SMALL DAMS

S mall concrete dams
Finally, during a flood on a ski-jump-shaped free overflow block, the water will exert
centrifugal force in the hollow of the ski jump and this beneficial force can be
considered.
Uplift in the foundation
Uplift is generally calculated in situation of design flood. When no drainage is to be
provided, the designer usually considers a trapezoidal diagram with full uplift (u m
) at
the upstream heel and uplift (u v
) equal to the water level at the downstream toe
(figure 2-a).
When there is no drainage, the uplift diagram may be less favourable than the
trapezoidal diagram (a) in figure 2 if cracking in the rock has any tendency to close
at the downstream toe. When the geological study gives reason to fear such a
phenomenon, drains must be drilled at the downstream toe.
If drainage is provided, and assuming that the drains are maintained regularly, it is
recommended to consider that drainage is 50% effective, which means uplift will be
cut in half at the drainage curtain:
u A
-u B
= (u A
-u C
)/2 (figure 2-b).
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Fig. 2 - Indicative diagram of uplift:
(a) - with no grouting or drainage
(b) - with a grout curtain
(c) - with drainage
Similarly, if a grout curtain is placed in the foundation near the upstream heel, and
provided that the upstream heel is not subject to tensile stress, it is considered that the
effect of this curtain is to decrease uplift just downstream from it by one-third (versus
the trapezoidal diagram with full uplift on the upstream side):
u A
-u B
= (u A
-u C
)/3 (figure 2-a)