Construction workers are at risk of death or serious injury if they enter an unprotected trench and the walls col¬lapse. A trench is defined as a narrow underground excavation that is deeper than it is wide, and is no wider than 15 feet or 4.5 meters [OSHA]. Hazards associated with trench work and excavation are well defined and preventable.

Full article: Preventing trenching fatalities
Source: Safety Best Practices