Environmental Engineering Reference
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(1974), Seed (1979b, 1983) and Seed et al. (1978, 1985b). These studies show that based
on the data available at that time:
- Even at short distances from the epicentres, there have been no complete failures of
embankments built of clay soils, but several dams have come close to failure;
-Well constructed dams of clay soils on clay or rock foundation, not susceptible to strain
weakening, can withstand extremely strong shaking resulting from earthquakes of up
to Magnitude 8.25 with peak ground accelerations ranging from 0.35 g to 0.8 g; (They
suffer cracking but few, if any, have failed catastrophically.)
- Dams which have suffered complete failure as a result of earthquake shaking have been
constructed primarily with saturated sandy materials or on saturated sand foundations.
Liquefaction is a major contributory factor in these failures.The dams most susceptible
to failure under earthquake loading are hydraulic fill dams and tailings dams constructed
using upstream methods, because they are susceptible to liquefaction if the fill or tailings
are granular and saturated;
- In dams constructed of saturated cohesionless soils the primary cause of damage or fail-
ure is the build up of pore water pressure (liquefaction) under the earthquake loading;
- There are very few cases of dam failures during the earthquake shaking. Most of the
failures occur from a few minutes up to twenty-four hours after the earthquake.
However, cracking and displacements do occur during the earthquake.
Based on the above and similar observations, the US Bureau of Reclamation (1989)
classifies embankment dams into two main categories: (1) not susceptible to liquefaction
and (2) susceptible to liquefaction. For the purposes of seismic stability assessment they
also identify two types of analyses: deformation analysis and post earthquake liquefaction
analysis. They recommend that deformation analysis be performed on dams not suscepti-
ble to liquefaction and post earthquake liquefaction analysis be performed on dams sus-
ceptible to liquefaction. The authors recommend that a similar approach be adopted but
add the proviso that, where significant strain weaking of non liquefiable soils such as
overconsolidated high clay content soils may occur due to displacements induced by the
earthquake, post earthquake stability should be analysed taking account of the strain
weakening. The authors also emphasise that the post earthquake piping risk must not be
ignored as it may be a critical condition.
12.6.2
Pseudo-static analysis
Up until the 1970s, the pseudo-static analysis was the standard method of stability assess-
ment for embankment dams under earthquake loading. The approach involved a conven-
tional limit equilibrium stability analysis, incorporating a horizontal inertia force to
represent the effects of earthquake loading. The inertia force was often expressed as a prod-
uct of a seismic coefficient “k” and the weight of the sliding mass W. The larger the inertia
force, the smaller the safety factor under the seismic conditions. In this approach a factor of
safety (FOS) of
1 represents seismically safe conditions.
The seismic coefficients used in this approach were typically less than 0.2 and were
related to the relative seismic activity in the areas to which they apply.
The US Corps of Engineers (1984b) used the basic pseudo-static method for dams not
susceptible to liquefaction. They recommended use of a seismic coefficient equal to one-
half of peak ground acceleration and the use of undrained conditions for cohesive soils
and drained conditions for free draining granular materials, with a 20 percent strength
reduction to allow for strain weakening during the earthquake loading. They required a
factor of safety greater than 1.0. If a dam failed to satisfy this, they recommended more
accurate and detailed analyses. Their approach has been calibrated against a large num-
ber of deformation analyses and they state that up to 1 m of deformation may occur.
1 implies failure, whereas FOS
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