The following are excerpts from the Occupational Safety & Health Administration Requirements for Protective Systems - 1926.652

1926.652(a) - Protection of employees in excavations.

1926.652(a)(1) - Each employee in an excavation shall be protected from cave-ins by an adequate protective system designed in accordance with paragraph (b) or (c) of this section except when:

1926.652(a)(1)(i) - Excavations are made entirely in stable rock; or

1926.652(a)(1)(ii) - Excavations are less than 5 feet (1.52 m) in depth and examination of the ground by a competent person provides no indication of a potential cave-in.