Environmental Engineering Reference
In-Depth Information
e.g. displacements of CFRD face slab, and new construction materials, e.g. asphaltic
concrete core, geotextiles.
When setting up a surveillance framework it is vital to ensure that:
- The inspection and monitoring program is planned by qualified dam engineers and
engineering geologists, taking account of the potential failure modes for the dam. A
detailed failure modes analysis should be a required part of establishing and reviewing
an inspection and monitoring program;
- The monitoring data is reviewed by qualified dam engineers and engineering geologists
in an ordered manner, so that unusual behaviour can be identified and appropriate
action taken;
- The responsibilities of the owner, operator and government authority are clearly
defined, with lines of communication established.
20.2.2
Is it really necessary?
That it is necessary to have a monitoring and surveillance system established, is high-
lighted by the number of dams which experience accidents and failure, often after many
years of operation. This is discussed in some detail in ICOLD (1983a), National Research
Council (1983), Foster et al. (1998, 2000a), and Douglas et al. (1998, 1999).
ICOLD (1983a) studied the approximately 14,700 dams which qualified for the
ICOLD register at that time. An extensive survey indicated that of these dams 1105
(7.5%) had suffered incidents and deteriorations of one or more type and 107 (0.7%) had
failed. In some cases the dam had to be completely abandoned and in others the dam was
repaired and brought back into use despite severe damage.
Figure 20.1
summarises the failure of dams and causes of failure of dams over 15 m
high built from 1900-1975.
Tables 8.1, 8.2 and
20.1
summarise the statistics of failures and accidents for embank-
ment dams.
It can be seen that for embankment dams:
(a) Overtopping and internal erosion and piping are the main causes of failure;
(b) Slope instability is a relatively minor cause of failure but is a significant cause of accidents;
(c) About two-thirds of piping failures and about half of piping accidents occur on first
filling or in the first 5 years of operation. However accidents and failures do still occur
in older dams;
(d) About two-thirds of slope instability failures and half of instability accidents occur
after 5 years of operation;
(e) The ratio of accidents to failures is quite different depending on the failure mode. In
particular, slope instability and piping from embankment to foundation are much more
likely than piping within the embankment to result in accidents compared to failures;
(f) The percentage of dams suffering accidents is not improving with time, but the per-
centage of failures is decreasing, reflecting improved technology of design, construc-
tion, monitoring and surveillance;
(g) More than 79% of all embankment dams are less than 30 m high. These suffer pro-
portionally more failures than higher dams, but fewer accidents. This may reflect bet-
ter design and monitoring and surveillance of the larger dams.
Douglas et al. (1998, 1999) showed that for concrete dams:
(a) Sliding, leakage and piping in the foundation are the main causes of failure. Accidents
are most likely to be high recorded uplift or leakage in the foundation;