SMALL DAMS PETITS BARRAGES

characteristic must be determined. This must be used as standard to compare the
characteristics of the placed and compacted materials.
When during compaction a layer dries out before the next layer is placed the layer must
be reworked and recompacted at the correct water content after watering and mixing. Badly
compacted layers can cause high seepage rates and possible piping failures.
5.5.5 Compaction in Confined Areas
In areas inaccessible to large compaction machines compaction by smaller machines in
thinner layers have to be performed. If necessary compaction by hand can be used but then at
2% of optimum water content to ensure proper wetting of all soil particles. Quality assurance
here is of most importance as many failures of earthfill dams occurred due to piping along say
the bottom outlet pipe caused by below standard compaction (Oosthuizen 1985).
Figure 5.6 shows a pipe with intact seepage collars, with the photograph taken after an
internal erosion failure of a small dam. The fact that anti-seep collars have not prevented many
failures implies that most of the crack features causing the failure occurred in the earthfill
outside the collars, probably as a consequence of the poor compaction of the soil between the
collars.
In some cases internal erosion occurred through hydraulic fractures in surrounding
embankment soils that were dispersive clays, but not along the conduit as is often presumed.
Fig. 5.6 – Pipe with intact anti-seepage collars shortly after failure of a small dam
(McCook/2004).
Seepage and piping along outlet pipes are a major cause of failure of small dams. As
pipes become larger in diameter it becomes more difficult to compact the soil properly, down
the sides of the pipe. A good precaution consists in placing draining or filtering granular
material on either side of the pipe over its downstream third, in order to block or drainage
strips in the fill.
5.5.6 Testing of The Pipe
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