Civil Engineering Reference
In-Depth Information
The tensiometer must be fully de-aired during installation, as one would expect, if accurate results are
to be obtained. The response time of the type of apparatus described is only a few minutes but it has
the disadvantage, until recently, that it could only be used to measure suctions up to about 100 kPa.
2.14 Earth dams
2.14.1 Seepage patterns through an earth dam
As the upper flow line is subjected to atmospheric pressure, the boundary conditions are not completely
defined and it is consequently difficult to sketch a flow net until this line has been located.
Part of such a flow net is shown in Fig.
2.17.
It has already been shown that the hydrostatic head at a
point is the summation of velocity, pressure and elevation heads. As the top flow line is at atmospheric
pressure the only type of head that can exist along it is elevational, so that between each successive point
where an equipotential cuts an upper flow line there must be equal drops in elevation. This is the first of
three conditions that must be satisfied by the upper flow line.
The second condition is that, as the upstream face of the dam is an equipotential, the flow line must
start at right angles to it (see Fig.
2.18a
), but an exception to this rule is illustrated in Fig.
2.18b
where
the coarse material is so permeable that the resistance to flow is negligible and the upstream equipotential
is, in effect, the downstream face of the coarse material. The top flow line cannot be normal to this
surface as water with elevation head only cannot flow upwards, so that in this case the flow line starts
horizontally.
The third condition concerns the downstream end of the flow line where the water tends to follow
the direction of gravity and the flow line either exits at a tangent to the downstream face of the dam
(Fig.
2.19a
) or, if a filter of coarse material is inserted, takes up a vertical direction in its exit into the filter
(Fig.
2.19b
).
Fig. 2.17
Part of a flow net for an earth dam.
Fig. 2.18
Conditions at the start of an upper flow line.
