D.6 Example of a correlation between compressive resistance of a single pile and cone penetration resistance
(1) In Tables D.3 and D.4 examples are given of established correlations between the results of static load tests and CPT results for coarse soil with little or no fines. Unit base resistance p_{b} and unit shaft resistance p_{s}, of cast in-situ piles are given as a function of cone penetration resistance (q_{c}) (CPT) and normalised pile head settlement.
Normalised settlement s/D_{s}; s/D_{b} | Unit base resistance p_{b}, in MPa, at average cone penetration resistance q_{c} (CPT) in MPa | |||
q_{c} = l0 | q_{c} = 15 | q_{c} = 20 | q_{c} = 25 | |
0,02 | 0,70 | 1,05 | 1,40 | 1,75 |
0,03 | 0,90 | 1,35 | 1,80 | 2,25 |
0,10(= s_{g}) | 2,00 | 3,00 | 3.50 | 4,00 |
NOTE Intermediate values may be interpolated linearly. In the case of cast in-situ piles with pile base enlargement, the values shall be multiplied by 0,75. s is the normalised pile head settlement D_{s} is the diameter of the pile shaft D_{b} is the diameter of the pile base s_{g} is the ultimate settlement of pile head |
Average cone penetration resistance q_{c} (CPT) MPa |
Unit shaft resistance p_{s} MPa |
0 | 0 |
5 | 0,040 |
10 | 0,080 |
≥ 15 | 0,120 |
NOTE Intermediate values may be interpolated linearly |
NOTE 1 The example was established from results of tests carried out with an electrical cone penetrometer.
NOTE 2 This example was published in DIN 1054 (2003-01). For additional information and examples, see X.3.1.
D.7 Example of a method to determine the compressive resistance of a single pile
(1) The following is an example of the determination of the maximum bearing resistance of a single pile on the basis of the q_{c} values from an electrical CPT. In case of over consolidation, or in case of an excavation after execution of a CPT, the q_{c} values should be reduced.
(2) The maximum compressive resistance of a pile follows from:
F_{max} = F_{max;base} + F_{max;shaft}
where
F_{max;base} = A_{base}× p_{max;base}
and
where
A_{base} is the cross sectional area of the base, in m^{2};
C_{p} is the circumference of the part of the pile shaft in the layer in which the base of the pile is placed, in m;
F_{max} is the maximum compressive resistance of the pile, in MN;
F_{max;base} is the maximum base resistance, in MN;
F_{max;shaft} is the maximum shaft resistance, in MN;
p_{max;shaft;z} is the maximum unit shaft resistance, at depth z, in MPa
p_{max;base} is the maximum unit base resistance, in MPa;
ΔL is the distance from the base of the pile to the bottom of the first soil layer above the base with q_{c} < 2 MPa; moreover ΔL ≤ the length of the enlarged part of the pile point if applied, in m;
z is the depth or vertical direction (positive downwards).
D_{eq} is the equivalent diameter of the base, in m;
where
a is the length of the smaller side of the base area, in m;
b is the larger side, in m, with b ≤ 1,5 × a;
(3) The maximum base resistance p_{max;base} can be derived from the following equations:
and
p_{max;base} ≤ 15 MPa
where
α_{p} is the pile class factor given in Table D.5,
β is the factor which takes account of the shape of the pile point as shown in Figure D.3;
β is found by interpolation between the borders given in Figure D.3
s is the factor which accounts for the shape of the pile base – and is established as follows:
where r is L/B
L is the larger side of the rectangular pile point;
B is the smaller side of the rectangular pile point;
φ' is the effective angle of shearing resistance.
q_{c;I;mean} is the mean of the q_{c;I} values over the depth running from the pile base level to a level which is at least 0,7 times and at most 4 times the equivalent pile base diameter D_{eq}, deeper (see Figure D.2);
with
0,7D_{eq} < d_{crit} < 4D_{eq}
At the critical depth the calculated value of p_{max;base} becomes a minimum;
q_{cII;mean} is the mean of the lowest q_{c;II} values over the depth going upwards from the critical depth to the pile base (see Figure D.2);
q_{cIII;mean} is the mean value of the q_{c;III} values over a depth interval running from pile base level to a level of 8 times the pile base diameter above the pile base, or, in case b > 1,5 × a up to 8 × a above the pile base. This procedure starts with the lowest q_{c;II} value used for the compulation of q_{c;II;mean} (see Figure D.2);
For continuous flight auger piles, q_{c;III;mean} cannot exceed 2 MPa, unless the results of CPTs, which were performed at a distance from the pile ≤ 1 m after pile fabrication are used for the calculation of the compressive resistance;
(4) The maximum shaft resistance p_{max:shaft;z} should be determined as follows:
p_{max;shaft;z} = α_{s} × q_{c;z;a}
where
α_{s} is the factor according to Tables D.5 and D.6;
q_{c;z;a} is the cut-off value of q_{c} at depth z, in MPa.
If q_{c;z} ≥ 12 MPa over a continuous depth interval of 1 m or more, then q_{c;z;a} ≤ 15 MPa over this interval.
If the depth interval with q_{c;z;a} > 12 MPa is less than 1 m thick, then q_{c} ≤ 12 MPa over this interval.
Pile class or type | α_{p} | α_{s} ^{a} |
Soil displacement type piles, diameter > 150 mm – driven prefabricated piles, – cast in place piles made by driving a steel tube with closed end. The steel pipe is reclaimed during concreting. |
1,0 1,0 |
0,010 0,012 |
Soil replacement type piles, diameter > 150 mm – flight auger piles, – bored piles (with drilling mud). |
0,8 0,6 |
0,006 ^{b} 0,005 |
^{a} Values valid for fine to coarse sands. For very coarse sands a reduction factor of 0,75 is necessary; for gravel this reduction factor is 0,5. ^{b} This value is used in the case of applying the results of CPTs which were carried out before pile installation. When CPTs are used that have been carried out in the vicinity of the flight auger piles, a, may be raised to 0,01. |
Soil type | MPa | α_{s} |
clay clay silt peat |
> 3 <3 |
< 0,030 < 0,020 < 0,025 0 |
Key
- 1 borderline 1; β = 1,0:
- 2 borderline 2; β = 0,9;
- 3 borderline 3; β = 0,8;
- 4 borderline 4; β = 0,7;
- 5 borderline 5; β = 0,6:
For H, D_{eq} and d_{eq} see Figure D.2
NOTE This example was published in NEN 6743-1. For additional information and examples, see X.3.1.