Trenches are among the leading causes of injury and fatalities on construction sites. They are almost always classified as a “Serious” violation because of the extreme weight of soil, and the difficulty of rescuing people after a cave-in. The unfortunate part, is almost always the incident could have been prevented.
Trenching exposes workers to several hazards like falls, falling loads, cave-ins, hazardous atmosphere, and mobile equipment accidents. OSHA Regulations for excavation work and how to protect workers are listed in Subpart P – 1926.651 and 1926.652. It is required that excavation contractors follow these regulations, to furnish a jobsite that is safe and free from recognized hazards.
What is a Trench?
A man-made cut in the ground. In general a trench is deeper than it is wide. When measured at the bottom, a trench is less than 15’ wide in the narrowest dimension (not the length).
Anything larger than 15’ is considered an Excavation, which has different means of protection.
Hazards with a Trench
Cave-ins pose the greatest hazard. Trenches typically have vertical walls and as the conditions changes large chunks of soil can break loose and fall in. At 90-110 pounds per cubic foot, a 3×3 chunk of soil can weigh as much as a small car.
Working in a trench puts people in a vulnerable position, you are lower than the materials, equipment, and soil pile. Materials being lowered into the trench can shift or fall in. Fumes from idling equipment can build up in the bottom of the trench creating an oxygen deficiency. And as the soil pile dries out throughout the day it will start to fall potentially into the trench.
How do you make it safe?
Any trench that is 5’ deep or greater require a protective system.
Trenches that are 20’ deep or deeper need to be designed by a registered professional engineer.
Competent Person – a designated employee who is capable of identifying existing and predictable hazards. They can classify the soil type and make the correct decision to Slope, Shore, or Shield.
Slope – There are 4 soil types to consider when digging. Each with its own rule for sloping the banks of the excavation. Stable Rock and Type A can be dug at a steep angle, while Type B and C need to be dug very wide. Type C soils are like dry sand at the beach, no support and tend to fall into the hole.
Shore – Support the sides with a mechanical device. For example, a metal plate on each side with a hydraulic press pushing outward on the banks.
Shield – A movable metal box that protects the worker from the sides of the excavation. A trench box.
Benching – This can be done in Stable Rock or Type A soil as an alternative to sloping, step the banks every 6’ vertically to limit the fall hazard into the hole.
Before you enter a Trench
- It has been inspected by a competent person.
- There is Cave-in protection (slope, shore, shield)
- There is a safe way to enter and exit (minimum 1 ladder every 25 feet)
- Equipment and materials are at least 2’ away from the edge
- It is free of standing water or atmospheric hazard (check the oxygen level)
A grading and excavation contractor sent a laborer to install underground PVC for a sump discharge. A trench was dug across the yard, at the deepest point it was only 3’ below grade. The laborer was crouched down inside the trench to glue and assemble 2 of the pipes together, a small piece of the bank broke loose and landed on his foot. The way he was positioned caused his ankle to twist and break. Though not a fatal injury, he was out of work for 2 months and required weeks of physical therapy, all at the expense of the contractor. Not all trenches are life threatening, but they are all dangerous.
Questions for you
- How often do you work in or around trenches?
- Do you have access to a Trench Box or Shoring equipment?
- If a trench is 40 ft long, how many ladders do you need?
1 ladder, set up in the middle no more than 25’ from each end.