Reviewing Trench Safety

July 1, 2011

Every day, workers are injured or killed in trench collapses. “Safety is no accident,” says Dave Adler, who recently retired from the fire service in Illinois, where he was deputy chief of operations and a technical rescue instructor. Adler has seen hundreds of trench collapses and has instructed the New York City, Chicago, and Los Angeles County Fire Departments in trench rescue operations.

Soil weighs from 100 to 140 pounds per cubic foot. So at 10 feet deep, the soil is pressing vertically with a force of 1,000 pounds per square foot-and it’s pressing horizontally with half that force, or 500 pounds, at the bottom. In an unprotected trench, the trench walls can fail in four ways, according to Adler:

  • The slough-in
  • A sidewall cracks off and fails
  • A shear-in, where the adjacent trench wall slides down and into the trench (a layer of sand or gravel may allow this to occur)
  • A spoil-in, where your spoil pile falls into the trench

“After the first collapse there is usually a secondary collapse,” says Adler. “This second collapse kills more people than the first one, because people jump into the trench after the first collapse to help, and then they’re killed or injured as well. Please! Don’t jump into the trench to help. Call 911 and your local fire department.

“The average trench collapse is 1.5 cubic yards, which weighs around 4,000 to 5,000 pounds, and it happens in one-tenth of a second. How far can you run in one-tenth of a second? It’s like getting hit by a small truck at 45 miles per hour-and you never see it coming,” says Adler.

OSHA Regulations
It’s worth a few minutes to review the basics of OSHA’s trench safety regulations. OSHA requires a “competent person” to be present on all trenching projects. A competent person has been trained and is knowledgeable about OSHA regulations. And a competent person has the authority to correct hazardous conditions if they arise.

When preparing a trenching project, you need to determine the soil type in which you’ll be working. OSHA classifies three types of soils. Type A soils are firm clay or stable rock. A Type B soil is a moderately stable clay or clay mixture. And Type C soils are soft material, sand, silt, or gravel.

If you choose to slope the trench sides as a protective measure, Type A soil must be sloped at 0.75 (horizontal) to 1.00 vertical on both sides. The regulation for Type B soils is 1:1; and for Type C, the weakest soil, its 1.5:1.0.

Prior to digging, the contractor needs to locate and identify all underground utilities such as sewer, telephone, fuel, electric, water lines, and the like that may be encountered during the excavation. All surface encumbrances such as signs, trees, fences, poles, and sidewalks that could create a hazard must be removed or supported during the excavation.

All excavating equipment must maintain a minimum clearance of 10 feet from overhead power lines rated 50 kV or less, with 0.4 inches of clearance added for every 1 kV over 50. Support systems must be provided to ensure the stability of adjacent structures endangered by excavation operations.

Adler says it’s usually not practical to use sloping for an urban trench, so you’re probably going to use a trench box or shoring. If a trench is over 5 feet deep, either sloping or a protective system such as a trench shield must be used to prevent a cave-in. The contractor must provide a safe means of entering or exiting any excavation over 4 feet deep. And, says OSHA, a means of egress from a trench such as a ramp or ladder shall be located within 25 feet of workers in the trench.

Hazardous Gases
In excavations of more than 4 feet deep, the contractor needs to recognize the potential for the accumulation of hazardous gases-such as hydrogen sulfide or carbon monoxide, oxygen levels and flammable gases. Hydrogen sulfide is easily recognizable because it smells like rotten eggs. But it will deaden your sense of smell, says Adler, and in elevated concentrations, three breaths can be fatal.

You also need to be aware that medical emergencies can happen in the trenches. Heart attacks, strokes, or diabetic emergencies occur in trenches as well as anywhere else.

The following guidelines summarize some of the more important OSHA regulations regarding shielding systems, according to a document published by Efficiency Production Inc., a maker of trench boxes and shielding:

  • Shielding systems shall be installed in such a manner to restrict lateral or other hazardous movement of the shield in the event of a sudden collapse.
  • Shielding systems and their components shall not be subjected to loads that they are not designed to withstand.
  • Shielding systems and their components shall be securely connected to prevent predictable failures.
  • Here’s one that often gets overlooked: Removed spoil shall not be stockpiled closer than 2 feet from the edge of excavation. Backfilling shall progress together with the removal of support systems from excavations.

If you’re digging with a backhoe and strike an electrical line, your tractor my not provide you with protection, Adler says. Stay on the machine, shut it off and call 911. Always treat an electric line as if it were energized. “If there’s an electrical arc, the temperature can reach 10,000 degrees,” he says. “It can rupture the hydraulic lines on your backhoe and cause a fire.” At this point you have a decision to make.

Some natural-gas pipelines are now made of plastic. If you strike one, immediately shut off the tractor and call 911. The gas can ignite without an outside ignition source. This is due to the particles in the discharging natural gas, which can create a static charge, Adler says.

A typical OSHA violation is that no protection system is used. “I have never seen a properly engineered and constructed trench box fail,” says Adler. “I have seen trench boxes sitting on the surface, beside the spoil pile, adjacent to where a trench collapsed has just occurred.” Failure to inspect a trench with a competent person is another typical violation.

“Don’t use the backhoe to dig people out of the trench,” says Adler. “Numerous people have been struck, injured, or even decapitated by a panicked operator. Digging a parallel trench is not the answer either. The bucket will pressurize the soil causing the original trench with the victim to collapse.

Fire Departments around the country are now using the Rescue Vac System to rapidly and safely remove soil in trench-collapse rescue operations. Old rescue technology included using hand shovels and 5-gallon buckets. This was time consuming and labor intensive. The Rescue Vac System utilizes an Air Knife to aerate and fracture the soil along with special procedures, vacuum hoses, and tips that are connected to a local sewer vacuum truck to remove the soil. This kit is affordable and utilizes local resources already in place. What is exciting is that rescue digging operations have been dramatically lowered from an average of six hours to only 20 minutes. This saves time, which saves lives. Adler recommends the use of the Rescue Vac System. More information on the Rescue Vac can be found at