Environmental Engineering Reference
In-Depth Information
Table 15.16.
Factors to be considered and methods for assisting in the estimation of the likelihood of
progression of piping in CFRD (Southcott et al., 2003).
Initiator
Factors to consider
Methods
All initiators
• Zoning and particle size distribution of
Assess for internal instability using
each zone, and potential for internal
Kenney and Lau (1985)
instability in zones 2D, 2E, 3A, 3B
leading to washing out of fines
• Flow rate and location of the
Judged from the initiating mechanism,
initial concentrated leak
precedent in other CFRD
• Potential for downstream face protection
Assess as a filter against the eroding
to intercept eroding material, leading
fines for no-erosion, excessive erosion
to slowing down of internal erosion
and continuing erosion criteria (Foster and
Fell, 2001)
•Presence of zone 1A and 1B upstream
As above
of the plinth, and whether it will act to
seal on zones 2F, 2D
• Ability to intervene to lower the
Account for location and capacity
reservoir level, or seal the opening
of outlet works, reservoir volume, river
in the face slab or joints
inflows; accessibility of site, availability
of barges, personnel, whether the joint
or opening is covered by debris,
zone 1A etc
• Potential for erosion, and formation
Judged, accounting for the initiating
of a pipe in alluvium left in the
mechanism, downstream toe details,
dam foundation
whether the alluvium “day lights”
downstream of the toe, whether
flow will be in alluvium
or in damfill
15.8.5
Assessment of the likelihood of a breach forming
Table 15.17 lists the factors to be considered and the methods which can be used to calculate
whether for example, unravelling will occur. The ability to intervene to stop the process by
drawing down the reservoir, or reducing the leakage by clogging it with silty-sandy-gravel
(for example) can be assessed depending on the particular circumstances of the dam. The
erosion process is not likely to be so rapid as for earth and earth and rockfill dams, but
may still progress in days or weeks, so detection and intervention need to be done soon
after the significant leak develops.
15.8.6
Concluding remarks
It will be apparent that what is described above is only a framework, and quantification
will be difficult and subject to considerable uncertainty. It may be sufficient in many cases
only to use qualitative terms, as was done by Bell et al. (2002) for the assessment of the
likelihood of piping in Eucumbene earth and rockfill dam, the process of formally con-
sidering all failure initiators and failure paths is a valuable one, even if the estimates of
likelihood are approximate.
These should only be considered very approximate as they are based on engineering
judgement and a “mapping scheme” linking probability in qualitative and quantitative
terms, with little data to aid in quantification.
 
 
Search WWH ::




Custom Search