where h is the retained height of the wall, measured from the underside of its base, and 9 is the angle of shearing resistance of the backfill. When this inequality is satisfied, no friction occurs along the virtual plane4 and the active effective earth thrust P'a is inclined at the same angle p to the horizontal as is the ground surface behind the wall.

When the inequality is not satisfied, calculations of wall strength and stability are normally based on the mechanism shown at the bottom of Figure 11.8. Backfill inside the inclined virtual plane is considered to be part of the wall and the forces acting on that virtual plane are treated as destabilizing actions.

The horizontal and vertical components of P'aare given by:

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