Environmental Engineering Reference
In-Depth Information
Coefficient of permeability k, cm/s (log scale)
10 2
10 1
10 1
10 2
10 3
10 4
10 5
10 6
10 7 10 8
10 9
10
Drainage
Poor
Good
Relatively impervious
Free
Soil types
Deep wells
Well points
Vacuum well points
Compressed air
Electroosmosis
GW
GP, SW, SP
SM, GM, ML, GC, SC, CL
CH
Freezing
Grouts:
Sand cement
Cement
Clay cement
Bentonite gel
Sodium silicate
Chrome-lignin
Resins
FIGURE 8.35
General applicability of some methods for controlling groundwater and seepage in soils as a function of grain-
size characteristics. (Other methods not shown include liners, walls, and drains.)
Walls
Compacted Earth Walls
Walls of compacted earth constructed to form a cutoff can include a homogeneous
embankment for a dam, a clay core within a dam, or a clay-filled core trench in a dam
foundation, as discussed in Section 8.4.7.
Concrete Walls
Walls of concrete are commonly used in foundation excavations and as tunnel linings
where a permanent water barrier is required. In addition, they provide high supporting
capacity to retain the excavation. Vertical concrete walls cast in braced excavations extend-
ing down to impervious material effect a positive cutoff beneath dams ( Figure 8.49e), but
their use in the United States has decreased in recent years because of their relatively high
cost compared with other methods.
Sheet Pile Walls
Sheet pile walls provide significant efficiency in seepage control only when the interlocks
are extremely tight, a difficult condition to ensure, especially when sheets encounter cob-
bles, boulders, broken rock, or other obstructions. Long sheets, in particular, tend to bend
and deflect. At best they provide only a partial cutoff. Commonly used to retain excava-
tions, they are normally combined in free-draining soils with a dewatering system, which
serves also to reduce lateral earth pressures. Thin “curtain” walls of sheeting have been
used for years as a cutoff for foundation seepage beneath dams, but their use is now much
less frequent, not only because of the difficulties in obtaining a tight barrier, but also
because of relatively high costs and their susceptibility to corrosion.
 
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