Active earth pressure and failure wedge formation: A failure wedge develops in the backfill behind a retaining wall when the wall primarily moves in which manner?

Introduction / Context:
Retaining-wall pressures depend on wall movement relative to the soil mass. Active movement reduces lateral stress until a limiting state is reached, mobilizing a distinct failure wedge in the backfill governed by soil strength parameters and wall friction assumptions (Rankine or Coulomb theories).

Given Data / Assumptions:

• Backfill is cohesionless or has known c–φ parameters.
• Wall is free to move sufficiently to mobilize active or passive states.
• Plane-strain conditions apply along the wall alignment.

Concept / Approach:
When the wall moves away from the backfill, lateral confinement decreases and shear failure initiates along a plane or wedge surface, forming the active failure wedge. Conversely, moving toward the backfill mobilizes passive resistance with a different wedge orientation and much higher force level. At-rest conditions involve minimal movement and no failure wedge formation.

Step-by-Step Solution:

Verification / Alternative check:
Classical Rankine/Coulomb solutions and model tests show distinct wedge geometry activated by wall movement direction and magnitude.

Why Other Options Are Wrong:

• B: Describes passive condition; wedge forms on the front side and not the stated backfill wedge.
• C/D/E: Do not describe the active mechanism producing a backfill failure wedge.

Common Pitfalls:
Assuming at-rest pressure produces a wedge; insufficient wall movement will not mobilize the limiting active state.

Final Answer:
Moves away from the backfill (active condition)