SMALL DAMS

F ill dams
72
If the latter is unaltered rock, after it has been cleaned and possibly its surface has
been smoothed, a first layer of wet (optimum moisture content (OMC) + 2 or 3) clay
a few decimetres thick is placed to guarantee good contact; it may be necessary to
set a filter between the downstream face of the trench and the permeable foundation
materials.
!" permeable foundation to a significant depth: grouting can be used both for a
loose foundation and for a more or less cracked rock mass, with the grout adapted to
the material being treated (bentonite-cement grout, specially designed grouts); the
cut-off will usually involve three lines of staggered drillholes; as grouting cannot be
effective at the surface, either the first few metres of grouting are relayed by the cutoff
trench, or treatment is started at a certain height in the fill. Another watertightening
technique is the diaphragm wall made of self-hardening bentonite cement grout or
plastic-concrete; this solution is quite conventional for loose ground but can also be
used in rock foundations with the more costly technique of the hydrodrill. It can cause
problems if major displacements occur, for example the fill may be punctured in the
event of severe settlement of a soft foundation (with the parallel risk of excessive
lateral friction); if it is located at the upstream base of the fill, the diaphragm may
suffer considerable shear at its top.
As concerns type 3 dams (with an artificial watertightening element), the connection
between the watertightening structure in the fill and watertightening in the foundation
is a difficult point, except in cases where a diaphragm wall provides all watertightening
from the dam crest (see under that heading hereafter, p. 82).
When the reservoir cannot be watertightened by a cut-off at the dam, the solution
consists in sealing the reservoir basin totally or partially with a geomembrane (see
that heading below, p. 81) or with a blanket of compacted clay materials (at least
two layers about 0.20 metre thick each), with the latter protected from any risk of
drying out. Such techniques always result in a high price per cubic metre of water
stored.
As concerns the support for these systems, it is necessary to:
!"meet filter conditions (see Filters and drains below, p.77) for an upstream blanket;
!" eliminate any rough areas that might puncture the geomembrane;
!" avoid any risk of uplift, in particular due to gases under the geomembrane.
FOUNDATION DRAINAGE
For drainage of flows from the foundation, the most satisfactory solution consists in
placing a drainage blanket at the base of the downstream shoulder, at the fillfoundation
contact, leading to the vertical or sloped drain in the central part of the fill
(see Fill drainage system below, p. 78).
This blanket, which may be compartmented in order to determine the behaviour of
each different zone, should be placed for any large structure (H 2 V > 700). For
smaller dams ( H 2 V < 700), where geological conditions permit, the drainage blanket
may be reduced by placement of draining strips (in particular in the areas judged to
be the most vulnerable in the river banks).