Retaining walls backfilled with cohesionless soils (sand and gravel) tend to rotate slightly around the base. Behind such a wall, a wedge of soil tends to shear along inclined plane. A. Coulomb determined that the ratio of sliding resistance to sliding force is a minimum when inclined plane makes an angle 45 degrees + ϕ/2 with with the horizontal, where ϕ is an angle of soils internal friction.