Geoscience Reference
In-Depth Information
2. Effect of grain size curve on S-wave velocity and
N
-value
Kokusho and Yoshida (1997) investigated the effect of particle gradations on
N
-values
and S-wave velocities by large scale soil container tests for sands and gravels with vary-
ing
Cu
.Artificialsoillayersweremadeinalargesteelsoilcontainer,2.0minsidediame-
terand1.5mheight,asshowninFigure8.5.Thesoillayersweresaturatedandvertically
loadedhydraulicallywithgivenoverburdenstressesbyarubberbaginstalledjustbeneath
the container cap. The overburden was initially set as 50kPa and then increased step by
stepeithertothemaximumof200kPainthefirstseriesoftest(namedhereasLCtest)or
tothemaximumof1MPainthesecondseriescalledhereasHCtest.Thestresscondition
in the soil layer was monitored vertically and horizontally by pressure cells installed at
the bottom and side walls of the container. The soils were placed in the container with
various initial density either by foot-tamping or by a mechanical tamper. Standard Pene-
trationTest(SPT)wascarriedoutthroughopeningsinthecontainercapintosoilsloaded
withvariousoverburden stressesunder
K
0
-condition.The
K
0
-value,
σ
h
/σ
v
,evaluatedas
σ
h
versustheverticalpressure
σ
v
aratioofthemeasuredhorizontaleffectivepressure
was
K
0
=
5 for very dense soils. Velocity of SH-wave
generated from a steel rod and vertically propagating upward was measured by using a
set of sensorsembedded inthe soillayer as depicted in Figure 8.5.
1
/
2-1
/
4 in most cases but
K
0
=
1
/
Five soils with different particle gradations shown in Figure 8.6 were used in the test;
two types of river sands (TS and TKS sands) and three types of gravels (G25, G50 and
G75 gravels) with different mean grain size
D
50
and different uniformity coefficient
Cu
.
Theparticlegradationswerechosentorepresenttypicalgranularsoilsinnaturecovering
poorly-graded sands to well-graded gravels. Gravel particles composing the materials
were sub-round and hard in quality. The maximum and minimum densities of the soils
were determined by using similar methods as will be explained later, but with a larger
mold of 30cm in diameter. InTable 8.1, physical properties of the soils are listed.
(a) Section
(b) Plan
2000 mm
Pressure cell
Pressure cell
Rubber
bag
Opening for
penetration
tests
Wave
sensor
S-wave source
Pressure cell
Fig. 8.5. Circular steel soilcontainer withoverburden pressure
(Kokusho and Yoshida, 1997)
