seepage, pressure
If water flows through a deposit of sand, the viscous drag tends to move the grains in the direction of the flow and produces a force, known as a seepage pressure between the grains. The seepage pressure is an intergranular stress. If upward flowing water produces a seepage pressure equal and opposite to the submerged weight of the sand above a given horizontal section, the intergranular pressure becomes zero and the strength of the sand becomes zero. The sand then is in a "quick" condition and is incapable to support a load on its surface. Where flowing water comes out of a cohesionless deposit, the seepage pressure has a component directed outward and away from the surface and tends to remove some of the smaller particles. If a process continues, a tunnel-shaped cavity or "pipe", may develop. The formation of such "pipes" below or within dams has led to several catastrophic failures. It may be prevented if we cover the surface, where the seepage emerges, with coarser materials that help the escape of the water but prevent the erosion of the fines. If the seepage pressure has a rather great upward component, it may be necessary to add weight to the top of the filter to counterbalance the upward forces.
