Standard penetration test (SPT)

F.1 Examples of correlations between blow counts and density indices

F.2 Examples of deriving values for the effective angle of shearing resistance

F.3 Example of a method to calculate the settlement of spread foundations

Annex F

(Informative)

(1) Examples of correlations of blow counts and density indices are given below.

(2) The relationship between the blow count (N₆₀), density index I_D = (e_max – e)/(e_max – e_min) and the effective total (initial) stress σ'_v0(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 < I_D < 0,85 and 0,5 < σ'_v0 < 2,5, in kPa × 10^–2. (see Skempton 1986, Table 8)

(3) For normally consolidated natural sand deposits, the correlation shown in Table F.1 has been established between the normalised blow count (N₁)₆₀ and I_D.

Table F.1 — Correlation between the normalised blow count (N₁)₆₀ and the density index I_D

	Very loose	Loose	Medium	Dense	Very dense
(N₁)₆₀	0—3	3—8	8—25	25—42	42—58
I_D	0 % — 15 %	15 % — 35 %	35 % — 65 %	65 % — 85 %	85 % — 100%

(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 F.2.

Table F.2 — Effect of ageing in normally consolidated fine sands

	Age years	(N₁)₆₀/I_D²
Laboratory tests Recent fills Natural deposits	10^–2 10 >10²	35 40 55

(6) Over-consolidation increases the coefficient b by the factor:

where

K₀ and K_0NC are the in-situ stress ratios between horizontal and vertical effective stresses for the over-consolidated 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, such as calcareous sands or even silica sands containing a non-negligible amount of fines, may lead to an underestimation of I_D.

NOTE These examples were published by Skempton (1986). For additional information and examples, sec X.3.3.

(1)Table F.3 is an example that can be used to derive values of the effective angle of shearing resistance of silica sands, (φ'), from the density index (I_D). The values of φ'are also influenced by the angularity of the particles and the stress level (see Table F.3).

Table F.3 — Correlation between the density index (I_D) and the effective angle of shearing resistance of silica sands, (φ'), in degrees

NOTE This example was published by the US Army Corps of Engineers (1993). For additional information and examples, see X.3.3.

Density index
I_D

Well-graded

Well-graded

Well-graded

(1) This is an example of an empirical direct method for the calculation of settlements in granular soil of spread foundations,

(2) The settlement for stresses below the over consolidation pressure is assumed to be 1/3 of that corresponding to the normally consolidated sand. The immediate settlement, s_i, in mm, of a square footing of width B, in m, for an over consolidated sand, if q' ≥ σ'_p, is then given by:

where

σ'_p

is maximum previous overburden pressure, in kPa;

is average effective foundation pressure, in kPa;

I_cc

is a_f/B^0,7;

a_f

is the foundation subgrade compressibility, Δs_i/Δq', in mm/kPa.

If q' ≤ σ_p then the equation becomes:

And for normally consolidated sands:

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

where

is the average SPT blow count over the depth of influence.

The standard error of a_f 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 (E_r)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 + 0,5 × (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 N is given by the arithmetic mean of the measured N-values over the depth of influence, z₁ = B^0,75, 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 25 or the bottom of the soft layer whichever is the lesser.

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

should be applied. The value of f_s 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, f_t should be applied to the immediate settlement given by:

f_t = (1 + R₃ + R_tlg t/3)

where

f_t

is the correction factor for time t ≥ 3 years;

R₃

is the time-dependent factor for the settlement that takes place during the first 3 years after construction; and R_t 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 R_t are 0,3 and 0,2 respectively. Thus at t = 30 years, f_t = 1,5. For fluctuating loads (tall chimneys, bridges, silos, turbines etc.), values of R₃ and R_t are 0,7 and 0,8 respectively so that at t = 30 years, f_t = 2,5.

NOTE This example was published by Burland and Burbridge (1985). For additional information and examples, see X.3.3.