Civil Engineering Reference
In-Depth Information
Figure 14.8
Flownet for steady state seepage through a dam.
The top of the impermeable rock AC is a flowline. The phreatic surface BE is not
precisely located by the geometry of the dam alone but its position will be fixed by
the geometry of the flownet. The phreatic surface is a flowline and, on the phreatic
surface, the pore pressure is zero. A roughly sketched flownet is shown in Fig. 14.8(b).
Again this satisfies the boundary conditions, flowlines and equipotential are orthogonal
and each element is more or less 'square'. Notice that the equipotentials intersect the
phreatic surface at equal vertical intervals (because
u
=
0 along the phreatic surface).
For this flownet,
N
f
=
5.
The level of water in a standpipe does not necessarily rise to the phreatic surface.
In Fig. 14.8(b) the tip of the standpipe is on the equipotential HJ. If the tip of a
standpipe is on the phreatic surface at J the water remains at J and so the level of water
in any standpipe on HJ must be at the level of J. For the standpipe at H the water rises
not to the phreatic surface but to the level of J as shown.
The flownets can be used to calculate the rates of leakage into the trench excava-
tion and through the dam using Eq. (14.9). Note that this contains the coefficient of
permeability
k
and the accuracy of the solution will depend more on how well you
can determine a value for
k
than on how well you can draw a flownet. The flownets
can also be used to calculate pore pressures. You will need these to calculate the loads
on the walls and props in Fig. 14.7 and the stability of the dam slopes in Fig. 14.8,
but to calculate pore pressures the flownet must be accurately drawn. Notice that the
geometry of a flownet and the pore pressures are independent of the value of coefficient
of permeability
k
.
The flownets shown in Figs. 14.7 and 14.8 were sketched by me very quickly using
a soft pencil and a good eraser. They are a bit rough - not all the elements are prop-
erly 'square' and sometimes the flownets and equipotential do not intersect exactly
2 and
N
d
=
