Environmental Engineering Reference
In-Depth Information
Figure 11.20.
Drawdown flownets (a) earthfill with chimney drain, low permeability foundation; (b)
earthfill with chimney drain, high permeability foundation; (c) earthfill with horizontal
drainage blankets; (d) sloping upstream core, low permeability foundation. Note that
flownets assume isotropic permeability and k
H
k
V
and the reservoir is completely
drawn down.
(iii) In Figure 11.20a the foundation permeability is low compared to that of the earthfill and
flow will be towards the upstream face. This gives relatively high pore pressures compared
to Figure 11.20b. In the latter case, the foundation is more permeable than the earthfill, so
flow lines will be near vertical and equipotentials near horizontal, giving pore pressures
which should be close to zero;
(iv) In Figure 11.20c horizontal drains are incorporated in the upstream slope. These
cause flow lines to be near vertical on drawdown (except near the low permeability
foundation) and should result in low pore pressures;
(v) In Figure 11.20d the flow lines are influenced by the presence of the filter zones and rock-
fill, giving lower pore pressures than for 11.20a, at least in the upper part of the slope.
