Earth Pressures: Soil when in contact with any structure exerts a lateral pressure to it. While designing the various retaining structures like, retaining walls, sheet piles or any earth retaining structures it is necessary to find out the amount of the earth pressure exerted by the soil to it. If the soil is present above the top of the structure lying in the horizontal plane over it, then it is called as the surcharge.
Types of Earth pressure based on deformation of retaining wall, lateral pressure:
(a) Active Earth Pressure: When there is excessive pressure of the retained soil on the retaining wall, it tends to move away from the retained soil or back fill. It results in the separation of a wedge of the soil just behind the wall from the whole soil mass and it results in the lessening of the soil mass on the retaining wall. This failure wedge of soil wedge tends to move along with the wall. This wedge is retained at its position by a shear force along the failure plane, which is acting away from the wall. So there is a minimum pressure which is exerted by the soil mass on the wall which is called as the active earth pressure.
(b) Passive Earth Pressure: Under any natural cause when the retaining wall tends to move towards the soil back-fill, it will have a tendency to move the failure wedge in the opposite direction or towards the retained soil. The soil will get compressed in itself. Shear stresses will get developed along the shear failure plane but now in a direction towards the retaining wall. Shear stresses will be developed up to a maximum value when it reaches the soil strength. So there will be a maximum pressure exerted by the soil mass on the retaining wall under the maximum shear stress conditions and it is known as the Passive Earth Pressure.
(c) Earth pressure at rest: When there is no movement of the retaining wall, neither towards the back-fill nor away from back-fill soil exerts a pressure on the retaining wall at rest. This pressure is higher than the active earth pressure and lower than the passive earth pressure. So this pressure exerted by the soil on the retaining wall at rest is known as the earth pressure at rest.
Types of Earth pressure based on deformation of retaining wall, lateral pressure:
(a) Active Earth Pressure: When there is excessive pressure of the retained soil on the retaining wall, it tends to move away from the retained soil or back fill. It results in the separation of a wedge of the soil just behind the wall from the whole soil mass and it results in the lessening of the soil mass on the retaining wall. This failure wedge of soil wedge tends to move along with the wall. This wedge is retained at its position by a shear force along the failure plane, which is acting away from the wall. So there is a minimum pressure which is exerted by the soil mass on the wall which is called as the active earth pressure.
(b) Passive Earth Pressure: Under any natural cause when the retaining wall tends to move towards the soil back-fill, it will have a tendency to move the failure wedge in the opposite direction or towards the retained soil. The soil will get compressed in itself. Shear stresses will get developed along the shear failure plane but now in a direction towards the retaining wall. Shear stresses will be developed up to a maximum value when it reaches the soil strength. So there will be a maximum pressure exerted by the soil mass on the retaining wall under the maximum shear stress conditions and it is known as the Passive Earth Pressure.
(c) Earth pressure at rest: When there is no movement of the retaining wall, neither towards the back-fill nor away from back-fill soil exerts a pressure on the retaining wall at rest. This pressure is higher than the active earth pressure and lower than the passive earth pressure. So this pressure exerted by the soil on the retaining wall at rest is known as the earth pressure at rest.
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