Geoscience Reference
In-Depth Information
5.4 Clean sand strength from SPT data
•
The values vary from corrected to uncorrected N values and type of sand.
•
The SPT - value can be used to determine the degree of compactness of a cohesion-
less soil. However, it is the soil friction angle that is used as the strength parameter.
Table 5.4
Strength from SPT on clean medium size sands only.
Description
Relative
SPT - N (blows/300 mm)
Strength
density D
r
Uncorrected field value
Corrected value
Friction angle
V. Loose
15%
N
4
(N
o
)
60
3
28
◦
<
≤
≤
φ
<
Loose
15-35%
N
=
4-10
(N
o
)
60
=
3-8
φ =
28-30
◦
Med dense
35-65%
N
10-30
(N
o
)
60
8-25
30-40
◦
=
=
φ =
Dense
65-85%
N
=
30-50
(N
o
)
60
=
25-42
φ =
40-45
◦
V. Dense
85%
N
50
(N
o
)
60
42
45
◦
-50
◦
>
>
>
φ =
100%
(N
o
)
60
60
50
◦
=
φ =
•
φ
by 5
◦
Reduce
for clayey sand.
•
by 5
◦
Increase
φ
for gravely sand.
5.5 Fine and coarse sand strength from SPT data
•
Fine sands have reduced values from the table above while coarse sand has an
increased strength value.
•
The corrected N value is used in the table below.
Table 5.5
Strength from corrected SPT value on clean fine and coarse size sands.
Description
Relative
Corrected SPT - N (blows/300 mm)
Strength
density D
r
Fine sand
Medium
Coarse sand
V. Loose
<
15%
(N
o
)
60
≤
3
(N
o
)
60
≤
3
(N
o
)
60
≤
3
φ
<
28
◦
Loose
15-35%
(N
o
)
60
3-7
(N
o
)
60
3-8
(N
o
)
60
3-8
28-30
◦
=
=
=
φ =
Med dense
35-65%
(N
o
)
60
7-23
(N
o
)
60
8-25
(N
o
)
60
8-27
30-40
◦
=
=
=
φ =
Dense
65-85%
(N
o
)
60
23-40
(N
o
)
60
25-43
(N
o
)
60
27-47
40-45
◦
=
=
=
φ =
V. Dense
85%
(N
o
)
60
40
(N
o
)
60
43
(N
o
)
60
47
45-50
◦
>
>
>
>
φ =
100%
(N
o
)
60
=
55
(N
o
)
60
=
60
(N
o
)
60
=
65
φ =
50
◦
•
Above is based on Skempton (1988):
-(N
o
)
60
/D
r
=
55 for Fine Sands.
-(N
o
)
60
/D
r
=
60 for Medium Sands.
-(N
o
)
60
/D
r
=
65 for Coarse Sands.
5.6 Effect of aging
•
The SPT in recent fills and natural deposits should be interpreted differently.
•
Typically the usual correlations and interpretations are for natural materials. Fills
and remoulded samples should be assessed different.
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