Geoscience Reference
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Fig. 11.11. Typical resultsofparametric numerical analyses. (a) Time history of excess
pore pressureratio, and (b) timehistory of liquefaction induced settlements
Liu and Dobry, 1997; Adalier et al., 2003; Coehlo et al., 2004, 2005; Fujiwara et al.,
2005) and field case studies (Cetin et al., 2002; Seed et al., 2003; Bird et al., 2006) and
will prove essential for the analysis of the liquefaction performance of foundations. For
instance, observe that:
(a) Theexcessporepressureratiointhefreefieldapproaches1.0,aclearindicationof
liquefaction, whileit does not exceed about 0.60 under the footing.
(b) Excess pore pressures under the footing reach a peak value and consequently
decrease while shaking is still in progress. Note that the same trend is observed
under undrained conditions as well (not shown here), indicating that this is not
only the result of excess pore pressure mitigation, from the high overburden area
under the footing towards the free field, as it was originally believed, but it is due
to dilation associated with the intense shearing which is induced by foundation
settlement.
(c) The accumulation of foundation settlement is essentially linear with time and
practicallystopsshortaftertheendofshaking,whilethedissipationofearthquake-
inducedexcessporepressuresisfarfromcomplete.Thisobservationindicatesthat
observed settlements are not due to densification of the liquefiable soil layer but
duetodynamicshearfailure,suchastheonepredictedbytheslidingblockmethod
of Newmark (1965). Note that Richards et al. (1993) made the same assumption
in an attempt to explain and predict analytically seismic settlements of surface
foundations on drysand.
4. Evaluation of degraded bearing capacity
In the present study, the degraded bearing capacity at the end of shaking, before any
significant dissipation of earthquake-induced excess pore pressures takes place, will be
evaluated analytically based on an extension of the method proposed by Cascone and
