Displacements of river bridge pier foundations due to geotechnical effect of floods

Safety of river bridge piers is analysed through the comparison of limit states of the soil (G)-foundation (F)-piers (P) system, potentially occuring during a flood. The displacements of a rigid, shallow F resting on compressible G, subjected to vertical, horizontal forces and destabilizing moments transmitted by P are first analysed through an original constitutive model (c.m.). The tilt and the stability of the G-F-P system are then investigated on the base of the c.m., taking into account the complex relationships among the tilt of P, the scour of G and the hydrodynamic forces (horizontal force on P and uplift on F). Results show that, depending on properties of G, geometry of P and F, water level during a flood and displacements history, the tilt of P may slowly increase towards a final, stable value, or quickly reach a critical value corresponding to the rotational instability. This one occurs before that the bearing capacity of the F-G sub-system is achieved.