NEGATIVE SKIN FRICTION AND SETTLEMENT OF PILES

A relative movement between a pile and a soil produces shear stress along the interface of the pile and the soil. Such movement can be induced by a push-load on the pile pressing it down into the soil, or by a pull-load moving it upward. ` A relative movement can also be induced when the soil settles in relation to the pile, or, in swelling soils, when the soil moves upward in relation to the pile. By definition, if the movement of the pile is downward, i.e., the shear stress induced in the pile is upward, the direction of the shear is positive. If the movement of the pile is upward, the shear stress direction is negative; accordingly, the induced shear stress is called positive or negative.