Design of embankment dams to withstand earthquakes - Geotechnical Engineering of Dams - page 492

Table 12.2. Example of assessing the probability of failure by earth-

quake.

Conditional (1)

B (2) Acceleration Annual probability probability (P BC )

0.075 g 0.874 0.0005 0.0004

0.075 g to 0.125 g 0.100 0.005 0.0005

0.125 g to 0.175 g 0.015 0.05 0.0007

0.175 g to 0.225 g 0.007 0.1 0.0007

0.225 g to 0.3 g 0.003 0.3 0.0009

0.3 g 0.001 0.5 0.0005

Total 1.000 0.0037

(1) Given the earthquake occurs.
(2) P B

AEP P BC .

National codes, or guidelines to practice, will often specify which method should be used.
The authors favour the probabilistic approach because it recognizes there is a chance a
dam will fail at loads less than the MDE from the MCE.
Whichever approach is taken, the bedrock ground motions need to be adjusted where
appropriate for amplification (or de-amplification) effects.

12.3.5 The need to get good advice

Seismic hazard should be assessed by a seismologist familiar with the seismic hazard in the
project area, and experienced in assessing seismic hazard for dams.

12.4 LIQUEFACTION OF DAM EMBANKMENTS AND FOUNDATIONS

12.4.1 Definitions and the mechanics of liquefaction

12.4.1.1 Definitions
The following definitions are based on USNRC (1985), Robertson and Fear (1995),
Robertson and Wride (1997), Lade and Yamamuro (1997) and Fell et al. (2000).
“ Liquefaction ”. All phenomena giving rise to a reduction in shearing resistance and
stiffness, and development of large strains as a result of increase in pore pressure under
cyclic or monotonic (static) loading of contractive soils.
“ Initial liquefaction ” is the condition when effective stress is momentarily zero during
cyclic loading.
“ Flow liquefaction ” is the condition where there is a strain weakening response in
undrained loading and the in-situ shear stresses are greater than the steady state
undrained shear strength.
“ Temporary liquefaction ” is the condition where there is a limited strain weakening
response in undrained loading; at larger strain the behaviour is strain hardening. It is impor-
tant to recognize that these phenomena apply to monotonic (static) as well as cyclic loading
and are apparent in contractive soils, both cohesionless and those with some clay content.
Robertson and Fear (1995) and Robertson and Wride (1997) define two further terms
which apply to cyclic loading.
“ Cyclic liquefaction ” is a form of temporary liquefaction, where the cyclic loading
causes shear stress reversal and an initial liquefaction (zero effective stress) condition
develops temporarily.