Theoretical investigation of the displacement ductility capacity of scoured fixed-head piles in cohesive soil

Scouring causes the exposure of structure foundations, which may influence their seismic performance. In this study, we develop a theoretical model to investigate the influence of foundation exposure caused by scouring on the displacement ductility capacity of fixed-head piles. The proposed theoretical model is based on consideration of a long pile in cohesive soil. The analytical solutions of the displacement ductility capacity and system overstrength ratio of scoured piles in linear soil are derived. The ultimate state of the pile is set as the state in which the pile-head moment reaches its ultimate capacity or when in-ground plastic hinging begins to occur. The proposed solutions are validated through numerical pushover analyses. A pile with a large scour depth is prone to in-ground plastic hinging. If no in-ground plastic hinging occurs, the displacement ductility capacity increases until an upper limit as the scour depth increases; however, the displacement ductility capacity decreases with increasing scour depth when in-ground plastic hinging occurs. The proposed model for linear soil provides a lower-bound estimate of the displacement ductility capacity. Soil nonlinearity can increase a pile’s ductility; however, this effect becomes negligible for large scour depths. The solutions for linear soil are modified for nonlinear soil by using an equivalent linear soil model.