Geoscience Reference
In-Depth Information
(b) Liquefaction settlements of footings take place during shaking, being associated
withplasticfailureofthesubsoil.Thus,theyaremuchhigher thanfreefieldsettle-
ments,whicharemerelyduetosoildensification,andconsequentlytheycannotbe
predicted with the same empirical methodologies. Furthermore, they may exceed
the allowable limits even though the degraded (at the end of shaking) static factor
ofsafety
FS
deg
ishigher than 1.0.
(c) Seismic settlements and degraded factors of safety are inter-related. Thus, it is
indeed possible to check the seismic performance of a shallow foundation using
a conventional static factor of safety, only that its value should be defined in terms
of the allowable settlements, the seismic motion characteristics and the anticipated
thickness of liquefied soil.
(d) Thepresenceofasoilcrustmaydrasticallyreduceseismicsettlementsandofstatic
bearingcapacitydegradation,sothattheuseofashallowfoundationmaybefound
adequate,withoutpriorsoilimprovement.Forinstance,typicalexamplespresented
herein have shown that 2-5m of clay crust with
C
u
40kPa can bear a strip
footing, of 3m width and 100kPa average contact pressure, with reasonably low
settlements. Based on preliminary findings for rectangular footings, not presented
herein, it appears that the above minimum requirements are further reduced as the
length of the footing decreases below 4
B
.
=
(e) There is indeed a critical thickness of the clay crust beyond which the bearing
capacity is not affected by liquefaction anymore. The exact value of
)
cr
depends on the shearing resistance of the crust, the average bearing pressure and
the intensity of the shaking. However, the value of
(
H
/
B
)
cr
does not become
higher than about 2.6. Furthermore, note that the above limiting value of
(
H
/
B
)
cr
decreases for rectangular footings, and becomes approximately equal to 1.3 in the
caseof square footings.
(
H
/
B
(f) The seismic settlements corresponding to
)
cr
are relatively low and may
satisfy design criteria for weak or moderate seismic motions and high ductility
structures. However, for strong seismic motions and low ductility structures some-
what larger
H
(
H
/
B
/
B
values may be necessary for an acceptable performance-based
design.
(g) Althoughtheresultsofthisstudyarestrictlyrelevanttoapurelycohesivecrust,they
may prove useful for a gross qualitative assessment of the effects of other types of
liquefaction resistant soil crust as well, such as dense sand and gravel mixtures.
Furthermore they may be extended in order to guide the selection of the optimal
depth of application of various liquefaction mitigation measures.
Itisfinallynotedthatourstudiessofarhavenotconsideredanumberofengineeringfac-
tors (soil-structure interaction, seismic excitation time history, cohessionless soil crust,
footingshapeandembedment,etc.),whichmayhaveapotentiallyimportanteffect.Thus,
