Particle migration between a dam and its foundation : Laboratory study

Rapid variations of the water level in a reservoir create considerable changes in the direction of the flow at the interface between the core of the dam and the foundation, particularly in the vicinity of the downstream side of the core. Experiments have been performed in a simple test cell that partly consisted of glass. The cell contained a foundation of fine grains and a core of coarse grains. Initially, the foundation was saturated and the core was dry. Migration was recorded with a video camera. The experiments showed that when water enters the core, fine grains from the foundation moved up into the core and consequently created a cavity in the foundation. When the water had penetrated the core horizontally, the hydraulic gradient changed direction and coarse grains of the core moved down into the cavity. Ultimately the core collapsed. During the collapse of the core, successive formation and destruction of arcs in the core took place. The risk for instability, which is associated with the change of direction of velocity at the interface, can be avoided if the rate of change of the level in the reservoir is much less than the ratio between conductivity and the porosity of the core.