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)
From D R and gradation curves estimates can be made for E (Young's modulus) from Table
3.25, k from Figure 3.14 and
φ
from Table 3.36 .
Correlations with consistency and U c Table 3.37 and Figure 3.94.
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
s u . Determinations of deformation property are discussed in Section 3.5.4. Test method 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.
 
 
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