Laboratory Teaching Exercises
21
Figure i
For the purpose of classifying various types of soil with respect to
permeability it has been found convenient to make use of a so-called,
"laboratory (ie. 'standard') coefficient of permeability" designated as K. K is
defined as the flow of water at 60 deg F in gallons per day through a one-
square-foot cross sectional area of the soil in question under an hydraulic
gradient of one foot per foot.
Classification of soils with respect to K
b. Flow
Nets
Flow Lines and Equipotential Lines
A flow net is a graphical representation of flow through soil (or any other
porous medium). From the flow net, information may be obtained on such
features as the amount of seepage or leakage below a dam or through an
earth dam, the uplift pressure caused by the water on the base of a concrete
dam (or, say a harbour wall); and the danger of a "quick" or liquefaction
condition at points where seepage water comes to the ground surface.
The path which a particle of water follows in its course of seepage through a
saturated soil mass is called a flow line. In isotropic soil, flow follows paths of
greatest hydraulic gradient, much as bodies rolling or sliding downhill tend to
pick paths having the steepest slope.
It follows from Darcy's Law, and from common sense that water can flow
through soil only if some head difference h exists between the places
between which the flow might occur. This head difference (which may be
made up of several components, see Figure iii) represents a certain amount
of potential energy which is transformed into the kinetic energy of the moving
