The excavation standard turns on one number and one person. The number is 5 feet. The person is the competent person, who has to be able to identify the hazards and has the authority to fix them (29 CFR 1926.650(b)).
The rule. Each employee in an excavation must be protected from cave-ins by an adequate protective system designed under paragraph (b) (sloping and benching) or paragraph (c) (support systems, shields, and other systems), except when the excavation is made entirely in stable rock, or when it is less than 5 feet deep and examination of the ground by a competent person provides no indication of a potential cave-in (29 CFR 1926.652(a)(1)). Protective systems must be able to resist, without failure, all loads that are intended or could reasonably be expected to be applied to them (29 CFR 1926.652(a)(2)).
Choosing the system. Sloping and benching configurations are selected under one of four options — a maximum allowable slope of 1½:1 (34 degrees) for any soil type, the appendices' soil classification and slope tables, tabulated data, or a registered professional engineer's design (29 CFR 1926.652(b)). Support systems, shields and other protective systems follow a parallel set of options (29 CFR 1926.652(c)).
The rest of the package. A protective system is not the only duty that attaches at depth. Trenches 4 feet or deeper need a stairway, ladder, ramp, or other safe means of egress within 25 feet of lateral travel for every employee (29 CFR 1926.651(c)(2)). Spoil piles and equipment stay at least 2 feet back from the edge, or are held by retaining devices (29 CFR 1926.651(j)(2)). Where a hazardous atmosphere could reasonably be expected, the air is tested before anyone enters an excavation more than 4 feet deep (29 CFR 1926.651(g)(1)(i)). And the competent person inspects the excavation, the adjacent areas and the protective systems daily, before the start of work, as needed through the shift, and after every rainstorm or other hazard-increasing occurrence (29 CFR 1926.651(k)(1)).
The common failure is not an absent trench box; it is a trench that was 4 feet deep at the morning inspection and 6 feet deep by lunch. Depth changes, and the duty changes with it.