Equipment calibration
In view of the above, it is clear that it is extremely important, in each investigation campaign, to know precisely the energy ratio, E_{K}, of the equipment used in the SPT tests. For this purpose, proof of recent calibration of the equipment should be requested.
Calibration methods are outside the scope of this book. In any case, it is anticipated that the energy transferred to the rods in each blow is equal to the integral of the applied force multiplied by the velocity, defined from the moment of impact of the hammer on the rods until the moment the integral reaches the maximum value, according to the equation:
in which F(t) and v(t) correspond to the force and velocity values, respectively, as a function of time.
The evaluation of the energy involved in each blow, therefore, requires the instrumentation of one or more rods by means of strain gauges (for obtaining the force, F) and of accelerometers (which allow determination of the propagation velocity of the shock wave, v). These should be linked to a data acquisition system, and its interpretation can be done using appropriate commercial software. The time corresponding to the greatest transferred energy determines the integration interval.
Likewise, it is also advisable to check the hammer drop height, sampler dimensions and sampler and hammer weights: since the SPT requires very simple equipment, it can be produced in nonspecialized workshops.
Correlations of (N1)60 with soil properties and parameters
Table 1.6 shows the correlation between (N_{t})_{60} and the relative density or density index for normally consolidated sands, as proposed by Skempton (1986).
Table 1.6 Relation between (N_{t})_{60} and density index for sands (Skempton, 1986).
(^{N}'L 
03 
38 
825 
2542 
>42 
M°/o) 
015 
1535 
3565 
6585 
85100 
Relative density 
Very loose 
Loose 
Medium dense 
Dense 
Very dense 
Notes:
 1. For /_{0} > 0.35 (N,)_{60} / Id = 60 .
 2. For coarse sands, N should be multiplied by 55/60.
 3. For fine sands, N should be multiplied by 65/60.
Figure 1.14 Correlation between (N)_{60} and density index for clean sands (Mayne et al., 2001).
Figure 1.14 (Mayne et al., 2001) essentially presents the same correlation together with a large number of experimental determinations obtained by several authors.
Figure 1.15 shows two correlations between (N[)_{60} and the angle of shearing resistance (peak values) proposed by Decourt (1989) and Hatanaka and Uchida (1996). It can be seen that the proposals are in reasonable agreement with each other.
Figure 1.15 Correlations between (N,)_{60} and the angle of shearing resistance for sands (Decourt, 1989; Hatanaka and Uchida, 1996).
Table 1.7 Correlation between the density index and the angle of shearing resistance for quartz sands (US Army Corps of Engineers, 1993).
'dW 
Fine sands 
Medium sands 
Coarse sands 

Uniform 
Wellgraded 
Uniform 
Wellgraded 
Uniform 
Wellgraded 

40 
34 
36 
36 
38 
38 
41 
60 
36 
38 
38 
41 
41 
43 
80 
39 
41 
41 
43 
43 
44 
100 
42 
43 
43 
44 
44 
46 
Table 1.8 Relation between N_{60} and the consistency of clays (Clayton et al., 1995).
N_{60} 
04 
48 
815 
1530 
3060 
>60 
Consistency 
very soft 
soft 
medium 
stiff 
very stiff 
hard 
Table 1.7 shows a correlation between the density index and the angle of shearing resistance for quartz sands (US Army Corps of Engineers, 1993).
The correlations between SPT results and liquefaction susceptibility of sandy soils will be discussed in Chapter 6 (see 6.5).
Table 1.8 includes a soil classification proposed by Clayton et al. (1995), based on the SPT for clayey soils, in terms of consistency.
Before concluding this presentation of the SPT, it should be pointed out that the estimate of mechanical parameters of the soil, based on correlations, such as those previously mentioned, does not deplete its usefulness for geotechnical design. In fact, numerous empirical methods of the design of shallow and deep foundations (piles), directly based on SPT results (N_{60} or (N,)_{60}), can be found in the literature.