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:
equipotential lines are smooth curves.
Flow lines and
equipotential lines meet at right angles to each other.
No two flow lines
cross each other.
No two flow or
equipotential lines start from the same point.
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 =
H = head difference between the upstream and
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
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
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.