One cubic yard of dirt can fit neatly in the bucket of a 100-hp excavator. And while it may not look like much, that dirt weighs on average 3,000 lbs. or, as Steve Winter at ProTec relates, as much as a Volkswagon. Since trenching involves a lot more than a single bucket load, protecting workers in the trench - who are surrounded by thousands of pounds of soil - is crucial.

The potential hazards of working in deep trenches are fairly obvious. But even a cave-in in a relatively shallow trench can be dangerous.

"It takes only a small amount of soil to create enough pressure to asphyxiate a worker, and a person does not need to be completely covered in soil to die," says Paul McDonnell, vice president - trench safety, United Rentals. "Furthermore, it only takes seconds for a cave-in to become deadly."

The federal standard (OSHA 29 CRF, Part 1926 Subpart P) requires any excavation deeper than 5 ft. to have some type of protective system. However, some states, cities and municipalities have more stringent guidelines, and require protection in trenches at shallower depths.

"There is simply so much weight," says Winter. "Even in a shallow trench, a worker's legs could get caught. And the weight and pressure of the dirt can cut off circulation."