Angle of repose and temporary excavation slopes

One-line orientation

The angle of repose is the steepest slope loose soil can maintain without support. A safe excavation slope also depends on soil classification, moisture, time, and applicable excavation rules.

Key points

• Definition: the steepest angle at which loose, unconsolidated grains (sand, gravel, soil) stay put without sliding under gravity.
• It sets the unsupported slope. A cut laid back at or below the repose angle stands on its own; steeper than that, the face needs help.
• Soil behavior affects the angle. Granular soils such as sand and gravel rely mainly on friction between particles. Cohesive soils such as clay also gain stability from cohesion. Use project-specific soil information and excavation requirements rather than a universal angle.
• Stabilization to stand steeper than repose:
  • Shotcrete sprayed on the face increases the effective stable angle and resists erosion.
  • Benching the slope into steps.
  • Shoring (engineered support) where a slope can’t be laid back — deep or space-constrained (urban) cuts.

Confusions / comparison

	Frictional / granular	Cohesive
Examples	Sand, gravel	Clay
Holds via	Particle friction only	Friction + cohesion
General behavior	Stability comes mainly from particle friction	Cohesion also helps the soil hold together
Unsupported cut	Must be laid back flatter	Can stand steeper

Related

→ Shoring systems (this module): what to do when you can’t lay the cut back · Excavation bracing (this module): bracing the shoring against earth pressure · Excavation dewatering (this module): the other thing that destabilizes a cut.