The sloping table
Appendix B to Subpart P sets the steepest slope allowed for each soil type, written as horizontal to vertical:
- Stable rock: vertical (90°)
- Type A: ¾ to 1 (53°). A simple slope that's open 24 hours or less and 12 feet deep or less can be ½ to 1 (63°).
- Type B: 1 to 1 (45°)
- Type C: 1½ to 1 (34°)
Without a soil classification, 1926.652(b)(1) allows sloping at no steeper than 1½ to 1, the same as Type C. When the face shows signs of distress, the slope has to be cut back at least another ½ horizontal to 1 vertical. The full text is in 29 CFR 1926 Subpart P.
An example
An 8-foot trench in Type B soil with a 4-foot bottom: at 1 to 1, each side lays back 8 feet, so the top of the cut is 4 + 8 + 8 = 20 feet wide. Spoil and equipment then go at least 2 feet back from that top edge.
When this doesn't apply
- Deeper than 20 feet: the protective system has to be designed by a registered professional engineer.
- Under 5 feet in depth, a protective system isn't required if a competent person examines the ground and finds no sign of a potential cave-in.
- Soil can't be Type A if it's fissured, subject to vibration from traffic or pile driving, previously disturbed, or sitting in layers that dip into the excavation.
- Surcharge loads from spoil, equipment, traffic or nearby structures mean a flatter slope, as a competent person decides.
- Benched, layered and vertically sided cuts have their own configurations in Appendix B. This calculator covers simple slopes only.