Geoscience Reference
In-Depth Information
2.2. SPT
N
-VALUE
Based on a number of the soil container tests for four different soils listed in Table 8.1
frompoorly-graded sand to well-graded gravels, measured
N
-values are normalized as
/
σ
+
σ
h
/
3
p
0
n
N
0
=
N
2
(8.3)
and plotted versus the void ratio in Figure 8.10. It was found that, on the full logarith-
mic chart, the data points may be approximated by parallel lines with different locations
depending ondifferent particlegradations.
The maximum and minimum
N
-values;
N
0
max
and
N
0
min
corresponding to
e
min
and
e
max
(see Table 8.1), defined as intersections of the straight lines for individual soils
are marked with large solid circles and large solid squares, respectively, in Figure 8.10.
N
0
max
tends to increase drastically for soils with higher uniformity coefficient, while
the minimum
N
0
min
stays almost constant. This implies that
N
-values of well-graded
gravels can be as small as poorly-graded loose sand if they are loose enough despite
tremendous differences in void ratio, whereas
N
-values can be considerably larger than
that of dense sand if they are dense enough. In Figure 8.11, the normalized
N
-values
N
0
areplottedversustherelativedensities
D
r
,indicatingthatwell-gradedgravelscantakea
widerrangeof
N
-valuesthanpoorly-gradedsandandthedifferencein
N
-valuesbetween
soils of different particle gradations widens for
D
r
larger than around 50%.
In Figure 8.12, the values of
N
0
max
and
N
0
min
are plotted against uniformity coefficient
Cu
in the full logarithmic scale, which may be formulated by straight lines shown in the
1000
e
min
TS(HC)
TS(HC)
G50(HC)
G75(HC)
TS(LC)
G25(LC)
G50(LC)
G75(LC)
N
0min
N
0max
100
10
e
max
1
0.1
1
Void ratio e
Fig. 8.10. Normalized SPT
N
-value versus void ratio for four tested soils
(Kokusho and Yoshida, 1997)
