geological structures it is possible to make a probabilistic assessment of
seismic risk in terms of specific intensities of seismic event. In the absence of
reliable historical information it will be necessary to monitor microseismic
activity as a basis for the probabilistic prediction of major seismic events.
Either process is imprecise and will at best provide only an estimate of the
order of seismic risk. As a measure of reassurance over seismicity it has been
suggested that most well-engineered dams on a competent foundation can
accept a moderate seismic event, with peak accelerations in excess of 0.2g,
without fatal damage. Dams constructed with or on low-density saturated
cohesionless soils, i.e. silts or sands, are, however, at some risk of failure in
the event of seismic disturbance due to porewater pressure buildup and liq-
uefaction with consequent loss of stability.
Seismicity is discussed further in Sections 2.7 and 3.1, with a brief
introduction to the application of pseudo-static seismic analysis.
1.6.4 Foundation investigations
Foundation competence of the dam site must be assessed in terms of stability,
load-carrying capacity, compressibility (soils) or deformability (rocks), and
effective mass permeability. The investigative techniques to be adopted will
depend upon the geomorphology and geology of the specific site.
(a) Dams on competent stiff clays and weathered rocks
Serious underseepage is unlikely to be a problem in extensive and uniform
deposits of competent clay. It is important, however, to identify and con-
sider the influence of interbedded thin and more permeable horizons
which may be present, e.g. silt lenses, fine laminations, etc. Considerable
care is required in the examination of recovered samples to detect all such
features. The determination of appropriate shear strength parameters for
evaluating foundation stability is of major importance.
For a foundation on rock positive identification of the weathered
rock profile may prove difficult. In situdetermination of shear strength
parameters may also be necessary, using plate loading tests in trial pits or
adits, or dilatometer or pressuremeter testing conducted within boreholes.
The latter techniques are particularly suitable in softer rocks containing
very fine and closely spaced fissures.
(b) Dams on soft cohesive foundations
The presence of superficial soft and compressible clay deposits normally
ensures that seepage is not a major consideration. The nature of such for-
mations also ensures that investigations are, in principle, relatively
straightforward.
28 ELEMENTS OF DAM ENGINEERING
