SMALL DAMS

Chapter IV
METHODS USING THE DYNAMIC BEHAVIOUR OF SOILS
The pseudo-static method is not satisfactory on the theoretical level and does not give
a correct understanding of problems when seismic activity is high.
The most suitable method in this case is that developed by H.-B. Seed, which consists
in doing a study of the dynamic behaviour of the dam and its foundation during an
earthquake. The dam is then considered stable when strains are limited and compatible
with the properties of the materials used.
This study can be necessary if > 0.2 to 0.25 and for most important dams.
SPECIAL CONSTRUCTION TECHNIQUES FOR THE FOUNDATION AND THE FILL
The foundation can cause problems in the event of a strong earthquake when it
contains materials that are likely to present a significant decrease in strength in relation
with strong development of pore pressures (phenomenon of liquefaction of
saturated loose sands) or high distortion (soft clays). As a general rule, these materials
also pose problems in "static" design of the dam and may have been replaced or
treated within the dam area. However, in this case it is still necessary to check the
influence of whatever material has been left in place or has not been treated beyond
the dam's upstream and downstream toes. In the case of sandy layers, classic treatment
consists in building a mesh of vertical drains (to drain overpressures generated by the
earthquake) or in improving the ground by densifying it (vibratory compaction,
weighted columns, solid grouts, etc.).
For the fill, modifications may be necessary to deal with any internal problems and
strains that may occur. Generally, a probable consequence of a strong earthquake is
the appearance of concentrated leakage through the dam. Such modifications concern
zoning of the fill, resistance of the materials to retrogressive erosion, and design of a
crest. The following precautions are worth mentioning:
!" avoid fine, cohesionless and uniform soils in saturated zones;
!"provide a chimney drain or enlarge it in the case of homogeneous fills;
!" design transition zones that are as wide as possible;
!" be especially prudent with the thickness of filters (place a layer of sand upstream
from the core to plug any cracks that may be caused by an earthquake);
!"build the dam or its core with materials offering good resistance to internal erosion
(plastic clays, sandy gravel with clay fines of a very continuous grading).
In addition, if the design earthquake is very strong, it may be wise to increase freeboard
and crest width.
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