Trench Shoring and Soil Class
The protective system your trench depth and soil class require under OSHA 1926 subpart P, the maximum safe slope, spoil set-back, ladder reach, water control, and the inspection duties.
A trench 5 ft deep needs a protective system: sloping, shoring, a box, or a system in a written excavation plan. Type C at 12 ft wants 1.5 horizontal to 1 vertical, spoils sit 2 ft back from the edge, and a ladder is within 25 ft of every worker.
How the math works
- The 5 ft rule (OSHA 1926.652(a)(1)): a protective system is required in all excavations 5 ft or deeper, with the two narrow exceptions: a slope stable in stable rock, and an excavation made in a stable rock face. Under 5 ft, a protective system is required when the competent person sees a caving risk (1926.652(a)(2)).
- Soil class (1926.652(a)(2) and Appendix A): Type A is stable rock or soil with the properties of aged stable soil, less than 30 percent passing the No. 20 sieve, no fissures, and drained. Type B is 30 percent or less passing No. 20, fissured, saturated, and disturbed, and it is the default class for most dig sites. Type C is 50 percent or more passing No. 20, submerged, running, or any soil below the water table. Class C at any depth is the class that kills.
- Maximum safe slope (1926.652(b)(1)): sloping is measured horizontal to vertical (H to V). Type A allows a vertical face to 12 ft, 1H:1V to 20 ft, and 1.5H:1V to 25 ft. Type B allows 1H:1V to 12 ft and 1.5H:1V to 25 ft. Type C allows 1.5H:1V to 20 ft and nothing above 20 ft without a shored or engineered system. Benching is allowed in A and B only (1926.652(b)(2)).
- Shoring and boxes (1926.652(c), Appendices A, B, C): timber shoring, hydraulic shoring, and trench boxes each have a tabulated data sheet that governs the spacing and the rated depth, a box is driven to the rated depth, the trench box is moved ahead of the crew only within the manufacturer rules, and a system outside the tables is designed by a registered professional engineer (1926.652(c)(1)).
- Edge loads, egress, and water (1926.651): spoils, pipe, and machines stay 2 ft back from the edge, a ladder, ramp, or stair wheel is within 25 ft of every worker, the ladder extends 3 ft above the landing, surface water and seepage are controlled and the trench is re-inspected after rain and thaw, and no one works in a trench with a suspended load overhead (1926.651(d), (c)(2)).
How deep can a trench be without shoring?
Under 5 ft is the depth rule, with two narrow exceptions: a stable rock face, and a slope stable in soil classified as Type A. Under 5 ft, the competent person can still require shoring for water, a fissure, a surcharge, a vibration, a thaw, and a class C face, and those are the trenches that cave.
What does 1H to 1V slope mean?
One foot of horizontal for every foot of vertical depth. A 10 ft trench in Type B sloped at 1H:1V takes a 10 ft bench at the top. 1.5H:1V on a 10 ft trench is 15 ft of bench, and a 25 ft Type C trench cannot be sloped at all under the table.
How far from the edge do spoil piles go?
2 ft minimum (Appendix B(1)(g)). The spoil is surcharge on the face, and a surcharge load on a shored trench can exceed the tabulated rating on the box. Pipe, pallets, stumps, a spoil box, a parked machine, and a fuel trailer are all surcharge.