Annex D.1

(informative)

# Standard Penetration Test (SPT)

Table D.1: Values of the energy ratios ERr of the common equipment used in various countries and the correction factors to apply for normalizing to ERr = 60 %
 Country Hammer Release ERr (%) ERr/60 North and South America DonutSafetyAutomatic 2 turns of rope 2 turns of ropeTrip 45 5555 to 83 0,75 0,920,92 to 1,38 Japan DonutDonut 2 turns of rope Auto-Trigger 65 78 1,08 1,3 China DonutAutomatic 2 turns of rope Trip 50 60 0,83 1,0 United Kingdom SafetyAutomatic 2 turns of ropeTrip 50 60 0,83 1,0 Italy Donut Trip 65 1,08

For additional information and examples see Annex M.

ANNEX D.2

(informative)

## Standard Penetration Test (SPT)

(1) Below examples of correlations of blow counts and density indices (Skempton, 1986) are given.

(2) The relationship between the blow count N60, density index ID = (emaxe) / (emaxemin) and the effective overburden pressure σ'v (kPa × 10-2) in a given sand can be represented by the expression:

The parameters a and b in normally consolidated sands are nearly constant for 0,35 < ID < 0,85 and 0,5 < σ'v < 2,5, in kPa × 10-2.

(3) For normally consolidated natural sand deposits the correlation shown in table D.2 has been established between ID and the normalized blow count (N1)60:

Table D.2: Correlation between the density index ID and the normalized blow count (N1)60
 ID 0 % 15 % 35 % 65 % 85 % 100 % Very loose Loose Medium Dense Very dense (N1)60 = 0 3 8 25 42 58

For ID > 0,35 it corresponds to (N1)60/ID2 60.

(4) For fine sands the N-values should be reduced in the ratio 55/60 and for coarse sands increased in the ratio 65/60.

(5) The resistance of sand to deformation is greater the longer the period of consolidation. This "ageing" effect is reflected in higher blow counts, and appears to cause an increase in the parameter a.

Typical results for normally consolidated fine sands are given in table D.3.

Table D.3: Effect of ageing in normally consolidated fine sands
 Age [years] (N1)60/ID2 Laboratory testsRecent fillsNatural deposits 10-210> 102 354055

(6) Overconsolidation increases the coefficient b by the factor

where:

K0 and K0NC are the in situ stress ratios between horizontal and vertical effective stresses for the overconsolidated and normally consolidated sand respectively.

(7) All the above mentioned correlations have been established for predominantly silica sands. Their use in more crushable and compressible sands like calcareous sands or even silica sands containing a non-negligible amount of fines, may lead to an underestimation of ID.

For additional information and examples see Annex M.

ANNEX D.3

(informative)

## Standard Penetration Test (SPT)

(1) This is an example of derivation of the angle of shearing resistance of silica sands, N', from the density index ID. The values of N' are also influenced by the angularity of the particles and the stress level.

Table D.4: Angle of shearing resistance of silica sands, N'
 Desity index ID Fine grained Medium grained Coarse grained [%] Uniform Well graded Uniform Well graded Uniform Well graded 406080100 34363942 36384143 36384143 38414344 38414344 41434446

For additional information and examples see Annex M.

ANNEX D.4

(informative)

## Standard Penetration Test (SPT)

(1) This is an example of an empirical direct method for the calculation of settlements in granular soils of spread foundations proposed by Burland and Burbidge (1985).

(2) The settlement for stresses below the overconsolidation pressure is assumed to be 1/3 of that corresponding to the normally consolidated sand. The immediate settlement, si, in mm, of a square footing of width B, in m, is then given by:

where:

σ'v0 is maximum previous overburden pressure, in kPa;

q' is average effective foundation pressure, in kPa;

IC is af/B0,7;

af is the foundation subgrade compressibility, Δsiq' in mm/kPa.

(3) Through a regression analysis of settlement records the value of IC is obtained through the expression:

where is the average SPT blow count over the depth of influence. The standard error of af varies from about 1,5 for greater than 25 to 1,8 for less than about 10.

(4) The N-values for this particular empirical method should not be corrected for the overburden pressure. No mention is made of the energy ratio (ERr) corresponding to the N-values. The effect of the water table is supposed to be already reflected in the measured blow count, but the correction N' = 15 + 1/2 × (N – 15) for submerged fine or silty sands should be applied for N > 15.

In cases involving gravels or sandy gravels, the SPT blow count should be increased by a factor of about 1,25.

(5) The value of is given by the arithmetic mean of the measured N-values over the depth of influence, zi = B0,75, within which 75 % of the settlement takes place, for cases where N increases or is constant with depth. Where N shows a consistent decrease with depth, the depth of influence is taken as 2B or the bottom of the soft layer whichever is the lesser.

(6) A correction factor fs for the length-to-width ratio (L/B) of the foundation

should be applied. The value of fs tends to 1,56 as L/B tends to infinity. No depth (D) correction factor has to be applied for D/B < 3.

(7) Foundations in sands and gravels exhibit time-dependent settlements. A correction factor, ft, should be applied to the immediate settlement given by:

ft = (1 + R3 + Rtlogt/3)

where ft is the correction factor for time t ≥ 3 years, R3 is the time-dependant factor for the settlement that takes place during the first 3 years after construction and Rt is the time-dependent factor for the settlement that takes place each log cycle of time after 3 years.

(8) For static loads conservative values of R3 and Rt are 0,3 and 0,2 respectively. Thus at t = 30 years, ft = 1,5. For fluctuating loads (tall chimneys, bridges, silos, turbines etc.) values of R3 and Rt are 0,7 and 0,8 respectively so that at t = 30 years, ft = 2,5.

For additional information and examples, see Annex M.

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