Influence of Friction between Soil and back of the Structure
The magnitude of active or passive earth pressure, respectively, depends not only on the selected solution theory but also on friction between the soil and the back of the wall and by the adhesion of soil to the structure face represented by the angle δ. If δ = 0 then the pressure σ acts in the direction normal to the back of the wall and the resultant of earth pressure P is also directed in normal to the back of the wall - see figure:
Distribution of earth pressure along the structure for δ = 0
Providing the friction between the soil and the back of the wall is considered in the analysis of earth pressures, the earth pressure σ and also its resultant P are inclined from the back of the wall by the angle δ. Orientation of friction angles δ from normal to the back of the wall must be introduced in accord with the mutual movement of the structure and soil. With increasing value of δ the value of active earth pressure decreases, i.e. the resultant force of active earth pressure deviates from the normal direction - see figure:
Distribution of earth pressure along structure for δ ≠ 0
The magnitude δ can be usually found in the range of δ ≤ 1/3φ to δ = 2/3φ. The values of the orientation of the friction angle δ between the soil and the structure are stored in the table of values of δ for various interfaces and in the table of recommended values for |δ| / φ. The value of δ ≤ 1/3φ can be used if assuming smooth treatment of the back of sheeting structure (foil and coating against groundwater). For an untreated face, it is not reasonable to exceed the value of δ = 2/3φ. When selecting the value of δ it is necessary to reflect also other conditions, particularly the force equation of equilibrium in the vertical direction. One should decide whether the structure is capable of transmitting the vertical surcharge due to friction on its back without excessive vertical deformation. Otherwise, it is necessary to reduce the value of δ, since only the partial mobilization of the friction on the back of the wall may occur. In case of an uncertainty it is always safer to assume smaller vale of δ.