

Each side goes back 10 x 1.5 = 15 feet. The top is 15 + 4 + 15 = 34 feet.

4. It rained hard overnight. The trench looks the same as it did yesterday. Can the crew go in at the start of the shift?

Not until the competent person has inspected it. An inspection is required before work and after a rainstorm.

5. A coworker is buried to the chest and calling out. What do you do?

Call 911 and say it is a trench rescue. Stay out of the trench, keep others back, and shut down nearby machines. The wall can fail again.

Key Takeaways

• Call 811 before digging, every time, and hand dig inside the tolerance zone.
• A trench 5 feet deep or more needs sloping, shoring or a shield unless it is entirely in stable rock.
• The competent person inspects daily and after rain, and can pull everyone out.
• Keep spoil 2 feet back, and have a way out within 25 feet in trenches 4 feet deep or more.
• Test the air when bad air is possible, and never work in standing water without protection.
• After a cave-in, call 911 and stay out.