ABSTRACT
In the middle years of the nineteenth century, Darcy showed, by means of experiment, that the rate of flow of fluid, either water or air (gas) is proportional to the gradient of pressure head in the direction of flow, i.e. the loss of pressure head per unit of distance in the direction of flow. Darcy’s law can be expressed as
v = ki, or k = v/i (6.1)
where v is the velocity of flow with dimensions [m/s] k is called the coefficient of permeability i is the gradient of the pressure head [m/m = dimensionless] the pressure head is the
pressure divided by the unit weight of water (N/m2.m3/N = m] Thus the units of k are [m/s]. In the engineering of waste storages, the permeabilities of both the construction
materials used to contain or drain the waste, as well as the waste itself need to be established. Construction materials could include the compacted soil used for preliminary earthworks – toe walls, erosion control paddocks, etc., as well as features such as compacted soil retaining dykes and embankments, compacted clay linings and filter drains. The permeability of the tailings also needs to be known in order to design drainage measures for the storage, construct flow nets etc. In order to construct a flow net, both the permeabilities for vertical (cross layer) flow and horizontal (along layer) flow need to be known.
