Earth pressure on retaining walls

Soil behind a wall pushes on it. How hard depends on whether the wall moves. If the wall cannot move at all, the soil stays in its at-rest state, with lateral pressure K₀ times the vertical effective stress. If the wall moves slightly away from the soil, the soil expands and the pressure falls to the active value. If the wall is pushed into the soil, the pressure rises to the passive value, which is many times larger but needs far more movement to mobilise.

Rankine's theory assumes a smooth vertical wall and level backfill. For a cohesionless soil, Ka = (1 − sin φ)/(1 + sin φ) and Kp is its inverse. Pressure increases linearly with depth, so the resultant thrust on a wall of height H is ½ Ka γ H², acting at H/3 above the base. Surcharge on the backfill adds a uniform pressure Ka q. Cohesion reduces active pressure and can produce a tension zone near the top. Soil cannot sustain that tension, so designers ignore it and often assume the resulting crack can fill with water. Coulomb's wedge theory allows wall friction, inclined backs and sloping backfill.

Water is often the bigger danger. If drainage fails, hydrostatic pressure acts in full on top of the reduced effective earth pressure, roughly doubling the load. Weep holes, a filter layer and a drain behind the wall are therefore essential. Earthquakes add a dynamic increment, commonly estimated with the Mononobe-Okabe method.