Environmental Engineering Reference
In-Depth Information
Clay tile with open joints
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Porous concrete pipe
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The pipes should be surrounded with a filter soil designed so as not to enter the holes
or slots. Cleanout points often are provided.
8.4.6
Surface Treatments
Purpose
Surface treatments are used to deter or prevent water infiltration so as to improve slope
stability, where a natural slope appears potentially unstable or a cut slope is being exca-
vated, and to prevent increased saturation of potentially collapsible or swelling soils.
Techniques
Slope stabilization is improved (see
Section 9.4.3)
by preventing or minimizing the surface
infiltration of water from upslope by:
Planting vegetation, which reduces the occurrence of shrinkage cracks that per-
mit water to enter, deters erosion, and binds the surface materials
●
Grading all depressions to prevent water from ponding and to allow runoff
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Sealing all surface cracks with a deformable material
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Installing surface drains to collect runoff and direct it away from the potentially
unstable area
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Areas adjacent to structures founded on potentially collapsible or swelling soils are
sometimes paved to prevent direct infiltration from rainfall.
8.4.7
Typical Solutions to Engineering Problems
Earth and Rock-Fill Dams
Embankment Control
Some schemes for the control of seepage through earth and rock-fill dams are illustrated
in
Figure 8.47.
The objective of the design is to retain the reservoir and prevent flow from
emerging from the downstream face (Figure 8.47a) where piping erosion may result.
A homogeneous embankment can be constructed from some natural materials such as
glacial tills or residual soils containing a wide range of grain sizes, which, when com-
pacted, provide high strengths and adequately low permeabilities. Low dams, in particu-
lar, are constructed of these materials. Most dams, however, are zoned to use a range of
site materials and require an impervious core to control flow within the shell, which pro-
vides the embankment strength (Figure 8.47e).
Internal drainage control of the phreatic surface is provided in all dams. Because earth
dams are constructed by compacting borrowed soil materials in layers, there is always the
possibility for the embankment to contain relatively pervious horizontal zones permitting
lateral drainage, which must be prevented from intersecting the downstream face. Various
types of drains are used, including longitudinal drains (Figure 8.47b), chimney and blan-
ket drains (Figure 8.47c), and toe drains (Figure 8.47d). Details of two types of toe drains
are given in
Figure 8.48.
Rock-fill dams may be provided with an impervious core section, or the upstream face may
be covered with a membrane of concrete, asphaltic concrete, or welded steel (Figure 8.47f).
