The hazard
Soil is heavy, and a trench wall can give way with little or no warning. A cave-in can bury or crush a worker before they can react, and a coworker who jumps in to help can be caught by the next collapse. These rules keep the walls off people and give everyone a way out.
The action
Before an excavation is opened, the estimated location of underground utilities such as sewer, telephone, fuel, electric and water lines must be determined. Utility companies or owners must be asked to locate their lines before digging, and the exact location must be found by safe and acceptable means as you get close (29 CFR 1926.651(b)(2) and (b)(3)). Calling 811 connects you to the local one-call center. (29 CFR 1926.651(b)(1))
ASK THE CREW
What protective system are we using, and where does it stop protecting us?
From the talk: Trenching and excavation. Read it in full, free, with the questions and the sign-in sheet.