generally be in this category, since the theoretical natural frequency for an
elastic long and uniform triangular embankment section is a function of
height, H, and shear velocity, Vs. For larger embankments it can be calcu-
lated that natural frequency will be of the order of fn1 Hz and above,
thus registering in the frequency band of most seismic energy release. This
has been demonstrated in field trials. (A concrete dam, by comparison,
will typically have fn10 Hz and will thus respond as a ‘rigid’ body.)
Certain ancillary structures, e.g. free-standing valve towers, gate
structures and critical items of operating equipment such as valves etc.,
may be particularly at risk, irrespective of type of dam.
The seismic events of primary interest in dam design are essentially
the outcome of tectonic and other natural ground movement, e.g. displace-
ment on a fault system etc. There are, however, a number of instances
where reservoir-induced seismic activity has been recorded, as at Koyna
(India) and Kariba (Zimbabwe/Zambia). Seismicity of this nature is gener-
ally associated with high dams (H100 m) and reservoirs of high capacity
(storage in excess of 100 106 m^3 ) located on tectonically sensitive sites.
Seismic categorization of a dam site can present difficulties, particu-
larly in parts of the world which do not have a significant history of seis-
micity. Many countries faced with this problem resort to zoning, e.g. the
USA has four zones defined by recorded levels of seismicity (see Table
2.7). The UK is similarly divided, into zones designated A, B and C, Zone
A being the most severe (Charles et al., 1991).
In categorizing a dam in the UK an empirical system of weighted
classification parameters embracing dam height, reservoir capacity, evacu-
ation requirements and damage potential is evaluated, the aggregated
value placing the site in one of four categories. Categories I to IV are in
turn linked by zone to suggested PGA values and notional SEE return
periods. For Category IV, the most severe situation and applicable to only
a very small number of UK dams, the SEE is defined as equivalent to
MCEorto an upper-bound return period of 3 104 years. The associated
PGA values range from 0.25 g (Zone C) up to 0.375 g (Zone A). At the
other end of the scale, seismic safety evaluation is not normally considered
necessary for Category I dams, but if deemed appropriate the SEE return
period is 1 103 years, with PGAs of 0.05 g (Zone C) and 0.10 g (Zone A)
(Charleset al., 1991).
SEISMIC LOADING
Seismic loads can be approximated in the first instance by using the sim-
plistic approach of pseudostatic, or seismic coefficient, analysis. Inertia
forces are calculated in terms of the acceleration maxima selected for
design and considered as equivalent to additional static loads. This
approach, sometimes referred to as the equivalent static load method, is
generally conservative. It is therefore now applied only to smaller and less