Active And Passive Earth Pressures

Active earth pressures

Active pressure is the condition in which the earth exerts a force on a retaining system and the members tend to move toward the excavation. Passive pressure is a condition in which the retaining system exerts a force on the soil. 

When the wall moves away from the backfill, there is a decrease in the pressure on the wall and this decrease continues until a minimum value has reached after which their is no reduction in the pressure and the value will become constant. This kind of pressure is known as active earth pressure.

Passive earth pressures

When the wall moves towards the back fill, there is an increase in the pressure on the wall and this increase continues until a maximum value has reached after which their is no increase in the pressure and the value will become constant. This kind of pressure is known as passive earth pressure.

All retaining walls are usually not placed on the ground surface on the front side but are laid at some depth. Hence, the retaining wall has soil to some depth on its front side. When the wall moves away from the backfill due to active earth pressure, it actually moves towards the soil on the front side.