In a cohesionless soil element located within the backfill of a retaining wall, the major principal stress is oriented how when the soil is in an active plastic equilibrium state?

Explanation

The major principal stress in cohesionless soil behind a retaining wall is vertical when the soil is in an active state of plastic equilibrium. If the soil is in a passive state, the orientation differs. Therefore, the correct answer is that the major principal stress is vertical under active plastic equilibrium conditions.