SMALL DAMS PETITS BARRAGES

flowing water. Flowing water with high power (kW/m 2 ) can remove stones if too small. Where
there has been poor maintenance, total breakdown of a Gabion will take place if most of a
panel has corroded and “broken down” before repair (or rehabilitation to its original functional
condition)
Stones and the “upper panels” of Gabions may be removed by vandalism.
A cut-off structure connected to rock should be considered.
7.3 TECHNIQUES FOR IMPROVING EMBANKMENT SAFETY
7.3.1 Techniques to Overcome Flood Handling Problem
7.3.1.1 Parapet Wall
A parapet wall can be used to raise the crest of a consolidated embankment crest. The
concrete parapet must be designed to accommodate water loads including uplift but the safety
factors against overtopping and shear failure if exposed to the PHC 3 floods can be selected
close to 1 in this case. An impermeable membrane must be designed between the impervious
core of the embankment and the base (or underside) of the parapet wall. The footing normally
have to be imbedded into the embankment crest.
7.3.1.2 Raise Embankment Crest
Generally (but not always) the lowest cost option to overcome inadequate flood handling
problem is to increase the total freeboard of the dam by raising the crest of the embankment.
This is normally achieved by adding earthfill to the downstream face of the embankment
starting from the toe of the embankment (see Figure 7.2). In addition to the fact that the
spillway must be designed to accommodate higher “recommended design and safety
evaluation check floods”, respectively, the three (3) topics that are listed below must be
addressed in the context of the structural integrity of the embankment:
The stability of the downstream slope (and “in-situ” foundation) must be analyzed
taking into account that the existing embankment (or portions thereof) are structural zones
(or components) of the raised embankment. The engineering properties of the earthfill
materials of the existing embankment and the “new” earthfill section should be determined with
reasonable confidence (i.e. a representative (or appropriate) sampling and laboratory testing
program) and used in any analyses.
A “sand chimney drain” must be considered at the interface between the downstream
face of the existing embankment and the “new” earthfill section (or zone). This will draw down
the phreatic surface and intercept any possible seepage through the embankment.
A new toe drain possibly connected to the existing toe drain must be considered.
An example is shown in Figure 7.2.
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