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for RS1 than for RS3. The residual strength reduction is much more considerable than
the cyclic strength reduction indicated in Figure 8.20. Thus, fines content in liquefiable
granularsoilshasmoresignificanteffectonpost-liquefactionresidualstrengththanlique-
faction strength. Figure 8.25 also indicates that, for the same increment of fines content,
thestrengthreductionoccursmoredrasticallyinwell-gradedsoilsthanforpoorly-graded
soils particularly for higher density.
5. Conclusions
Recent case studies indicate that gravelly soils can liquefy if they are loose enough no
matter how well graded and how coarse they may be. Gravelly soils liquefied during
recentearthquakeshave
N
1
-valuenolargerthan25andalsocontainrelativelylargequan-
tity of fines. It is also indicated that liquefaction strength of well-graded gravelly soils
largely deviates from that of poor-graded sand for
N
1
-value higher than 25-30, although
it shows almost the same stress ratio as poorly-graded loose sand for
N
1
smaller than
that. In order to understand the differences in liquefaction or post-liquefaction behavior
of well-graded soils in contrast to poorly-graded sands, experimental results of soil con-
tainer tests and undrained triaxial tests for soils with different particle gradations were
examined, yielding the following major findings:
(1) S-wavevelocitiesofgranularsoilsarenotdeterminedbyauniquefunctionofvoid
ratiobuthighlydependentonparticlegradation.MinimumS-wavevelocityofwell-
graded gravels remains at almost the same value as that of poorly-graded sands
despite much lower void ratio than sands. In contrast, maximum S-wave velocity
of well-graded gravels tends to increase considerably withincreasing
Cu
.
(2)
N
-value of well-graded gravels can be as small as poorly-graded loose sands if
theirrelativedensity
D
r
islowenoughdespitetremendousdifferenceinvoidratio,
whereas it can be considerably larger than that of sands for
D
r
50%. This may
explainwhytherelationshipbetweenliquefactionstrengthversusSPT
N
1
-valueof
well-graded soilsdeviates fromthat of poor-graded sands for
N
1
>
>
25-30.
(3) Liquefaction strength of granular soils defined by
R
L
20
(DA strain
5%) may
be uniquely evaluated by relative density,
D
r
, despite large difference in particle
gradations.
(4) Liquefactionstrengthdefinedby
R
L
20
(DAstrain
=
5%)obviouslydecreaseswith
increasing
Fc
particularly for larger
D
r
not only in poorly-graded sands but also
in well-graded gravels. The decrease for well-graded soils occurs in smaller
Fc
rangeinaccordancewithsmallercriticalfinescontentorsmallervoidratiothanfor
poorly-graded sands.
(5) Post-liquefactionstrengthforlargerstrainof20%-25%isnotuniquelydetermined
byrelativedensitybutlargelydependentonparticlegradations.Namely,soilswith
larger
Cu
and larger absolute density tend to exhibit considerably larger residual
strength.
=
