SMALL DAMS PETITS BARRAGES

7.3.4.1 Seepage Barriers and Cutoff Systems
This section provides a brief overview of commonly used seepage control systems.
Seepage cutoffs and barriers are intended to reduce the amount of seepage and
minimize downstream pressures and exit gradients. Depending on the site and foundation
conditions, seepage cutoff systems may provide either:
Positive cutoff that completely penetrates pervious zones and ties into impervious soil
layers or bedrock in the dam foundation and abutments, or
− Partial cutoff, which is sometimes used for deep pervious foundations to lengthen the
seepage path and reduce exit gradients to acceptable levels.
There are many different types of cutoff systems that are used for dam rehabilitation.
The primary objective is to provide a low permeability element within the dam and/or its
foundation, either as the primary seepage barrier or to supplement existing barriers. Cutoffs
may be classified according to their stiffness, the type of materials used, or construction
methods used.
The terms cutoff “wall” or “diaphragm” are often used to distinguish seepage barriers that
are thin compared to the surrounding embankment. Barriers employed for dam rehabilitation
may be constructed through the dam and foundation from the existing dam crest (diaphragm
or wall).
Alternatively barriers may be constructed using a liner system on the upstream face of
the dam in combination with vertical or horizontal seepage barriers extending from the
upstream toe.
Some of the seepage barriers that are commonly used for dam rehabilitation are listed in
Table 7.1 .Most of those solutions require lowering of the reservoir level significantly, in order
to reduce hydraulic gradients during rehabilitation and the risk of wash-out of cement and
bentonite. In case of construction of rehabilitation works on the upstream face of the dam, the
reservoir must be emptied.
Table 7.1 – Common Seepage cut-off measures identified for Alluvium and Earthfill
Foundations of small dams
Type of cutoff Method
Open trench is
Materials used
Remarks
References
and
Core trench:
excavated and, if
stable, filling with one of
the materials is done
Earthfill
Soil-bentonite
Concrete-bentonite
Depth limited to some
metres
Slurry trench: Slurry
wall, concrete wall,
tremmy
Open trench by
extended backhoe, kept
open and stabilized
with bentonite slurry
Concrete by tremmy
pipe. Primary and
secondary panels are
inserted once sufficient
strength has been
identified
Figure 7.4 and
Figure 7.9 and 7.10
Sheet piles curtain Inserted by vibratory Steel Curtain not taken to
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