Side friction is developed when a shaft is provided to resist Vertical loads. The side friction between soil and shaft results relative movements between them. The maximum side friction is often developed after relative small displacements less than 0.5 inches. Side friction is limited by the adhesion between the shaft and the soil or else the shear strength of the adjacent soil, whichever is smaller.
Side friction often contributes the most bearing capacity in practical situations unless the base is bearing on stiff shale or rock that is much stiffer and stronger than the overlying soil.
Side friction is hard to accurately estimate, especially for foundations constructed in augered or partially jetted holes or foundations in stiff, fissured clays.