Environmental Engineering Reference
In-Depth Information
theoretically stable dam at all stages of an analysis, including, in the case of a step-by-step
numerical integration FEM, at the time-step for the maximum displacement of the dam.
However this may be impractical for high seismic load areas.
Calculated permanent displacements of, say, 100 mm and above seem at odds with
these large, stiff structures whose maximum displacement under elastic loading are prob-
ably only up to 10-15 mm. Such large permanent displacements imply the development of
considerable distress in the dam and much remedial work if it is to be retained.
Engineers should not be too carried away with any of these analyses, no matter how
complex. The words of Newmark and Rosenbleuth (1971) from some 30 years ago still
hold true:
“We must also face uncertainty on a large scale, for it is our task to design engineering
systems
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about whose pertinent properties we know little
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to resist future earthquakes
and tidal waves
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about whose characteristics we know even less.”
16.9.3.10
Post-earthquake analyses
As emphasised by FERC (2000) these analyses are vital to any gravity dam that could be
subjected to earthquake loading. They should be done carefully, with conservative
strength factors allowing for the deformations and cracking induced by the earthquake
and proper consideration of changed uplift pressures.
16.9.3.11
Dams on rock foundations with potentially dee-seated failure mechanisms
For most of the foregoing discussions, the tacit assumption has been that the dam-
foundation interface zone is the critical foundation component in the analyses. If there
are known defects and weaknesses, such as existing bedding surface shears, that could
give rise to a potential deep-seated failure, the analytical problem increases again in
complexity.
Without going into highly complex coupled foundation-dam analytical models, the likely
steps are:
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The dam-foundation mechanism presumably can be proven to safe under static loads.
Therefore calculate the dam's maximum response base moment and shear using the
methods just described, and apply them to the potential foundation failure mechanism. If
there is an adequate safety margin, with conservatively chosen strength parameters, the
dam-foundation interface can be accepted as the critical foundation zone.
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If movement does occur on the deep-seated failure mechanism, a yield acceleration
should be calculated, allowing for the dam's loading, and a Newmark analysis done to
estimate the total permanent displacement. This type of approach may be reasonable
as during sliding on the foundation mechanism, the impact of the earthquake will not
be transmitted through to the dam above (USBR 1989).
The difficulty posed by this particular problem clearly illustrates the need for very care-
ful geological exploration and assessment of the damsite. Any very stiff structure like a
gravity dam could be under serious threat if it were to be subjected to potential displace-
ments from ground movement.
16.9.3.12
Dams on foundations that could be subjected to ground displacement
None of the above methods can hope to cope with major ground dislocation. Clyde dam
in New Zealand does include a design feature to allow for some differential displacement,
but even that feature would not handle the displacement of 9 m vertically and 1.5 m lat-
erally that occurred at Shikang dam in Taiwan in 1999 (see
Figure 16.29
).
