SMALL DAMS

Chapter IV
MAN-MADE WATERTIGHTENING SYSTEMS
Geomembranes 1 (see photo 10 p. IV)
Geomembranes are thin, flexible, continuous watertight products at least one millimetre
thick. The products on the market vary widely, with the main categories being:
!" bituminous geomembranes (blown bitumen or bitumen modified by addition of
polymers);
!" plastomer geomembranes such as PVC (polyvinyl chloride), HDPE (high-density
polyethylene), etc.
! " elastomer geomembranes such as butyl, EPDM (ethylene propylene diene
monomer), etc.
Good design and careful installation can guarantee satisfactory integrity of such
products over time; the first practical applications of them date from the end of the
1960s.
Geomembranes are manufactured in a factory and delivered in rolls measuring
several metres in width or in shop-assembled panels measuring 200 to 1000 m 2 . The
rolls or panels are connected together on site by welding or gluing. This must be done
with great care and in compliance with certain rules depending on the products (e.g.
temperature limits). On a dam face, horizontal joints must be prohibited because of
the loss of strength versus the tensile stresses that could develop.
As concerns support for the geomembrane, two elements must be considered:
!" risk of puncture or tearing due to an excessively aggressive support, either during
placement or later under the weight of the water; in addition to efforts to improve the
surface of the granular backing, a protective geotextile is often added, whether it is
separate from the geomembrane or incorporated into it in the factory;
!" risk of uplift when the reservoir is emptied due to leakage, which is always possible,
and insufficiently permeable fill material; a continuous layer of good draining
material under the geomembrane and outlets at the base of the dam must be provided
to discharge water at the dam's downstream base.
For reservoir basins sealed with geomembranes, a system to discharge the gases that
are likely to form under the geomembrane must also be installed.
Surface protection from ultraviolet radiation and from traffic on a geomembrane,
wave action, floating debris and vandalism can be provided by rip-rap or concrete
slabs. A transition is required to protect the geomembrane, and geotextiles fulfil this
function very well.
It is important to verify stability against sliding at the various interfaces and avoid any
tensile stress on the geomembrane.
It is not unfeasible to provide no surface protection for geomembranes at small dams,
but this implies at least minimum surveillance; repairs are easy to make, but
geomembranes will be more vulnerable, they will age more rapidly, and any efforts
to pull a person or an animal accidentally fallen into the reservoir out again can be
very difficult.
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1. See Bibliography, references 5 and 6, p. 111.