Slope Stability: Angles, Failure & Prevention

What is Angle of Repose?

The steepest angle at which loose soil stays in place (doesn't slide). Varies by soil type. Understand this = prevent slope failures.

Material	Angle	Ratio (H:V)	Safe for Equipment?
Sand/Gravel	30-35°	1.5:1 to 2:1	Yes, with caution
Clay	35-40°	1.3:1 to 1.5:1	Risky, avoid
Rock (blasted)	45-50°	1:1 to 1.2:1	Very risky

Shallow failure: Top 1-2 ft slides off. Often triggered by rain, vibration, or oversteepening.

Deep failure: Large mass fails along curved surface. Usually clay soils, groundwater triggered.

Dozer/grader on slope: Don't operate on slopes >2:1 (steeper than 2 horizontal per 1 vertical). Tip-over risk.
Pushing material downslope: Never push material down steep slopes (unstable, can trigger failure).
Soil saturation: Rain or water makes slopes unstable. Delay work, let site drain.
Surcharge: Heavy equipment on slope top adds weight, increases failure risk. Keep clear.

Flatter angles: Make slope gentler (2:1 or flatter). Most reliable.

Benching: Create horizontal steps on slope. Reduces overall height, adds stability.

Soil nailing: Install anchors (passive reinforcement). For steep cut slopes.