(d) Foundation seepage control
Seepage flows and pressure within the foundation are controlled by cut-
offs and by drainage. Cut-offs are impervious barriers which function as
extensions of the embankment core into the foundation. They are gener-
ally located directly under the core, but can also be located a short dis-
tance upstream and connected to the core by an impervious horizontal
blanket under the shoulder. The cut-off may penetrate to impervious
strata (a ‘fully penetrating’ cut-off ) or, if pervious material occurs to con-
siderable depths, may terminate where the head loss across the cut-off is
sufficient to effect the required degree of control (a ‘partially penetrating’
cut-off ). Older cut-offs were frequently constructed as very narrow clay-
filled ‘puddle trenches’, many proving vulnerable to hydraulic fracture (see
Section 2.7.2), seepage damage and erosion. The principal variants of cut-
off now employed are illustrated schematically in Fig. 2.9.
The relatively wide and shallow trench cut-off (Fig. 2.9(a)) is filled
with rolled clay and forms the base to the core above. It is very effective,
particularly if supplemented by a deep grout curtain, but excavation costs
limit it to maximum trench depths of the order of 10–20 m even for the
largest dams.
The grouted zone type cut-off, shown in Fig. 2.9(b), is now applicable
to a wide range of foundation conditions due to developments in grouting
technology, e.g. alluvial grouting techniques. The cut-off is formed by
several parallel lines of staggered grout holes spaced at 2–3 m centres.
Cement-based grouts are generally employed, but more sophisticated and
costly chemical grouts are available for particularly difficult conditions.
Grout cut-offs are most effective in fractured rock and in coarser-grained
soils, where they may reduce permeability by one to three orders of mag-
nitude. Cut-offs of this type have been constructed to depths in excess of
100 m. They can be installed, or improved retrospectively, by drilling
through the body of a dam, but are generally relatively expensive.
The thin diaphragm-type cut-off, the result of advances made in
geotechnical processes, is illustrated in Fig. 2.9(c). The cut-off is formed by
excavation, in panel lengths, of a narrow slurry-stabilized trench which is
subsequently backfilled with a permanent clay–sand–bentonite mix. A
relatively weak and deformable, ‘plastic’ concrete backfill may alterna-
tively be employed to form the impervious element. The diaphragm cut-
off is very effective in alluvium and finer-grained soils, and can be
constructed economically to depths exceeding 30–40 m.
Diaphragm walls of sheet piling may be driven to depths of up to
20–25 m to form a cut-off under low-head structures. The cost of this type
of cut-off is moderate, but its efficiency is low unless supplemented by
upstream grouting, e.g. with bentonite slurry.
Control of seepage downstream of the cut-off is also assisted by the
almost universal provision of a horizontal blanket drain at ground level
amelia
(Amelia)
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