equivalent static thrust, P 8 , is shown. Seismic load is considered
further within Section 2.7 in relation to embankment dams, and
Section 3.1 with respect to concrete dams.
- Tectonic effects.Saturation, or disturbance following deep excavation
in rock, may generate loading as a result of slow tectonic movements.
This lies beyond the scope of this text and is not considered further,
but reference may be made to Golzé (1977).
1.7.3 Concepts of loading
The concurrent and persistent presence of all primary loads acting on an
operational dam is self-evident. Some secondary loads, e.g. sediment load,
will develop gradually and persist in concert with the primary loads. Other
secondary loads will provide an infrequent and temporary additional load,
e.g. ice thrust. Of the exceptional loads, tectonic action can generate a
permanent addition to the overall loading on a dam. Seismic activity,
however, is essentially random with a low probability of occurrence, gen-
erating an extreme but transient dynamic loading.
Study of the nature of the various individual loads and loading pat-
terns leads to the logic of expressly defined load combinations being
employed in analysis. Each load combination is related to a particular
service condition and defines those loads considered to be concurrently
operative on the dam. This logic can be extended to effecting a considered
reduction in design safety margins to parallel the diminishing probability
of occurrence of more rigorous load combinations.
The important elements of embankment dam and concrete dam
design are introduced in Chapters 2 and 3 respectively. In anticipation of
the analytical methods presented there a historical conceptual difference
in the assumed mode of action of the primary loads should be identified.
In embankment dam analyses water, seepage and self-weight loads are
considered, correctly, to operate through distributed internal or body pres-
sures and forces, as shown schematically on Fig. 1.9. The classical stability
analysis of concrete dams, however, was developed on the basis of
38 ELEMENTS OF DAM ENGINEERING
Fig. 1.9 Schematic of internal body forces: embankment dam
