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 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 wall and the resultant of earth pressure P is also directed in normal to the back of wall - see figure:
Distribution of earth pressure along structure for δ = 0
Providing the friction between the soil and the back of wall is considered in the analysis of earth pressures, the earth pressure σ and also its resultant P are inclined from the back of wall by the angle δ. Orientation of friction angles δ from normal to the back of wall must be introduced in accord with the mutual movement of 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 orientation of the friction angle δ between the soil and the structure are stored in table of values of δ for various interfaces and in 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 ground water). For 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 partial mobilization of friction on the back of wall may occur. In case of uncertainty it is always safer to assume smaller vale of δ.