Environmental Engineering Reference
In-Depth Information
Table 10.17.
Influence of factors on the likelihood of “blow out” (Foster, 1999; Foster and Fell, 1999b).
Influence on likelihood
Factor
More likely
Neutral
Less likely
Foundation conditions
Low permeability
High or low permeability
at the downstream toe
layer overlying high
layer only
permeability layer
Observed behaviour
Sand boils at downstream toe
No sand boils
“Quick sand” conditions
Factor of safety for zero
F UT
1.2
F UT
1.5
F UT
2.0
effective stress condition
F UT vo /u
(c) “Blow-out” (or “heave”, “liquefaction”). The likelihood of blow-out can be assessed
by calculation as described in Section 10.3.3, Table 10.17 summarizes this and the
other factors which influence the likelihood of blowout.
10.7.2.2 Continuation of erosion
A necessary requirement for piping in the foundation to occur is the presence of an unfil-
tered exit point for the seepage, which allows continuing removal of eroded materials
(Von Thun, 1996). Figure 10.37 showed filtered and infiltered exit points.
Good dam design sets out to ensure that all seepage in the foundation is collected in fil-
tered exit points such as:
- Horizontal filter drains;
-Toe drain;
- Relief wells.
Natural filtering can also happen due to stratification in the alluvium.
The approach should be to characterise the foundation geology, particle size distribu-
tions and other factors, and use the no erosion, excessive erosion and continuing erosion
filter criteria (Section 9.3.2) to assess the likelihood of continuation of erosion.
10.7.2.3 Progression of erosion
The statistics of dam incidents, described in Foster et al. (1998, 2000a), suggests that
piping initiating in the foundation is less likely to progress to failure than piping
through the embankment. The progression of piping in the foundation is related to the
ability of the foundation soils to support a roof and to the factors influencing pipe
enlargement.
The formation of an open pipe through the foundation would be expected to be largely
influenced by the foundation soil types, soil stratigraphy and by the presence of geologi-
cal features such as cemented layers and infilled scour channels in cohesive soils.
Homogeneous cohesionless materials are unlikely to maintain an unsupported roof and
are therefore unlikely to be susceptible to piping unless they are overlain by an artificial
roof, such as the base of a concrete spillway structure, or a cohesive material. (Terzaghi
and Peck, 1967) (Sherard et al., 1972a). Well graded sandy gravels may be able to support
a roof by arching action between the larger gravel particles.
Given that cohesive embankment materials can support open pipes, it is feasible that
the base of the embankment dam could form a roof if piping developed along the
 
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