Environmental Engineering Reference
In-Depth Information
Table 20.2.
A method for the approximate estimation of the time for progression of piping and
development of a breach, for breach by gross enlargement, and slope instability linked to
development of a pipe (Fell et al., 2001, 2003).
Factors influencing the time for
progression and breach
Ability to
support a
Rate of
Upstream
Breach
Approximate likely
Approximate likely
roof (1)
erosion (2)
flow limited (3)
time (4)
time-qualitative
time-quantitative
Yes
R or VR
No
VR or R-VR
Very Rapid
3 hours
Yes
R
No
R
Very Rapid to Rapid
3-12 hours
Yes
R-M
No
VR
Yes
R
No
R-M
Rapid
12-24 hours
Yes
R-M, or M
No
R
Yes
R
Yes
R or VR
Yes
R
No
M or S
Rapid to Medium
1-2 days
Yes
R-M, or M
No
M or M-S
Yes
R or R-M
Yes
R or R-M
Yes
M or R-M
No
S
Medium
2-7 days
Yes
R-M or M
Yes
S
Yes
M
Yes or No
S
Slow
Weeks - even months
or years
Notes: Estimated using (1)
Table 20.3
(2) Using
Table 20.4
(3) Using
Table 20.5
(4) Using
Table 20.6
.
VR
Very Rapid; R
Rapid; M
Medium; S
Slow.
20.2.4
Time for development of piping failure of embankment dams and ease of
detection
Fell et al. (2001, 2003) describe an approximate method for estimating the time from first
concentrated leak to initial breach of the dam. The times are for situations where failure
occurs. Dams with properly designed and constructed filters, or even filters providing only
some degree of protection, will usually not fail or the piping will develop more slowly,
increasing the likelihood of intervention.
The method is summarized in Tables 20.2 to 20.6.
Table 20.2 is applicable to cases where the breach mechanism is gross enlargement. It is
considered to be reasonably applicable to cases where the final breach is by slope instability,
following development of a pipe. The factors in Table 20.3 should be also taken into consid-
eration. For cases where the primary breach mechanism is by sinkhole leading to crest settle-
ment, the rate of erosion should be assessed from Table 20.4 with consideration of the other
factors in Table 20.5. Breaching by crest settlement is usually slow to medium, although
unless addressed, it can lead to overtopping with a resultant rapid breach. For unravelling or
sloughing, the rate of erosion should be assessed from Table 20.4, and consideration given to
the other factors in Table 20.5. These processes are expected to be slow to medium, but may
be rapid if the erosion rate is rapid.
The times are from the first sign of a concentrated leak to first breach of the dam crest.
They therefore cover the progression phase of the internal erosion and piping process.
These methods for assessing the rate of development of internal erosion and piping are
very approximate and should be only used as a general guide to performance. Where the
consequences of failure of the dam are large, a cautious approach should be adopted in
making and using these estimates.
It should be recognized that the time for progression and development of a breach does
not include the time for the dam to be emptied of water, i.e. it does not include the time for
