SMALL DAMS PETITS BARRAGES

t
5.7.1.4 Practical Considerations
h
d
Blanket (k 1)
x xd
Pervious foundation (k )
Impervious foundation
Figure 5.9 – Artificial Blanket Design Method
Water can seep through horizontal layers especially where compaction standards are
not met. The provision of a chimney drain can intercept seepage. In case of small dams
expensive drains are sometimes omitted. In case of dispersive earthfill materials drains,
especially the chimney drain, must be inserted and extended into the core trench, because the
correctly designed chimney filter not preventing migration of small soil particles is then a
requirement. In case of differential settlement between embankment material zones or cracks
a chimney drain is a requirement. Sherard (1984) showed that based on laboratory tests filter
systems can heel concentrated leaks.
Collars of sand must be provided around bottom outlets on the downstream side to
prevent piping formation when seepage occurs through below standard compaction areas.
The thickness of a drain can be determined using Darcy’s law (Q = kiA) by determining
the area required. The hydraulic gradient can be taken as the relation of the height difference
between the beginning and end of the system and the length of the system. The following
aspects are also important:
� Construction constraints, e.g. the limitation of machines to place filter materials in thin
layers, cause the minimum thickness to be 25 cm.
� Safety factors regarding theoretical capacity and design capacity of drains of 10 to
about 100 are warranted as provision must be made for unknowns, decreases in
permeabilities due to over compaction, ageing of materials, cementation of sand in filters.
5.7.1.5 Filter Material Criteria
Based on the bi-lateral functions of filter and drain systems namely maximum retention
of soil material of the base and maximum through flow of water, limits of grain sizes for base
and filter material were established. In 1922 Terzaghi developed piping criteria which are still
applicable. Elges, HFWK (1986) describes natural filters. Sherard et al, 1985.
5.7.1.6 Piping Prevention Criteria
The criteria for the prevention of removal of particles and failure of the base material
including dispersive materials by a filter are shown in Table 5.9.
Table 5.9 – Criteria regarding piping
Criteria Nº Description
1 D15 filter / D85 basis = 5 or less
2 D15 filter / D50 basis < 25
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