The risk of such cracking modes arising can be moderated by careful
detail design, e.g. avoidance of major irregularities in foundation excava-
tions, use of wetter and more plastic core materials, and careful zoning,
with wide transitions adjacent to the core.
2.7.3 Seismicity and seismic load effectsSEISMICITY
Dynamic loads generated by seismic disturbances must be considered in
the design of all major dams situated in recognized seismic ‘high-risk’
regions. The possibility of seismic activity should also be considered for
dams located outside those regions, particularly where sited in close proxi-
mity to potentially active geological fault complexes.
The sites for major dams are normally subjected to rigorous seismo-
logical appraisal. Seismicity is assessed through a specialist review of
regional and local geology in conjunction with historical evidence. Where
a risk of seismic activity is confirmed, estimates of probable maximum
intensity provide the basis for selecting seismic design parameters. In the
case of smaller concrete dams, or dams on ‘low-risk’ sites, it is generally
sufficient to specify a nominal level of disturbance for design purposes.
Seismic risk to UK dams is reviewed in Charles et al.(1991) and ICE
(1998).
Seismic activity is associated with complex oscillating patterns of
accelerations and ground motions, which generate transient dynamic loads
due to the inertia of the dam and the retained body of water. Horizontal
and vertical accelerations are not equal, the former being of greater
intensity. For design purposes both should be considered operative in the
sense least favourable to stability of the dam. Horizontal accelerations are
therefore assumed to operate normal to the axis of the dam. Under reser-
voir full conditions the most adverse seismic loading will then occur when
a ground shock is associated with:
- horizontal foundation acceleration operating upstream, and
- vertical foundation acceleration operating downwards.
As a result of 1, inertia effects will generate an additional hydrodynamic
water load acting downstream, plus a further inertia load attributable to
the mass of the dam and also acting in a downstream sense. Foundation
acceleration downwards, 2 above, will effectively reduce the mass of the
structure. The more important recurring seismic shock waves have a
frequency in the range 1–10 Hz. Seismic loads consequently oscillate very
rapidly and are transient in their effect.