The slope table
Appendix B to 29 CFR 1926 Subpart P, Table B-1, sets maximum allowable slopes for excavations 20 ft or less deep, horizontal:vertical:
- Type A: 3/4:1 (53°).
- Type B: 1:1 (45°).
- Type C: 1-1/2:1 (34°).
Reading them backwards is the classic trap.
Under 1926.652(b)(1), sloping Option 1 with no soil classification may be no steeper than 1-1/2H:1V (34°), the Type C slope. An 8 ft excavation therefore runs back 1.5 × 8 = 12 ft on each side. Traps: 6 ft uses Type A, 8 ft Type B, 5.3 ft reads the ratio backwards.
Top width of a sloped trench
Top width = bottom width + run on each side, where run = H ratio × depth. For a 10 ft Type C trench with a 4 ft bottom: each side runs 1.5 × 10 = 15 ft, so the top is 4 + 15 + 15 = 34 ft. Traps: 24 ft uses 1:1; 17.3 ft reads the ratio backwards; 19 ft slopes one side only.
Type A special cases
- Short-term slope: 1/2:1 applies only to Type A excavations 12 ft or less deep that are open 24 hours or less. A 14 ft excavation open 12 hours stays at 3/4:1.
- Vertical lower portion: no more than 3-1/2 ft high, unsupported. For depths over 8 ft up to 12 ft, the slope above it is 1:1, not the plain 3/4:1.
Distress, shields and depth limits
Where there are signs of distress, the slope must be cut back to at least 1/2H:1V less steep than the maximum. For Type A: 3/4 + 1/2 = 1-1/4:1. 1:1 adds only 1/4, 1/4:1 subtracts, and 1-1/2:1 is flatter than the minimum cutback.
In Type B and Type C soil, a vertically sided lower portion must be shielded or supported to at least 18 in. above the top of the vertical side.
Appendix B slopes stop at 20 ft. A 24 ft Type B excavation needs a sloping system designed by a registered professional engineer; flattening to 1-1/2:1 does not replace that design.