SMALL DAMS PETITS BARRAGES

around soil particles, through fractured zones or cracks etc. Hydraulic gradient refers to nondimensional
unit change in hydraulic potential between two points in the flow medium (water).
The basic practice in seepage control design is to provide a number of seepage control
measures to ensure maximum water storage capability of a dam. Defensive design is
necessary to accommodate a series of unknown factors which can be as follows:
� Unknown characteristics of geological and foundation materials due to limited
available data.
� Degradation or ageing of seepage control measures or materials.
� Change in the operations method of a dam.
� Non conforming quality control during construction e.g. below standard compaction.
Dispersive clay materials susceptible to piping can be used in the central part of the
embankment as core to provide an impervious medium and be protected with semi-pervious
materials not susceptible to piping in the shells.
5.7.1.1 Measures for Pervious Ground
In a case where it is impossible for engineering reasons or uneconomical to insert an
impervious zone to seepage resistant ground, the quantity of seepage must be maintained in
the allowed range and seepage water must be safely discharged outside the dam with
reference to Table 5.8 considering on-site conditions.
The blanket method is an example of this Seepage Control Measures.
Thickness of
pervious layer
Thin
Table 5.8 – Treatment of Pervious Ground (Japanese Criteria)
Design
method
Impervious
zone
Medium Sheet pile
Sheet pile
Sketch Applications
Impervious zone
Pervious layer
Sheet pile
Impervious zone
Pervious layer
Provides complete
impervious effects. But it is
used when the pervious
layer thickness is within 1/3
of the present dam height
above the ground.
Provides incomplete
impervious effects. It is not
suitable for a layer
containing pebbles. It is
effective in fine sand and
silt layers.
66