Annex D.1
(informative)
Standard Penetration Test (SPT)
Country | Hammer | Release | ER_{r} (%) | ER_{r}/60 |
North and South America | Donut Safety Automatic |
2 turns of rope 2 turns of rope Trip |
45 55 55 to 83 |
0,75 0,92 0,92 to 1,38 |
Japan | Donut Donut |
2 turns of rope Auto-Trigger |
65 78 |
1,08 1,3 |
China | Donut Automatic |
2 turns of rope Trip |
50 60 |
0,83 1,0 |
United Kingdom | Safety Automatic |
2 turns of rope Trip |
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 N_{60}, density index I_{D} = (e_{max} − e) / (e_{max} − e_{min}) 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 < I_{D} < 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 I_{D} and the normalized blow count (N_{1})_{60}:
I_{D} | 0 % | 15 % | 35 % | 65 % | 85 % | 100 % | ||||||||||||
Very loose | Loose | Medium | Dense | Very dense | ||||||||||||||
(N_{1})_{60} = | 0 | 3 | 8 | 25 | 42 | 58 |
For I_{D} > 0,35 it corresponds to (N_{1})_{60}/I_{D}^{2} 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.
Age [years] | (N_{1})_{60}/I_{D}^{2} | |
Laboratory tests Recent fills Natural deposits |
10^{-2} 10 > 10^{2} |
35 40 55 |
(6) Overconsolidation increases the coefficient b by the factor
where:
K_{0} and K_{0NC} 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 I_{D}.
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 I_{D}. The values of N' are also influenced by the angularity of the particles and the stress level.
Desity index I_{D} | Fine grained | Medium grained | Coarse grained | |||
[%] | Uniform | Well graded | Uniform | Well graded | Uniform | Well graded |
40 60 80 100 |
34 36 39 42 |
36 38 41 43 |
36 38 41 43 |
38 41 43 44 |
38 41 43 44 |
41 43 44 46 |
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, s_{i}, 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;
I_{C} is a_{f}/B^{0,7};
a_{f} is the foundation subgrade compressibility, Δs_{i}/Δq' in mm/kPa.
(3) Through a regression analysis of settlement records the value of I_{C} 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 (ER_{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 + ^{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, z_{i} = 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 2B 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 + R_{3} + R_{t}logt/3)
where f_{t} is the correction factor for time t ≥ 3 years, R_{3} is the time-dependant 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 R_{3} 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 R_{3} and R_{t} are 0,7 and 0,8 respectively so that at t = 30 years, f_{t} = 2,5.
For additional information and examples, see Annex M.