Geoscience Reference
In-Depth Information
Fig. 11.6. Liquefaction settlement of light structures resting ontop of aclay crustin
terms of (a) the degraded factor of safety
Fs
,and (b) the degraded bearing capacity
index
Xs
(after Naesgaard et al., 1997)
A different approach for the computation of liquefaction-induced displacements has
been proposed by Yasuda et al. (1999) under the abbreviated name ALID (analysis
for liquefaction-induced displacements). This methodology is proposed in connection
with the numerical analysis of various geotechnical structures and foundations, but its
basicprinciplecanbereadilyextendedtoanalyticalcomputationsaswell,basedoncode
provided simplified methods for static settlement computation. For the case of surface
foundations,
- Settlements are first computed for the static loads applied to the foundation, using
the initial(prior toliquefaction) shear moduli of the subsoil layers.
- The shear moduli are consequently reduced, in order to account for the effects of
liquefaction, on the basisof the empirical charts shown in Figure 11.7.
- The static analyses are repeated, using the reduced shear moduli, and liquefaction-
induced settlements are finally computed after subtracting the static settlements of
the firststep.
ALIDhasbeensuccessfullyusedinanumberofcasestudies(Yasudaetal.,2001;Yasuda,
2004, 2005) and proved to be well suited for case specific practical applications. Never-
theless, it does not provide immediate insight to all factors which control the liquefac-
tion performance of footings and consequently it cannot be easily used to establish more
general design criteria.
Acknowledging the uncertainties associated with the very soft and simultaneously
extremely non-linear shear stress-strain response of liquefied sands (e.g. Figure 11.7),
as well as the objective difficulties encountered in applying the finite element method
under such conditions, Acacio et al. (2001) propose an analytical prediction method for
liquefaction-inducedsubsidence,wheretheliquefiedsoilwasmodeledasaviscousfluid.
Although application of this method is also computer aided, it does not presume the use
of any non-linear numerical (finite element or finite difference) algorithm. Comparison
withobservedsettlementsofbuildingsinDagupanCityafterthe1990Luzonearthquake
