Civil Engineering Reference
In-Depth Information
(ii) The junction between a permeable and an impermeable material, which is also a flow line; for flow
net purposes a soil that has a permeability of one-tenth or less the permeability of the other may be
regarded as impermeable.
(iii) The horizontal ground surfaces on each side of the dam, which are equipotential lines.
The procedure is as follows:
(a) Draw the first flow line and hence establish the first flow channel.
(b) Divide the first flow channel into squares. At first the use of compasses is necessary to check that in
each figure b
=
1, but after some practice this sketching procedure can be done by eye.
(c) Project the equipotentials beyond the first flow channel, which gives an indication of the size of the
squares in the next flow channel.
(d) With compasses determine the position of the next flow line; draw this line as a smooth curve and
complete the squares in the flow channel formed.
(e) Project the equipotentials and repeat the procedure until the flow net is completed.
As an example, suppose that it is necessary to draw the flow net for the conditions shown in Fig.
2.9a
.
The boundary conditions for this problem are shown in Fig.
2.9b
, and the sketching procedure for the
flow net is illustrated in Figs c, d, e and f of Fig.
2.9.
If the flow net is correct the following conditions will apply:
(i) Equipotentials will be at right angles to buried surfaces and the surface of the impermeable layer.
(ii) Beneath the dam the outermost flow line will be parallel to the surface of the impermeable layer.
Fig. 2.9
Example of flow net construction.
