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Could anyone please explain what is the difference between active and passive earth pressure when we talk about weight retaining walls.

Both are lateral pressure exerted by the soil on a retaining wall, but why do we call one active and the other passive ?

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If a retaining wall is immobile, it incurs lateral soil pressure from both sides, and that is the so called resting pressure.

When the retaining wall, due to load from the retained soil massive, which tends to slide down (wedge of failure), starts moving towards the soil in front of the wall (and thus off the retained soil), then the resting pressure from the retained soil becomes active pressure; while the pressure from the soil in front of the wall, resisting against the wall moving towards it, becomes passive pressure (displacement wedge pressure).

Active pressure is less than resting pressure, while passive pressure always exceeds resting pressure. Practically, some centimeters movement is sufficient to completely realize passive pressure; as well as practically not any retaining wall undergone active pressure remains completely immobile.

As far as the wall so moves, the shifting force (due to active pressure) is reducing, while resisting force (due to passive pressure) is increasing.

As soon as growing passive pressure load becomes equal to active pressure load, the wall stops moving. As a rule, if the retaining wall is designed adequately, the passive pressure does not reach its maximally possible value.

To summarize: active pressure acts actively, it holds, let's say, 'first-mover right' trying to 'push off' the retaining wall. While the passive pressure will not 'respond', until it is 'attacked' by wall pushed by active pressure.

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Active earth pressure is the one that is exerted by the soil that tends to overturn or slide the retaining wall. It is unfavorable to stability of the wall.

Passive earth pressure is the one exerted from the other side and that tends to stabilize it. Generally, it is applied near the "toe" of the wall.

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  • $\begingroup$ This answer is incomplete. Anchored retaining walls will generate passive earth pressure. $\endgroup$ – Wasabi Aug 22 '16 at 11:28

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