The earth pressures are defined as follows:

Consider a retaining wall with a plane vertical face, as shown in Fig. 2.1, which is backfilled with cohesionless soil. If the wall does not move even after filling the materials, the pressure exerted on the wall is known as pressure for the at rest condition of the wall. If suppose the wall gradually rotates about the point A and moves away from the backfill, the unit pressure on the wall gradually gets reduced and after a particular displacement of the wall at the top, the pressure reaches a constant value. This pressure is the minimum possible. The pressure is termed as the active earth pressure since the weight of the backfill is responsible for the movement of the wall. If the wall is smooth, the resultant pressure acts normal to the face of the wall. If the wall is rough, the resultant pressure acts at an angle of to the normal to the face. This angle is known as the angle of wall friction . As the wall moves away from the backfill, the soil also tends to move forward. When the wall movement is sufficient, a soil mass of weight W ruptures along a surface AC'C shown in fig. This surface is slightly curved. If the surface is assumed to be plane surface AC, analysis would indicate that this surface would make an angle of with the horizontal.

If the wall is now rotated about A towards the backfill, the actual failure plane AC'C is also a curved surface. However, if the failure surface is approximated to a plane AC, this makes an angle with the horizontal and the pressure on the wall increases from the value of at rest condition to a maximum possible value. The maximum pressure that is developed is termed as passive earth pressure. The pressure is called passive earth pressure because the weight of the backfill opposes the movement of the wall. It also makes an angle of with the normal if the wall is rough.

The gradual increase or decrease of the pressure of the wall with the movement of the wall from the “at rest condition” may be depicted as shown in Fig.2.2. The movement required to develop passive state is considerably larger than the required for active case.