Geoscience Reference
In-Depth Information
2
Pleistocene gravel (Tanaka et al. 1992)
Mori Debris gravel (Kokusho et al. 1995)
Kobe DG Gravel (Inagaki et al. 1996)
Idaho Holocene gravel (Estimated from PGA)
(Andrus 1994)
1.5
Poorly graded fine sand
(Yoshimi et al.1989)
1
0.5
0
0
20
40
60
80
100
120
140
160
Normalized SPT
N
- value N
1
Fig. 8.4. Normalized SPT
N
1
-value versus cyclic stress ratio
of well-graded gravelly soils
recovered by in situ freezing sampling. For data points of Idaho gravel represented by
solid circles with arrows in the chart, the stress ratio was evaluated from PGA (Peak
GroundAcceleration)duringthe1983BorahPeakearthquake,indicatingthatthecritical
curve for the onset of liquefaction may be located somewhere below the points.
On the same chart, the similar relationship for poorly-graded clean sand with uniformity
coefficient
Cu
6 on average proposed by Yoshimi et al. (1989) is drawn with a
dashed curve. It is remarkable that the solid curve approximating well-graded gravelly
soilshavealmostthesamestressratioascleansandfor
N
1
smallerthan20-25.Thesolid
curve in Figure8.4, however, tends to largely deviate from the curve of sand as
N
1
-value
getslargerthanaround25-30.Itisalsonotedthatallthegravellysoilsactuallyliquefied
during recent earthquakes have
N
1
-value no larger than 25 and also contains measurable
fines; fines content
Fc
=
1
.
=
8%-18% for Kobe DG gravel,
Fc
=
8
.
5% for Mori debris
gravel and
Fc
=
3%-18% for Idaho fluvial gravel.
In order to understand differences in liquefaction behavior between poorly-graded and
well-graded soils from the view point of particle gradation, results of a series of basic
laboratory tests carried out by the present author and his colleagues are addressed here;
(a) soil container tests for the effect of grain size distribution on
N
-value and
Vs
,
(b) undrained triaxial tests for the effect of grain size distribution on liquefaction and
post-liquefactionstrengthand(c)undrainedtriaxialtestsfortheeffectoffinescontenton
liquefaction and post-liquefaction strength.
