This calculator helps plan trench slope setback and top width from OSHA Appendix B maximum allowable slope tables after a competent person classifies the soil or rock deposit under 29 CFR 1926 Subpart P Appendix A. It supports sloping and benching geometry planning, and it pairs well with the excavation calculator when you need both trench quantity and the widened top-of-cut footprint.
Geometry from this calculator alone does not establish compliance, does not select a protective system, and does not replace a competent person's assessment of layering, water, surcharge loads, adjacent structures, vibration, weather, or other site conditions.
OSHA Soil Classification
OSHA classifies soil/rock into four categories based on stability, from most stable to least:
- Stable Rock: Vertical walls permitted (90°)
- Type A: 3/4H:1V slope (53°) — cohesive soils, ≥1.5 tsf
- Type B: 1H:1V slope (45°) — cohesive soils, 0.5–1.5 tsf
- Type C: 1-1/2H:1V slope (34°) — granular/weak soils, ≤0.5 tsf
OSHA Appendix A requires a competent person to classify the deposit from at least one visual and one manual analysis. This calculator is a geometry aid after that classification step, not a substitute for the field analysis itself or for protective-system selection.
If the soil call is changing or the trench geometry keeps shifting in the field, review the excavation mistakes guide alongside the OSHA trench safety guide before treating the setback as final.
Slope Setback Formula
The setback per side = depth × horizontal slope ratio. The top-of-trench width = bottom width + 2 × setback. For example, a 10 ft deep trench with 2 ft bottom width in Type C soil has a setback of 10 × 1.5 = 15 ft per side, for a top width of 32 ft. The excavation formulas guide shows the same geometry in reusable estimating form.
Benching Rules
For Type A planning outputs, the first rise above the trench bottom is capped at 4 ft and each additional rise is capped at 5 ft, with a 2 ft horizontal bench-width assumption.
For Type B planning outputs, the first rise and each additional rise are both capped at 4 ft. Type B benching applies only to cohesive Type B soil. If the deposit is non-cohesive Type B, layered unfavorably, or otherwise outside Appendix B assumptions, the benching result should not be treated as a protective-system selection or compliance determination.
Type C benching is not permitted under OSHA Appendix B, and stable rock is handled outside benching because the allowable configuration may be vertical.
PE Requirement
OSHA Appendix B sloping and benching configurations apply to excavations 20 ft or less in depth. Deeper sloping or benching requires a registered professional engineer design under 29 CFR 1926.652(b), unless you are following another allowed written design option such as approved tabulated data.
Support systems and shield systems are handled separately under 29 CFR 1926.652(c). Those systems must stay within the applicable manufacturer instructions or approved tabulated data, or they need a registered professional engineer design.
Primary OSHA Sources
Review the governing OSHA text before relying on any planning output: 29 CFR 1926.652, Appendix A, Appendix B, and the OSHA Technical Manual.