Environmental Engineering Reference
In-Depth Information
C: Downstream zone, which maintains stability and may, if permeable compared to the
core and filter, control pore pressures;
D: Horizontal filter drain - which controls exit gradients, erosion of the foundation and
pore pressures (but may actually increase the amount of seepage because the gradients
are increased);
E: Upstream low permeability blanket which increases the seepage path, reduces seepage
and seepage exit gradients;
F: Slurry trench (or other types of cutoffs) which will reduce (or almost stop if fully pen-
etrating to a low permeability base) seepage and seepage exit gradients (usually in soil
or extremely weathered rock);
G: Grouting to reduce seepage and seepage exit gradients. Grouting is usually in rock,
since grouting in soil is usually costly and ineffective;
H: Pressure relief wells - control exit gradients where confined aquifers in the foundation
reduce the effectiveness of the horizontal drain;
I:
Weighting berm - which improves downstream slope stability and overcomes poten-
tial liquefaction or “blow-up” of the foundation (with H).
Of course, not all these features are needed for every dam. For example, a central core
earth and rockfill dam on a strong, non erodible rock foundation will usually have grout-
ing, but will not need a horizontal filter or drain.
10.2
SOME PARTICULAR FEATURES OF ROCK AND SOIL FOUNDATIONS
WHICH AFFECT SEEPAGE AND SEEPAGE CONTROL
Seepage occurs through all embankment dams and their foundations. The permeability of
most compacted earthfill core materials is less than 10 8 or 10 9 m/sec. By comparison
virtually all rock foundations have rock mass permeability greater than 1 lugeon (1
lugeon
1 litre/second/metre of drill hole at a pressure of 1000 kPa) or approx.
10 7 m/sec, and most rocks have greater than 5 lugeons (approx. 5
10 7 m/sec).
Foundations of alluvial or deeply weathered and lateritised rock, may have mass perme-
abilities as high as 10 3 -10 5 m/sec. As a result most of the seepage is through the foun-
dation, not the embankment, and significant rates of seepage may occur.
The following details some particular features of rock and soil foundations which influ-
ence seepage in the foundation and the design of seepage control measures.
(a) Rock foundation . As discussed in detail in Chapters 3 and 4, the permeability and
seepage in rock foundations is controlled by joints, bedding, shears and faults and in
some situations other features such as solution cavities.
The detailed characterisation and understanding of the hydrogeological model is
vital to design of foundation grouting, assessing the likely magnitude of the under
seepage and erodibility of the foundation and hence the need for filters, drains or other
high permeability zones in the dam. It should be noted that clay infilling of joints and
stress relief at depth can give marked stratification of permeabilities in weathered sur-
faces.
Table 10.1 shows the assessed rock mass permeability model for the Ben Lomond
Tailings dam site which had steeply dipping meta-sediments in the foundations.
Similar effects were present at the Ranger Tailings dam, giving a confined aquifer
about 4 metres below the surface, below the clay infilled joints in the upper weathered
rock. This necessitated construction of weighting berms to maintain adequate factors
of safety against “blow-up”, or erosion into rockfill in the dam if filters are not pro-
vided over the foundations.
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