On earth retaining structures like retaining walls, sheet piles etc., back fill maintains the soil mass, the lateral pressure is known as earth pressure. If the position of the back fill is above a horizontal plane at the height of the top of the structure, it is called overload. The inclination towards the horizontal of the overload is called the overload angle. The magnitude of lateral earth pressure also depends on the speed of the retaining wall relative to the back fill and the type of soil.
Lateral earth pressure can be one of the following 3 types.
- active earth pressure
- idle earth pressure
- earth pressure at rest
1. active earth pressure
Due to the excessive pressure of the retained soil, the retaining wall tends to move away from the back fill. As a result, a certain part of the back fill located just behind the retaining wall is separated from the rest of the soil mass and hence the earth pressure on the retaining wall is reduced. The wedge-shaped portion of the back fill that tends to move along the wall is called the failure wedge. The retaining wall is kept in equilibrium by the resistance force developed due to the 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 can extend beyond the back fill, thereby limiting the pressure. The minimum pressure exerted by the soil on the retaining wall is called active earth pressure,
2. idle earth pressure
Whenever the retaining wall moves towards the back fill due to any natural cause, the earth pressure increases as the retaining soil gets compacted and the resultant shearing strength develops along the plane of failure wedge in the direction of the retaining wall. The pressure reaches its maximum when the shear resistance of the soil is fully activated. The maximum pressure of the earth due to maximum shear stress on the retaining wall is called idle earth pressure,
3. earth pressure at rest
We know that with active earth pressure the retaining wall moves away from back fill and with passive earth pressure the retaining wall moves towards back fill. Thus, there is an intermediate state when the retaining wall does not move due to the pressure of the earth, but remains completely stationary. The pressure developed due to back fill at zero speed is called earth pressure at rest, Its value is greater than limiting active pressure but less than passive pressure.