Geoscience Reference
In-Depth Information
CHAPTER 8
LIQUEFACTION STRENGTHS OF POORLY-GRADED AND WELL-GRADED
GRANULAR SOILS INVESTIGATED BY LAB TESTS
Takaji Kokusho
Civil Eng. Department, Faculty of Science & Engineering,
Chuo University, Tokyo Japan
kokusho@civil.chuo-u.ac.jp
Abstract.
Inordertounderstanddifferencesinliquefactionbehaviorofwell-gradedgravellysoils
compared to poorly-graded sands, a series of lab tests was performed on granular soils with dif-
ferent particle gradations or fines content having different relative densities reconstituted in labo-
ratory. Large soil container tests indicated that SPT
N
-value of well-graded gravels of relative
densityhigherthan50%isconsiderablylargerthanthatofsandofthesamerelativedensity,result-
ing in lower liquefaction strength of gravelly soils than that of poor-graded sand under the same
corrected
N
-value,
N
1
,for
N
1
>
25-30. Cyclic triaxial tests on reconstitutes specimens indicated
that relative density can serve as a proper index to uniquely evaluate liquefaction strength cor-
responding to 5% DA strain for variety of granular soils having different gradations. In contrast,
post-liquefactionundrainedresidualstrengthforlargerstrainisnotuniquelydeterminedbyrelative
density but largely dependent on particle gradations. Also found was that the liquefaction strength
clearly reduces with increasing fines content
Fc
both in well-graded and poorly-graded soils but
the reduction occurs in a smaller range of
Fc
in accordance with smaller critical void ratio for
well-graded soils than for poorly-graded sand. Increase in
Fc
also reduces post-liquefaction resid-
ual strength of granular soils particularly for higher relative density. Greater reduction occurs in
smaller
Fc
rangeforwell-gradedsoilsthanforpoorly-gradedsandbecauseofthedifferenceinthe
critical void ratio.
1. Introduction
So far, liquefaction research on granular soils has been focused on poorly-graded sandy
soils. However, liquefaction of well-graded gravelly soils, though less frequent in past
records than that of sands, has increasingly been witnessed during recent earthquakes.
Duringthe1995HyogokenNambuearthquakeinJapan,reclaimedgroundinKobefilled
with decomposed granite sandy soil called Masado containing large quantity of gravel
andfinesfractionliquefiedextensivelydespiteawidelyacceptedperceptionthatgravelly
soilwashardertoliquefythansandbecauseoflargeruniformitycoefficientandlargerdry
density.TheSPT
N
-valueofthegravellysoiluncorrectedbyoverburdenstresswasaslow
as5to15.Duringthe1993HokkaidoNansei-Okiearthquake,rockdebrisavalanchesoil
containing large size rocks as well as sands and silts liquefied in Mori town in Hokkaido
causing differential settlements of wooden houses. The SPT
N
-value was 8-16 and the
S-wave velocity was unbelievably low as 60-90m/s (Kokusho et al., 1995). During the
1983 Borah Peak earthquake in Idaho, USA, fluvial sandy gravel liquefied extensively
