Geoscience Reference
In-Depth Information
where
D
r
islogarithmic relative density defined hereas
D
r
=
log
(
e
max
/
e
)/(
e
max
/
e
min
)
(8.5)
The values of power
n
is plotted against
D
r
and approximated by a simple function as
n
D
r
=
27
D
r
−
0
.
4
(Kokusho and Yoshida, 1997). Thus
N
-value can be expressed
by uniformity coefficient
Cu
, logarithmic relative density
D
r
and confining pressure
σ
m
as
0
.
8
7
3
C
u
0
.
46
D
r
σ
m
/
p
0
n
(
D
r
)
N
=
5
.
.
(8.6)
This equation is superposed inFigure 8.11, indicating afairly good coincidence withthe
test results.
Thus, the empirical formula have been determined from the large soil container tests.
Needless to say, the soil conditions may not be exactly the same as natural soils and the
constants presented in Eqs. (8.2) and (8.6) have to be revised if additional data from nat-
uralsoildepositsbecomesavailable.However,thebasictrendspresentedintheempirical
equationsseemtoholdinevaluatingtheeffectofparticlegradationsonS-wavevelocities
and
N
-values for granular soils ingeneral.
3. Effect of grain size distribution on cyclic strength
As demonstrated in Figure 8.2 and by other data (Kokusho and Tanaka, 1994), grav-
elly soils in nature are mostly well-graded with smooth grain size curves. Undrained
cyclic strength of well-graded gravelly soils is not understood so well- as poorly-graded
sands withregard totheirdensity,particlegradations, etc.,though theyhave significance
in liquefaction potential evaluation for seismic design. Consequently, systematic cyclic
undrained triaxial tests have been performed (Hara and Kokusho, 2000; Kokusho and
Komiyama, 2001; Kokusho et al., 2004) for granular soils with different relative densi-
ties
D
r
, different
Cu
and different fines content
Fc
.
3.1. SOILMATERIALS
Granularsoilstestedwerethreetypes,showninFigure8.13andnamedhereasRS1,RS2
and RS3, reconstituted from sands and gravels originated from a river, particle shapes
of which are sub-rounded and hard to crush. Uniformity coefficients of RS1, RS2 and
RS3are
Cu
=
1
.
44,3.79and13.1respectively.InRS1andRS3,fines(finerthan75
µ
m)
were mixed stepwise to reconstitute soils containing fines up to
Fc
(fines content)
=
30% as shown in Figure 8.13. The fines were silty and clayey soils with low plasticity
index of Ip
=
6 sieved from decomposed granite in reclaimed ground of the Kobe city,
Japan.
