Environmental Engineering Reference
In-Depth Information
the partially saturated permeability is lower than the saturated permeability. Hence as
seepage begins to flow through the core of a dam, the flow regime is controlled by the
contrasting permeability. It is hypothesised that the phenomenon is aided by dissolved
gas in the seepage water coming out of solution in the lower furthermost downstream
- part of the core where the pore pressures are lowest.
(f)
Clogging of the filters
. Peck (1990) suggests that fine particles from the core may
migrate to the downstream filter, clog it and also lead to high pore pressure gradients
on the downstream part of the core. This is more likely to be a problem if the soil in
the core is internally unstable (see Chapter 9).
From the above discussion it will be apparent that it is difficult to predict steady seep-
age pore pressures. There are several mechanisms which can lead to pressures equivalent
to those where a high k
H
/k
V
ratio is assumed, i.e. reservoir head persists through the core.
The authors' advice is to design all embankments on the assumption that k
H
/k
V
is
15. For
larger, more critical dams, internal zoning should control seepage to such a degree that the
stability is not greatly sensitive to k
H
/k
V
. The authors' own practice for an earth and rock-
fill embankment with a vertical drain is to ignore the flownet effect, i.e. assume the flow-
lines are horizontal. This is conservative as it implies k
H
/k
V
but only affects the factor
of safety marginally over that if k
H
/k
V
9 were used and allows one to be confident that
pore pressures have been estimated conservatively. In some cases this allows adoption of
a slightly lower factor of safety for the design.
For existing dams, there is often piezometer data on which to assess the pore pressures.
The data need to be considered carefully, particularly in regard to whether equilibrium
has been reached and whether there is sufficient instrumentation to define the pore pres-
sures over the whole of the dam. The implication of the high k
H
/k
V
on pressures should be
considered.
11.4.1.2
Pore pressures under flood conditions
Pore pressures in the low permeability zones in a dam under flood conditions are very dif-
ficult to predict. There are two broad issues:
-Will pore pressures in the body of the dam, for example in the core, rise with the flood
water level?
-Will pore pressures in the vicinity of the dam crest above normal Full Supply Level rise
with the flood water level?
Considering these in turn:
(a)
Pore pressures in the body of the dam
. There are some general observations:
(i) Whether the pore pressures will respond rapidly to the reservoir rise depends on the
degree of drainage which can be assessed as described in Section 11.4.3.3. If a zone is
assessed to be free draining (Time Factor T
3) or in the intermediate zone (Time
Factor T 0.01 to 3), then it should be assumed the pore pressures will respond as the
reservoir level rises in the flood;
(ii) For non-draining zones (Time Factor T
0.01) the pore pressures will respond
slowly, so in most flood conditions there will be little or no rise in pore pressures;
(iii) Where monitoring of the dam shows a rapid response to reservoir level, regardless
of the assessments above, assume the pore pressures will respond to the reservoir
level.
(b)
Pore pressures near the crest of the dam
. The pore pressures near the crest of the dam
will depend on whether the zones are free draining or not, as detailed above, but
also on whether the core is affected by cracking due to differential settlement and
