Environmental Engineering Reference
In-Depth Information
Table 10.14.
Influence of factors on the likelihood of breaching (Foster and Fell, 2000).
Influence on likelihood of breaching
Factor
More likely
Neutral
Less likely
(a) Gross enlargement
Zoning
Homogeneous type
Zoned type dam,
Zoned type dam with a
zoning
downstream zone of
downstream zone of
sand or gravel with
gravel or rockfill
fines
Zoned type dam with
a downstream zone
able to support a roof
Storage volume
Large storage volume
Small storage volume
(b) Crest Settlement/Sinkhole
Freeboard at time of
2m freeboard
3m freeboard
4m freeboard
incident
(1)
Crest width
Narrow crest
Average crest width
Wide crest
Downstream zone
(2)
Fine grained, erodible
Fine grained,
Rockfill
non-erodible
gravel
(c) Unravelling or sloughing
Downstream zone
Silty sand (SM)
Fine grained rockfill.
Coarse grained rockfill
Silty gravel (GM)
Gravel with some
Cohesive soil (CL, CH)
fines
Flow-through capacity
Qc
estimated
Qc
Qp
Qc
Qp
(Qc) of downstream zone
flow-through due to
piping (Qp)
(d) Slope Instability
(d1) Initiation of slide
Existing stability
Analysis and/or
Analysis and evidence
evidence indicates
indicates significant
existing marginal stability
margin of safety against
instability
Downstream zone
Cohesive soils (CL, CH)
Clayey/silty sands
Free draining rockfill
(SC, SM)
or gravel
(d2) Loss of freeboard
and overtopping
Freeboard at time of
2m freeboard
3m freeboard
4m freeboard
incident
Crest Width
Narrow crest
Average crest width
Wide crest
(d3) Breaching given
Fine grained, erodible
Fine grained,
Rockfill
overtopping occurs
(2)
non-erodible
Downstream zone
Gravel
Notes:
(1)
Much more likely if
5 m.
(2)
Minor influence.
1 m, very unlikely
By contrast, in the piping failure of Teton dam (Independent Panel, 1976) and the major
piping incident at Fontenelle Dam (Bellport, 1967), relatively small initial leakages of
300-400 litres/sec eroded the downstream zones of sandy gravel. Particles size distribu-
tions for these are shown in
Figure 10.47
.
The relatively fine grading, and low permeability of these zones would have been quite
critical to the behaviour of the dams.
