Active and passive earth coefficients - Coulomb and Rankine

In the most general case, the pressures acting on a wall consist of the sum of the hydrostatic pressure, which is the result of the weight of water on the structure, and the effective pressure exerted by the soil particles. The coefficient of soil pressure is defined as the ratio of horizontal effective stress to vertical stress, and when there is no lateral deformation, it is known as the coefficient of static coefficient pressure, K0. For example, a basement wall is a good example of static pressure.

However, walls are not infinitely rigid and they deform. Depending on whether the lateral deformation is negative (soil "unloading") or positive (soil "loading"), active thrusts, Ka, and passive thrusts, Kp, can be distinguished, with the relationship Ka < K0 < Kp. Mobilizing passive soil pressures requires significantly greater displacements of the wall against the soil than those necessary to reach a state of active soil pressures, with movements towards the excavation area.

When the passive pressure is favorable, it is common to neglect its effect due to the imprecision in determining its actual value. For safety reasons, a reduction coefficient, such as 1.5, can also be applied to account for possible uncertainties in the calculation and ensure a more conservative design.