OSHA has been keenly aware of the danger to workers in open trench work since it was rated as the fourth deadliest job in the US sometime the middle of the last decade. Collapsing walls that bury trench workers resulting in death and injury continue to plague the water, sewer, pipeline and construction industry.
OSHA comments on this issue: “Because of the continuing incidence of trench collapses and accompanying loss of life, the agency has determined that an increased OSHA enforcement presence at worksites where such operations are being conducted is warranted.”
Because it continues to be one of the highest areas of fatality as well as the one with the highest number of safety violations, OSHA has made it a high priority.
Additional trenching hazards include electrocution, explosions, toxic fumes, and drowning.
OSHA defines “Excavation” as any man-made cut, cavity, trench, or depression in an earth surface, formed by earth removal and a “trench” as a narrow excavation (in relation to its length) made below the surface of the ground. In general, the depth is greater than the width, but the width of a trench (measured at the bottom) is not greater than 15 feet (4.6 m). If forms or other structures are installed or constructed in an excavation so as to reduce the dimension measured from the forms or structure to the side of the excavation to 15 feet (4.6 m) or less (measured at the bottom of the excavation), the excavation is also considered to be a trench.
OSHA provides plenty of help for anyone who is involved in trenching and excavation.
http://www.osha.gov/SLTC/trenchingexcavation/index.html contains etools, quickcards, downloads and much more. Spend some time studying the proposed solutions and answers the include identifying underground cables and utilities, monitoring for harmful gases and displaced oxygen as well as a number of other tools.