On earth retaining structures such as retaining wall, sheet piles, etc, the back fill retained mass of soil, exerts lateral pressure known as earth pressure. If the positions of the back fill lies above a horizontal plane at the elevation of the top of the structure, it is called surcharge.
The inclination of the surcharge to the horizontal is called surcharge angle. The magnitude of the lateral earth pressure depends upon the movement of the retaining wall relative to the back fill and also upon the type of the soil.
TYPES OF EARTH PRESSURE
Lateral earth pressure may be one of the following 3 types.
- Active earth pressure
- Passive earth pressure
- Earth pressure at rest
1. ACTIVE EARTH PRESSURE
Due to excessive pressure of the retained soil, the retaining wall tends to move away from the back fill. Consequently a certain portion of the back fill located immediately behind the retaining wall, gets separated from the rest of the soil mass and hence the earth pressure on the retaining wall decreases.
The wedged shaped portion of the back fill tending to move with the wall, is called the failure wedge. The retaining wall is kept in equilibrium by resisting force developed due to shear strength of the soil along the plane of the failure wedge in a direction away from the retaining wall.
There is a limit within which the retaining wall may move from the back fill, thereby limiting the pressure. The minimum pressure exerted by the soil on the retaining wall, is called Active Earth Pressure.
2. PASSIVE EARTH PRESSURE
Whenever the retaining wall moves towards the back fill due to any natural cause, the earth pressure increases because the retaining soil gets compressed and the resulting shearing strength develops along the plane of the failure wedge in direction towards the retaining wall.
The pressure reaches a maximum limit when the shearing resistance of the soil has been fully mobilized. The maximum earth pressure due to maximum shear stress on the retaining wall is called Passive Earth Pressure.
3. EARTH PRESSURE AT REST
We know that active earth pressure is accompanied by the movement of the retaining wall away from the back fill and passive earth pressure is accompanied by the movement of the retaining wall towards the back fill.
Thus, there occurs an intermediate situation when the retaining wall does not move due to earth pressure but remains perfectly stationary. The pressure which develops due to back fill at zero movement is called Earth Pressure at Rest. Its value is higher than limiting active pressure but less than the passive pressure.