Environmental Engineering Reference
In-Depth Information
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(d)
(a)
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Grout curtain
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(b)
(e)
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(c)
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FIGURE 8.49
Controlling seepage in pervious materials beneath dams: (a) uncontrolled foundation seepage causes water
loss and uplift at (1); (b) rolled Earth cutoff; (c) upstream blanket to reduce head; (d) grout curtain in fractured
rock; (e) concrete wall or slurry-trench cutoff; (f) downstream relief wells or trench. (Sections are schematic
only; upstream slopes are normally about 2½ to 3½:1 and downstream 2 to 3:1.)
the dam. (Flood control dams, for example, are not a concern regarding water loss, as long
as seepage pressures are not excessive.)
A core trench , formed by extending the embankment core zone of compacted impervi-
ous soils through the pervious foundation, is often the most economical cutoff solution
(Figure 8.49b). Ideally, the core trench should extend into impervious materials. Sherard
et al. (1963) state that compacted fill cutoffs are commonly constructed to depths of about
75 ft (22.5 m) when necessary, but become increasingly expensive below this depth.
Because the trench is excavated in granular soils or other free-draining materials in a
stream valley, the primary problem to overcome is maintenance of a dry excavation dur-
ing core construction.
A blanket formed of impervious materials, extending some distance upstream from the
embankment (Figure 8.49c), reduces toe-seepage forces in materials with moderate per-
meability by increasing the length of flow paths. Usually, the same material used for the
embankment core zone is used for the blanket. Thickness and length are dependent upon
the permeability of the blanket material; permeabilities, stratification, and thickness of the
pervious stratum; and reservoir depth (Bennett, 1952). The thickness usually ranges from
2-10 ft (0.7-3 m).
A grout curtain , extending from a core trench or the core wall, can be installed to almost
any practical depth, and has the added advantage of reducing the amount of excavation
and dewatering necessary. The controlling factors to be evaluated in the determination of
the necessity of grouting a rock foundation (Figure 8.49d) are the rock-mass characteristics
and the height of water contained by the dam. From a historical viewpoint, dams with a
height of less than 50 ft (15 m) on rock that is not excessively fractured have not been
 
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