Most contractors know how much a trench box costs to buy or rent. But do you know how much the alternative costs?
Without Proper Use,
Trench Box is No Protection
A recent fatality in a Cincinnati suburb highlights the importance of careful planning and the need for proper tools and equipment in trench work.
The contractor was digging a 30-foot deep trench and installed a 20-foot steel trench box to prevent collapse (see diagram). However, the trench caved-in below the box and killed a 28 year-old worker who was working at the bottom. It took rescue crews four and a half hours to recover the victim.
“It seems obvious from this example,” says LHSFNA Occupational Safety and Health Division Director Scott Schneider, “but trench boxes must reach very near to the bottom of the trench. Otherwise, in unstable earth, the sides can collapse and slip in below the box. If the gap between the floor of the trench and the bottom of the box is wider than a worker is tall, the worker has little chance of survival in a full-scale collapse.”
More information on trench safety is available from the LHSFNA OSH Division.
According to OSHA regulations, if trench boxes or other shoring are not used, the sides of the trench must be sloped to prevent cave-ins. How much they must be sloped depends on the kind of soil. OSHA recognizes three soil types:
- Type A – Most stable: clay, silty clay and hardpan (resists penetration); no soil is Type A if it is fissured, is subject to vibration of any type, has previously been disturbed or has seeping water.
- Type B – Medium stability: silt, sandy loam, medium clay and unstable dry rock; previously disturbed soils unless otherwise classified as Type C; soils that meet the requirements of Type A soil but are fissured or subject to vibration
- Type C – Least stable: gravel, loamy sand, soft clay, submerged soil or dense, heavy unstable rock, and soil from which water is freely seeping