Default slope: 1.5 horizontal to 1 vertical
29 CFR 1926.652(b)(1): when no other sloping option is used, the excavation must be sloped not steeper than 1.5 horizontal to 1 vertical, which is 34 degrees measured from horizontal, following the Type C soil slopes in Appendix B. arctan(1 ÷ 1.5) = 33.7°, rounded to 34°.
Traps:
- Reversing the ratio to 1 horizontal to 1.5 vertical, or 1.5 vertical per 1 horizontal, is the common error. It gives a far steeper wall.
- A 1 to 1 slope at 45 degrees is steeper than the default allows.
Computing the horizontal run
Each foot of depth needs 1.5 ft of horizontal run:
run = 1.5 × depth
8 ft trench: 1.5 × 8 = 12 ft from the bottom edge of each wall to the top edge of the slope.
- 8 ft uses a 1 to 1 slope, steeper than allowed.
- 5.3 ft reverses the ratio (8 ÷ 1.5).
- 16 ft uses 2 horizontal to 1 vertical, flatter than the rule requires.
Deeper than 20 ft: a registered engineer
Appendices A through D apply only to excavations 20 ft (6.1 m) or less. A trench 20 ft or deeper needs a protective system designed by a registered professional engineer, or based on tabulated data prepared or approved by one (1926.652(b)(4), (c)(4)).
For a 22 ft trench:
- A competent person using Appendix B cannot design it; the appendix does not reach past 20 ft.
- The default 1.5H:1V slope relies on the appendices, so it does not cover 22 ft either.
Manufacturer's tabulated data: support and shields only
Under 1926.652(c)(2), manufacturer's tabulated data is a recognized option only for support systems, shield systems and other protective systems.
Sloping cannot use it. Sloping uses the default configuration, Appendices A and B, or other tabulated data, and otherwise needs a registered professional engineer's approval (1926.652(b)(4)). Benching follows the same options as sloping.
Trap: saying any system qualifies as long as the trench is under 20 ft. The limit is the type of system, not the depth.