Earth pressure theories
Rankine's earth pressure theory assumes horizontal backfill and a frictionless wall. Coulomb's theory adds wall friction and sloped backfill, giving more realistic forces for design.
- Active pressure Ka = (1−sinφ)/(1+sinφ) for cohesionless soil with horizontal backfill.
- Passive pressure Kp is the inverse— often a 3× to 5× multiplier depending on φ.
- At-rest pressure K₀ ≈ 1−sinφ applies to walls that cannot deflect (rigid structures, basement walls).
Stability checks
A retaining wall must resist overturning (moment check), sliding (shear at base), and bearing failure (soil stress under footing).
- Factor of safety against overturning ≥ 1.5 (gravity walls) to 2.0 (cantilever).
- Include water pressure separately — saturated backfill can double total lateral force.
- Passive resistance at the toe may be ignored if forward movement is possible.
Drainage is the most important detail
Hydrostatic pressure behind a retaining wall is often larger than earth pressure. Adequate drainage prevents this buildup and is the single most critical construction detail.
- Perforated pipe and free-draining gravel backfill eliminate hydrostatic pressure.
- Weep holes must be kept clear and sized to pass peak infiltration rates.
- Geotextile filter fabric prevents migration of fines into drainage layer.