Environmental Engineering Reference
In-Depth Information
Table 15.15.
Factors to be considered and methods for assisting in the estimation of the likelihood of
continuation and progression of a concentrated leak in CFRD.
Methods for assisting in the estimation
Initiator
Factors to consider
of likelihood of concentrated leak
(A)
Continuation of concentrated leakage
Perimetric joint opens
• As constructed particle size
Assess filters against no-erosion,
distributions and filtering
excessive erosion and continuing
characteristics of zones 2F, 2D,
erosion criteria (Foster and Fell 2001);
2E, 3A (depending on zoning,
Assess for internal instability (Kenney
allowing for segregation)
and Lau, 1985)
Vertical joint opens,
• As constructed particle size
As above
face slab cracks,
distributions and filtering
crest wall joint opens
characteristics of zones
2D, 2E, 3A.
Piping under plinth
• Presence of extent of filter(s)
As above
under rockfill downstream of
plinth. If present, particle size of
filter and erodible material in the
foundation
All joints, cracks
• Displacement/disruption of zone
Judged from estimate of overall
2D by seismic loading
settlement using Swaisgood (1998)
or other methods
Southcott et al. (2003) give the conditional probabilities used by Hydro Tasmania in
their risk assessment.
15.8.3
Assessment of the likelihood of continuation of a concentrated leak
Table 15.15 summarizes the factors to be considered, and methods for assisting in esti-
mation of the likelihood of continuation of a concentrated leak. The factors affecting con-
tinuation are relatively readily quantifiable using the methods outlined in Foster and Fell
(2001) and Foster et al. (2002).
In this assessment, the particle size distribution of Zones 2D versus 2E, 2E versus 3A,
and 2A versus 3A have to be assessed against the no erosion, excessive erosion and con-
tinuing erosion criteria in Foster and Fell (2001) and Foster et al. (2002). However, since
in many existing dams the Zones 2D and 2E will be internally unstable, assessed using the
criteria of Kenney and Lau (1985), it is necessary to consider first whether the zone is
internally unstable, and if so, what size particles will be washed from the zone through the
adjacent zone before the adjacent zone will eventually act as a filter.
In many older dams, this may show that quite extensive internal erosion may occur
before the filter action works, and the leakage rate may be quite high.
15.8.4
Assessment of the likelihood of progression to form a pipe
Table 15.16
lists the factors to be considered. In the case of CFRD it is unlikely a hole or
pipe will form as would happen in an earthfill core. Rather, what is likely to happen is a
continuing erosion of the finer fraction from the zones in the dam, including the rockfill.
The erosion of the finer material will cause settlement, a likely greater opening of the joint
or crack which initiated leakage, and so on unless there is intervention.
