5.5 Interpretation of the results

5.5.1 Introduction

(1)P Existing design methods of foundations based on the SPT are of empirical nature. Operating methods have been adapted to obtain more reliable results. Therefore, the application of appropriate correction factors for interpreting the results shall be considered. In the following paragraphs, correction factors are given for energy delivered to the drive rods, for the effect of overburden pressure and other circumstances.

5.5.2 Energy delivered to the drive rods

(1) Energy losses are induced by the hammer assembly due to frictional and other parasitic effects, which cause the hammer velocity at impact to be less then the free fall velocity. Further losses of energy are originated by the impact on the anvil, depending on its mass and other characteristics. The type of machine, skill of the operator and other factors can also influence the energy delivered to the drive rods.

(2)P The value of the blow count, N, in sands is inversely proportional to the energy ratio ERr, so that:

Na × ERr,a = Nb × ERr,b

(3)P The ERr-value of the equipment used has to be known if the N-values are going to be used for the quantitative evaluation of foundations or for the comparison of results. A certificate of calibration of the ERr-value imediately below the driving head or anvil shall be available.

(4)P For general design and comparison purposes in sands, the N-values shall be adjusted to a reference energy ratio of 60 %, by the following expression:


N is the blow count and ERr, is the energy ratio of the specific test equipment.

(5) In table D.1 (annex D) an example is given of the energy ratios of the equipment commonly used in various countries and the corresponding correction factors for normalizing to ERr = 60 %.

(6)P If a design method for sands has been elaborated for a value of ERr different from 60 %, the corresponding corrected N-value shall be determined based on the expression shown in 5.5.2 (2).

5.5.3 Energy losses due to the length of rods

(1) If the length of rods is less than 10 m, the energy reaching the sampler is reduced and the correction factors shown in table 5.1 should be applied to the blow count for sands.

Table 5.1: Correction factors in sands due to rod length
Rod length
below the anvil
Correction factor,
>10 1,0
6—10 0,95
4—6 0,85
3—4 0,75

5.5.4 Effect of overburden pressure in sands

(1) The effect of the overburden pressure in the N-value in sands with reference to the density index ID, may be taken into account by applying to the measured N-value the correction factor CN given in table 5.2.

(2) Table 5.2: Correction factors CN due to the effective overburden pressure in sands
Type of sand Density Index ID CN
Normally consolidated 40 to 60 2

1 + σ'v
60 to 80 3

2 + σ'v
Overconsolidated   1,7

0,7 + σ'v
(σ'v, in kPa × 10-2)

For an effective overburden pressure of 100 kPa, σ'v = 1 and hence CN = 1 and the value of N is then defined as the normalized value N1.

(2) Values of the correction factor CN larger than 2,0 and preferably 1,5 should not be applied.

5.5.5 Other correction factors

(1) If the inner diameter of the sampler is 3,0 mm larger than that of the shoe, as in the deviation to this test procedure mentioned in 5.1 (7) no correction is necessary if a liner of appropriate thickness is used, such that the inside of the whole sampler is practically flush to a uniform diameter of 35 mm. Nevertheless, attention should be paid to the eventual damage of the liner during driving and its influence on the corresponding blow count. If the liner is omitted, the additional clearance of the inside of the barrel with reference to the shoe leads to N-values between 10 % and 20 % lower in sands.

5.5.6 Use of the correction factors

(1)P Several correction factors have been mentioned in the previous paragraphs. As the existing design methods of foundations based on the SPT are of an empirical nature, only the corresponding correction factors shall be used, unless duly justified.

(2) If all the correction factors corresponding to this test procedure are applied for a design method based on an energy ratio of 60 %, the following value for the final blow count would be obtained (without including the one mentioned in 5.5.5 (1)):


  • λ is the correction factor for energy losses due to the rod length in sand
  • CN is the correction factor for the effective overburden pressure in sand.

Eurocode 7 Geotechnical design — Part 3: Design assisted by fieldtesting