Workers who dig or excavate trenches are at risk of death if they enter an
unprotected trench and the walls collapse. However, hazards associated with
trench work and excavation are well defined and preventable. There is no
reliable warning when a trench fails. The walls can collapse suddenly, and
workers will not have time to move out of the way. Even though small amounts of
dirt may not seem treacherous, a single cubic yard of dirt can weigh more than
3,000 pounds, which can fatally crush or suffocate workers. Even small, solid
pieces of dirt can cause serious
injuries.

The US-American National Institute for Occupational Safety and Health (NIOSH)
recommends in a detailed publication engineering controls, protective
equipment, and safe work practices to minimize hazards for workers.

More info

AplusA-online.de - Source: National Institute for Occupational Safety and Health (NIOSH)