A document (.pdf file) from NIOSH offers recommendations for protecting workers from trench cave-ins.

Released June 6, the NIOSH guide advises employers to conduct pre-job planning, which includes calling 811 to identify utilities, and have a competent person determine the appropriate protective system and monitor the worksite for hazards. Workers should exit a trench at the slightest hint of instability and never enter an unprotected trench.

Data from the Bureau of Labor Statistics indicates that 350 workers died in trenching or excavation cave-ins from 2000 to 2009. According to NIOSH, an OSHA review found a lack of a protective system was the leading cause of trench-related deaths.