Permeability and seepage

Permeability measures how easily water flows through soil. It ranges over many orders of magnitude: clean gravel lets water through almost freely, while clay may take years to pass the same amount. That range governs drainage, dewatering, seepage under dams and how fast a clay layer settles.

Darcy's law states that, for slow laminar flow, the flow velocity is proportional to the hydraulic gradient, the head loss per unit length of flow path. The constant is the coefficient of permeability k.

In the laboratory, the constant-head test suits coarse soils and the falling-head test suits fine soils. In the field, pumping tests from wells give a more representative value because they sample a large volume of ground including its layers and fissures.

For two-dimensional seepage under a dam or around a sheet pile, a flow net of flow lines and equipotential lines gives the seepage quantity and the pore pressures. The exit gradient at the downstream toe must stay well below the critical gradient to prevent piping.