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
Every worker in an excavation must be protected from cave-ins by an adequate protective system, such as sloping, benching, shoring or a trench shield. The only exceptions are excavations entirely in stable rock, or less than 5 feet deep where a competent person has examined the ground and found no sign of a possible cave-in (29 CFR 1926.652(a)(1)(i) and (ii)). Not sure it is protected? Stay out and ask. (29 CFR 1926.652(a)(1))
ASK THE CREW
Who is our competent person today, and what do they check?
From the talk: Trenching and excavation. Read it in full, free, with the questions and the sign-in sheet.