Environmental Engineering Reference
In-Depth Information
8
Embankment dams, their zoning and selection
8.1
HISTORIC PERFORMANCE OF EMBANKMENT DAMS AND THE
LESSONS TO BE LEARNED
Much of the history of development of embankment dams can be related to failures and
accidents.
ICOLD (1974) define failures accidents and incidents as follows:
Accident
: Three categories of accidents are listed by ICOLD:
Accident Type 1 (A1): An accident to a dam which has been in use for some time but
which has been prevented from becoming a failure by immediate remedial measures,
including possibly drawing down the water.
Accident Type 2 (A2): An accident to a dam which has been observed during the initial
filling of the reservoir and which has been prevented from becoming a failure by immedi-
ate remedial measures, including possibly drawing down the water.
Accident Type 3 (A3): An accident to a dam during construction, i.e. settlement of foun-
dations, slumping of side slopes etc., which have been noted before any water was
impounded and where the essential remedial measures have been carried out, and the
reservoir safety filled thereafter.
Failure
: Collapse or movement of part of a dam or its foundation, so that the dam can-
not retain water. In general, a failure results in the release of large quantities of water,
imposing risks on the people or property downstream (ICOLD, 1995). Incidents to dams
during construction are only considered as failures when large amounts of water were
involuntarily released downstream.
Two categories of failures are listed by ICOLD (1974):
Failure Type 1 (F1): A major failure involving the complete abandonment of the dam.
Failure Type 2 (F2): A failure which at the time may have been severe, but yet has per-
mitted the extent of damage to be successfully repaired, and the dam brought into use again.
Incident
: Either a failure or accident, requiring major repair.
ICOLD (1974, 1995), carried out extensive surveys on dam incidents, and from this
developed some overall statistics.
Foster, Fell and Spannagle (1998, 2000a, 2000b), used the ICOLD data, and informa-
tion on the dams which had experienced incidents to develop historic performance data
for dams constructed up to 1986.
They showed that about 1 in 25 large embankment dams constructed before 1950, and
about 1 in 200 of those constructed after 1950 failed.
Table 8.1
summarises the statistics
of failure of embankment dams during operation (i.e. excluding failures during construc-
tion, showing that after overtopping, internal erosion and piping is the most important
mode. Slope instability accounts for only 5.5% of failures and only 1.6% for dams built
after 1950. Their data shows that for dams in Australia, USA, Canada and New Zealand
designed and constructed after 1930, about 90% of failures were related to internal ero-
sion and piping.
If 2 failures of hydraulic fill dams, and two of unknown zoning are excluded, only 1 in
5000 dams have failed by instability. However 1 in 200 have experienced an instability
accident. This reflects the fact that it is possible to reliably assess the factor of safety of a
