Geoscience Reference
In-Depth Information
friction angle (
inEq.11.4). The effect ofthe firsttworefinements proved of secondary
importance.Onthecontrary,theuseofareducedbuoyantunitweightledtomuchhigher
values of
ϕ
, shown with dotted lines in Figure 11.4b, leaving essentially open the ques-
tion of how to model the degraded shearing resistance of sand upon partial or complete
liquefaction.
ζ
It is noteworthy that none of the solutions for bearing capacity degradation reviewed
above has been thoroughly documented against experimental and numerical evidence.
2.3. LIQUEFACTION-INDUCED FOUNDATION SETTLEMENTS
The literature on liquefaction settlements is richer compared to that on degraded bearing
capacity. However, before proceeding to a brief review of relevant methodologies, it is
importanttoclarifythatseismicsettlementsoffootingscannotbepredictedusingempir-
ical methodologies developed for free field conditions (e.g. Tokimatsu and Seed, 1987;
IshiharaandYoshimine,1992)asthecontrolingmechanismsforthetwoeventsarediffer-
ent.Namely,freefieldsettlementsareduetovolumedensification,andconsequentlythey
take place during the dissipation of earthquake-induced excess pore pressures, mostly
aftertheendofshaking.Onthecontrary,footingsettlementsareassociatedwithfailurein
thefoundationsoil,causedbythecombinedactionofstaticandinertiafoundationloads,
shear strength degradation of the liquefiable soil and earthquake-induced shear stresses
in the soil. Thus, the latter are significantly larger than the former, and they take place
mostly during (not after) shaking. A number of experimental studies (Liu and Dobry,
1997; Adalier et al., 2003; Coehlo et al., 2004, 2005, etc.) and field observations (Cetin
et al., 2002; Seed et al.,2003; Birdet al., 2006) bear witness on the above differences.
Basedonfieldevidence,Figure11.5showsawidelyknownempiricalrelationshipforthe
beneficial effect of the normalized footing width
B
/
Z
liq
on the respective seismic foun-
dation settlements
S
Z
liq
, where
Z
liq
is the thickness of the liquefied soil. This chart
includes data regarding building settlements in Niigata City, Japan, after the 1964 earth-
quake (Yoshimi and Tokimatsu, 1977), and Dagupan City, Philippines, after the 1990
Luzon earthquake (Adachi et al., 1992), as well as tank settlements during the 1983
Nihonkai-chubu,Japan,earthquake(YasudaandBerrill,2000).Theimportanteffectofa
soilcrustover theliquefied subsoil isaddressedbyIshiharaetal.(1993), whouse obser-
vations from Dagupan City in order to define the minimum soil crust required to prevent
surfaceevidenceofliquefactionintermsofthecorrespondingthicknessofliquefiedsoil.
Later on, Acacio et al. (2001) extended the above relation in order to define safety limits
against excessive foundation settlements and tilting, using also field evidence from the
Dagupan event.
/
Alternative means for the evaluation of liquefaction-induced settlements of light struc-
tures based on a clay crust are provided by Naesgaard et al. (1997), based on a consider-
able number of static and dynamic numerical analyses. These analyses were performed
in terms of total stresses, assuming an elastic-perfectly plastic shear stress (
)
relationship for the liquefied soil, fitted to the response of liquefied sands (Byrne, 1991;
τ
)-strain (
γ
