When a protective system is required
29 CFR 1926.652(a)(1) requires an adequate protective system against cave-ins, with only two exceptions:
- the excavation is made entirely in stable rock, at any depth; or
- it is less than 5 ft (1.52 m) deep AND a competent person's examination shows no indication of a potential cave-in.
The trap: 4 ft is the egress trigger in 1926.651, and 6 ft and 8 ft do not appear in the rule.
Under 5 ft is not automatic
Both conditions must hold. Treating under 5 ft as an automatic exemption is the common error: a 4 ft trench where the competent person sees fissures and sloughing indicating a potential cave-in needs a protective system. Nothing limits protection to Type C soil.
- A 7 ft excavation entirely in stable rock: exempt.
- A 6 ft trench in unfissured Type A clay: 5 ft or more, so protected.
- A trench exactly 5 ft deep with a clean examination: not exempt, because the exception needs less than 5 ft.
Digging below a shield: 2 ft
1926.652(g)(2) for shields, and (e)(2)(i) for support systems, allow digging up to 2 ft (0.61 m) below the bottom of the shield in a trench, only when:
- the shield is designed for the forces at the full depth of the trench; and
- there is no sign of soil loss from behind or below it.
The traps: 18 in. is the Appendix B shield extension above a vertical side in Type B or C soil, and 3-1/2 ft is the Type A vertical lower portion.
The under-designed shield
A shield rated for 8 ft, set on a ledge with the last 2 ft of a 10 ft trench dug below it, does not comply: the gap is within 2 ft, but the shield fails the full-depth condition. The 2 ft spoil setback in 1926.651(j)(2) is a separate rule and does not cure it.