In principle, the analysis of seepage behaviour is represented by a network

of flow lines and equipotential lines, constituting a flow net. A flow line is

a line along which a water particle will travel from upstream to the downstream

side in the permeable soil medium, whereas an equipotential line is a line

along which the potential head at all points is equal.

The properties of a flow net:

1.

Flow and

equipotential lines are smooth curves.

2.

Flow lines and

equipotential lines meet at right angles to each other.

3.

No two flow lines

cross each other.

4.

No two flow or

equipotential lines start from the same point.

5.

The flow elements

formed are approximate squares.

Figure: A completed flow net.

The figure above shows an example of a completed flow net. The curves

drawn are flow lines, whereas the dotted curves are equipotential lines. In the

figure, Nf is the number of flow channels (strip between any two

adjacent flow lines) and Nd is the number of potential drops (drop

in the piezometric level between any two adjacent equipotential lines). The total

rate of seepage through the permeable layer per unit length (q) is given by:

where k =

hydraulic conductivity

H = head difference between the upstream and

downstream sides

A hydraulic gradient is required for pore water flow (seepage). When there

is a change in pore water pressure in conditions of seepage flow within the

ground, the potential driving the water flow is the hydraulic gradient between

the two points. In steady state seepage, the gradient remains constant. The hydraulic

gradient (i) is given by:

where Dh = difference in pressure head between two

points

L = distance between two points / length of

flow over which the loss of head occurred

Figure: Nature of variation of v with hydraulic gradient, i

Although the hydraulic gradient varies with different kinds of flow, the

flow of water through the void spaces can be considered laminar in most soils.

Hence, velocity (v) is directly

proportional to hydraulic gradient (i).

Hydraulic conductivity is also another key parameter governing the seepage

of soil. It is also known as coefficient of permeability. It describes the ease

of pore water flow through soil. The hydraulic conductivity of increases

rapidly with the degree of saturation of soil, thus high hydraulic conductivity

means water flows easily through the soil. Seepage velocity is the average

velocity at which the water flows through the soil pores. The formula for

seepage velocity (vs) is

given by:

where k = hydraulic conductivity

i = hydraulic gradient

n = porosity of a soil

For a given soil, the hydraulic conductivity is a function of its

porosity. As soils become denser, porosity reduces, compression of soil will

therefore alter its permeability.