What lateral earth pressure theory to use?
There are two well-known classical earth pressure theories: Rankine and Coulomb. Both theories provide expressions for the pressure states described below. Rankine is a simplified version of Coulomb. Rankine ignores the friction between the wall and the soil, and it assumes a vertical wall face. Coulomb takes all these factors into consideration.
Depending on the relative movement of the wall and the backfill, three different states of lateral earth pressure may develop:
- At-rest Ko – It occurs when the wall experiences no lateral movement.
- Active Ka – It occurs when the wall tilts away from the soil sufficiently to mobilize its shear strength.
- Passive Kp – It occurs when the wall is pushed into the soil, compressing the mass and mobilizing its shear strength.
In order to develop the full active or passive pressures, the wall has to move, as shown below. For granular soils, the full active pressure is developed when the wall top moves between 0.001H-0.004H, being H the wall height. Most cantilever walls will move this amount as the sum of the stem deflection plus the base rotation, therefore it’s common practice to use the active pressure in the design of cantilever retaining walls, otherwise the at-rest pressure should be used.