A trench is a cut through ground that was holding itself up. Remove the soil in the trench and the walls lose the support that kept them in place. Whether the wall stands for a minute, a day or a month depends on the soil's strength, the forces pushing on it, and what changes while the trench is open.
The forces at work
- Weight of the soil itself. Soil is heavy. OSHA notes that a cubic yard can weigh as much as a car, about 3,000 pounds. Try the soil load calculator to see the weight of a typical collapse wedge.
- Lateral pressure. The soil behind the wall pushes sideways toward the opening, and the pressure increases with depth.
- Surcharge. Spoil piles, equipment, trucks, stored pipe and nearby foundations add weight at the top, increasing pressure on the wall.
- Water pressure. Water in the soil adds weight and pressure and reduces the friction and cohesion that hold soil together.
The wall stands only as long as the soil's internal strength (cohesion and friction) exceeds these forces.
How walls fail
| Failure type | What happens | Typical clue |
|---|---|---|
| Sliding wedge | A wedge of soil shears along an inclined plane and slides in | Cracks at the surface, a slumped edge |
| Toppling | A slab separated by a tension crack tips into the trench | Tension cracks parallel to the edge, about 0.5 to 0.75 times the depth back |
| Spalling or sloughing | Chunks drop from the face | Drying, cracking face; pieces falling |
| Bottom heave | The floor rises as soft soil is squeezed up | Floor bulging, shores tilting |
| Boiling | Water flows up through the floor carrying soil | Sand boils, cloudy water in the bottom |
| Undermining | Soil washes or falls out from behind or below a shield | Voids behind the box, sinking surface next to the trench |
What triggers a collapse
Many cave-ins happen in trenches that stood fine for hours or days. Something changed:
- Rain or groundwater saturates the soil, turning it into Type C material and adding pressure.
- Drying causes cohesive soils to crack and spall.
- Freeze and thaw loosens soil structure.
- Vibration from traffic, compactors, pile driving or a running excavator.
- New surcharge such as a spoil pile, a delivered stack of pipe or a dump truck parked at the edge.
- Previously disturbed soil around existing utilities or old backfill.
- Layered soil with a weak sand or gravel seam, or layers sloping into the trench.
- Time. The longer a trench stays open, the more the soil relaxes and dries or soaks.
- Undercutting the bottom or digging below a shield beyond the 2-foot allowance.
Warning signs: get out now
You don't need permission to get out. The competent person must remove workers when these signs appear (1926.651(k)(2)).
How fast it happens
A wall failure takes seconds. Workers bending over to work on pipe, or kneeling, can't get up and out in time. That's why OSHA's protection must be in place before anyone enters, not installed "after we check it out."
Prevention, in one sentence
Classify the soil, use the right protective system for the depth and conditions, keep weight and water away from the edge, inspect daily and after every change, and keep a way out within 25 feet.
Learn more in the free lessons on soil classification, sloping and inspections.
Sources and official references
Last reviewed: October 25, 2026 · Free general education based on OSHA 29 CFR 1926 Subpart P. It does not replace the site-specific training and competent person your employer must provide, and it does not issue a certificate. Nothing you enter on this page is saved or sent.