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, EK, 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 non-specialized workshops.

Correlations of (N1)60 with soil properties and parameters

Table 1.6 shows the correlation between (Nt)60 and the relative density or density index for normally consolidated sands, as proposed by Skempton (1986).

Table 1.6 Relation between (Nt)60 and density index for sands (Skempton, 1986).

(N'L

0-3

3-8

8-25

25-42

>42

M°/o)

0-15

15-35

35-65

65-85

85-100

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.
Correlation between (N|) and density index for clean sands (Mayne et al., 2001)

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.

Correlations between (N,) and the angle of shearing resistance for sands (Decourt, 1989; Hatanaka and Uchida, 1996)

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

Well-graded

Uniform

Well-graded

Uniform

Well-graded

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 N60 and the consistency of clays (Clayton et al., 1995).

N60

0-4

4-8

8-15

15-30

30-60

>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 (N60 or (N,)60), can be found in the literature.

 
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