Why design for active pressure only

Why walls are designed for active pressure

A rigid retaining wall holds soil on the back side. The retained soil pushes the wall outwards; the wall yields very slightly (about 0.001 to 0.005 H for sand, up to 0.01 to 0.02 H for clay) and the soil reaches the active state, the minimum lateral pressure. This is the destabilising force that causes overturning and sliding, so the wall must be designed against it ($K_a = \tan^2(45^\circ - \phi/2)$).

Why passive pressure is neglected

Passive pressure acts on the toe side of the embedded part of the wall and resists sliding. It is usually ignored, or only a fraction (about 50%) is used, because:

1. Large movement is needed: full passive resistance needs a movement of about 0.01 to 0.05 H toward the soil, much more than the small wall movement allowed at working load. At working movement only a small part is mobilised.
2. Soil in front may be removed: by scour, erosion, future excavation, utility trenches or other construction.
3. Soil may soften or shrink: wetting by surface water, seasonal moisture change, frost action and cracks reduce its strength; the top 0.5-1 m is particularly unreliable.
4. Construction disturbance: the toe backfill is often loose or poorly compacted.
5. Compatibility of strains: active state forms at very small strain, passive state at large strain, so both limit states cannot occur together.
6. Safety: neglecting passive resistance gives a conservative design, because $K_p$ is large and an overestimate is dangerous (unsafe).

Where passive resistance is used (e.g. a shear key), it is taken only below the depth of possible disturbance and with a factor of safety of at least 2 on $P_p$.