Active Earth Pressure of Soil

Definition: The lateral pressure exerted by the soil on a retaining wall when the wall tends to move away from the backfill is called Active Earth Pressure.

Explanation:

Retaining walls are structures that are used to retain soil or other materials and prevent them from sliding or collapsing. The soil behind the wall exerts lateral pressure on the wall which tends to push the wall forward. This pressure is known as Earth Pressure.

Active Earth Pressure is the maximum lateral pressure exerted by the soil on the retaining wall when the wall tends to move away from the backfill. This happens when the wall is pushed away from the backfill due to external forces such as soil settlement or earthquake.

The active earth pressure is a critical design parameter for retaining walls as it determines the stability of the wall and the required strength of the wall.

Factors affecting Active Earth Pressure:

- The angle of repose of the soil behind the wall
- The height of the wall
- The slope of the backfill
- The type of soil

Calculation of Active Earth Pressure:

Active earth pressure can be calculated using the Rankine or Coulomb method. The Rankine method assumes that the soil behind the wall is cohesive and has no internal friction. The Coulomb method assumes that the soil has internal friction and is a granular material.

The formula for calculating active earth pressure using the Rankine method is:

Pa = Ka γ H² / 2

Where,
Pa = Active Earth Pressure
Ka = Coefficient of Active Earth Pressure
γ = Unit weight of soil
H = Height of the wall

The formula for calculating active earth pressure using the Coulomb method is:

Pa = Kp γ H / sin²(φ + δ)

Where,
Pa = Active Earth Pressure
Kp = Coefficient of Active Earth Pressure
γ = Unit weight of soil
H = Height of the wall
φ = Angle of internal friction of soil
δ = Angle of wall inclination

Conclusion:

Active Earth Pressure is an important design parameter for retaining walls. It determines the stability of the wall and the required strength of the wall. The calculation of active earth pressure can be done using the Rankine or Coulomb method depending on the soil type and the design requirements.