Kazem Fakharian - Assistant Professor Dept. of Civil Engrg., Amirkabir University of Technology, Tehran

Reduction in maximum shear stress and degradation of shear strength of an interface between a steel plate and a medium crushed silica sand under two-way cyclic tangential-displacement-controlled and one-way shear-stress-controlled experiments are investigated. The tests were conducted under the condition of constant stifhess in the normal direction to the interface plane. The results provided an explanation for the degradation of shaft resistance of piles embedded in sand. It is shown that the reduction in maximum shear stress and degradation of shear strength are attributed not only to the reduction of normal stress with cycles, but also to the amount of mobilized sliding displacement. The reduction in maximum shear stress is very large when failure is experienced at the interface between the pile shaft and the surrounding soil mass.