Environmental Engineering Reference
In-Depth Information
SPT N-Value Corrections
In recent years, it has become a practice to adjust the
N
value by a hammer-energy ratio or
hammer efficiency of 60%, and much attention has been given to
N
-values because of its
use in liquefaction studies. Additional corrections have been proposed (Skempton, 1986;
Robertson and Wride, 1998) for hammer type (donut and safety), borehole diameter, rod
lengths, and sampler. The largest correction factor appears to be for the donut hammer
(
C
E
0.5-1.0). Therefore
(
N
1
)
60
NC
N
C
E
(3.48)
Applications of the N value
Correlations with compactness and D
R
— see Table 3.23. Marcuson and
Bieganousky (1977) give an expression for computing
D
R
which accounts for
N
,
σ
o
, and
C
u
the coefficient of uniformity. (equation. (3.3)) as follows:
●
D
R
11.7
0.76
{|
222
N
1600
53
σ
o
50
C
u
2
|}
1/2
(3.49)
3.25,
k
from
Figure 3.14
and
φ
from
Table 3.36
.
●
Allowable soil-bearing value for foundations
— correlations with
N
(see,
●
e.g.,Terzaghi and Peck, 1967, p. 491)
Limitations
The value of
N
must always be considered as a rough approximation of soil compactness
or consistency in view of the various influencing factors. Even under controlled laboratory
testing conditions, a single value for
N
can represent a spread of
15% in
D
R
(Marcuson
and Bieganousky, 1977b).
Cone Penetrometer Test (CPT) (ASTM D5778)
Purpose
The correlations for strength properties are made between the cone tip resistance
q
c
,
and
soil classification
are discussed in
Section 2.3.4.
φ
TABLE 3.23
Correlations for Cohesionless Soils between Compactness,
D
R
, and
N
Compactness
Relative Density D
R
a
N
(SPT)
Very loose
<0.15
4
Loose
0.15-0.35
4-10
Medium dense
0.35-0.65
10-30
Dense (compact)
0.65-0.85
30-50
Very dense
0.85-1.0
50
a
From Gibbs, H.J. and Holtz, W.G., Proceedings of the 4th International Conference on Soil Mechanics and
Foundation Engineering, London, Vol. I, 1957, pp. 35-39.