Geoscience Reference
In-Depth Information
targetvalues
D
r
30%,50%and70%byatampingmethod.Thespecimenwasisotrop-
ically consolidated by the effective stress of 98kPa with the back-pressure of 294kPa.
The Skempton's B-value larger than 0.90 was measured in all tests indicating almost
perfect saturationconsidering that well-gradedsoilswithsmallvoidratios have theoreti-
callysmaller
B
-valuesthanpoorly-gradedsoilseveniftheyarefullysaturated(Kokusho,
2000). The axial stress was cyclically controlled by sinusoidal waves with the frequency
of0.1Hz.Themembranepenetrationeffectonundrainedcyclicstrengthwasfoundgene-
rally small, less than 10% even for the coarsest RS3 material, because the specimen sur-
face were actually smooth because of richcontent ofsand.
=
3.3. EFFECT OFPARTICLE GRADATION FOR CLEAN GRANULAR SOILS
σ
c
;
Figure8.16exemplifiestypicalrelationshipsbetweenthecyclicstressratio,
R
L
(
σ
d
/
2
σ
c
σ
d
effective confining stress), for attaining 5%
double amplitude (DA) strain versus the number of loading cycles
N
L
for the three soils
for relative density
D
r
=
single axial stress amplitude,
=
50%. This stress ratio defined by 5% DA strain is almost iden-
tical with that defined by nearly 100% pore-pressure buildup at least for
D
r
≈
60% or
smaller(HaraandKokusho,2000).InFigure8.17,thestressratiosfor5%DAstraincor-
responding to
N
L
≈
, which are often used as liquefaction strength
in engineering practice in Japan, are plotted versus relative density
D
r
. The data points
for soils RS1, RS2 and RS3 are located nearer to each other although some differences
are visibleat around
D
r
=
20
,
R
L
20
(
DA
=
5%
)
=
50% and 90%.
In order to see the effect of particle gradations on the strength more clearly, Figure 8.18
shows the relationship between
R
L
20
and
Cu
for different
D
r
. Small differences in
D
r
0.5
RS1 (Cu=1.44)
RS2 (Cu=3.79)
RS3 (Cu=13.1)
0.4
0.3
0.2
0.1
∼
Dr
50%
σ
c
'=98 kPa
0
0.1
1
10
100
1000
Number of cycles N
L
Fig. 8.16. Liquefaction strength(stressratio) versus number of cycles compared among
threesoilsof different particlegradation
