Mechanical effects of floods on river bridge pier foundations

Limit states of the soil (G)–foundation (F)– river bridge pier (P) system, potentially occurring during a flood, are analysed. An original constitutive model (c.m.) first allows to simulate the displacements of a rigid, shallow F resting on compressible G, subjected to vertical, horizontal forces and destabilizing moments transmitted by P. The tilt and the stability of the G-F–P system are then modelled on the base of the c.m., by 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). It is shown that the tilt of P may increase towards a stable value, or reach a critical value (rotational instability) that occurs before the bearing capacity of the F–G sub-system is achieved.