Permeability and Flow of Water in Soils - Elements of Soil Mechanics - page 60

Fig. 2.19 Conditions at the downstream end of an upper flow line.
Fig. 2.20 Types of seepage through an earth dam.
Fig. 2.21 Flow net for a theoretical earth dam.
2.14.2 Types of low occurring in an earth dam
From Fig. 2.19 it is seen that an earth dam may be subjected to two types of seepage: when the dam
rests on an impermeable base the discharge must occur on the surface of the downstream slope (the
upper flow line for this case is shown in Fig. 2.20a ), whereas when the dam sits on a base that is perme-
able at its downstream end the discharge will occur within the dam (Fig. 2.20b ). This is known as the
underdrainage case. From a stability point of view underdrainage is more satisfactory since there is less
chance of erosion at the downstream face and the slope can therefore be steeper but, on the other hand,
seepage loss is smaller in dams resting on impermeable bases.
2.14.3 Parabolic solutions for seepage through an earth dam
In Fig. 2.21 is shown the cross-section of a theoretical earth dam, the flow net of which consists of two
sets of parabolas. The flow lines all have the same focus, F, as do the equipotential lines. Apart from the