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 pb and unit shaft resistance ps, of cast in-situ piles are given as a function of cone penetration resistance (qc) (CPT) and normalised pile head settlement.

Table D.3 — Unit base resistance pb of cast in-situ piles in coarse soil with little or no fines
Normalised settlement s/Ds; s/Db Unit base resistance pb, in MPa, at average cone penetration resistance qc (CPT) in MPa
qc = l0 qc = 15 qc = 20 qc = 25
0,02 0,70 1,05 1,40 1,75
0,03 0,90 1,35 1,80 2,25
0,10(= sg) 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 Ds is the diameter of the pile shaft Db is the diameter of the pile base sg is the ultimate settlement of pile head
Table D.4 — Unit shaft resistance ps of cast in-situ piles in coarse soil with little or no fines
Average cone penetration resistance qc (CPT)
MPa
Unit shaft resistance ps
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 qc values from an electrical CPT. In case of over consolidation, or in case of an excavation after execution of a CPT, the qc values should be reduced.

(2) The maximum compressive resistance of a pile follows from:

Fmax = Fmax;base + Fmax;shaft

where

Fmax;base = Abase× pmax;base

and

where

Abase is the cross sectional area of the base, in m2;

Cp is the circumference of the part of the pile shaft in the layer in which the base of the pile is placed, in m;

Fmax is the maximum compressive resistance of the pile, in MN;

Fmax;base is the maximum base resistance, in MN;

Fmax;shaft is the maximum shaft resistance, in MN;

pmax;shaft;z is the maximum unit shaft resistance, at depth z, in MPa

pmax;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 qc < 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).

Deq 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 pmax;base can be derived from the following equations:

and

pmax;base ≤ 15 MPa

where

αp 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.

qc;I;mean is the mean of the qc;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 Deq, deeper (see Figure D.2);

with

0,7Deq < dcrit < 4Deq

At the critical depth the calculated value of pmax;base becomes a minimum;

qcII;mean is the mean of the lowest qc;II values over the depth going upwards from the critical depth to the pile base (see Figure D.2);

qcIII;mean is the mean value of the qc;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 qc;II value used for the compulation of qc;II;mean (see Figure D.2);

For continuous flight auger piles, qc;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 pmax:shaft;z should be determined as follows:

pmax;shaft;z = αs × qc;z;a

where

αs is the factor according to Tables D.5 and D.6;

qc;z;a is the cut-off value of qc at depth z, in MPa.

If qc;z ≥ 12 MPa over a continuous depth interval of 1 m or more, then qc;z;a ≤ 15 MPa over this interval.

If the depth interval with qc;z;a > 12 MPa is less than 1 m thick, then qc ≤ 12 MPa over this interval.

Table D.5 — Maximum values of αp and αs for sands and gravely sands
Pile class or type αp αs 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.
 
Table D.6 — Maximum αs values for clay, silt and peat
Soil type MPa αs
clay
clay
silt
peat
 > 3
<3
 
 
 < 0,030
< 0,020
< 0,025
0
Explanation of q
Figure D.2 — Explanation of qc;I, qc;II and qc;III

 

Pile foot shape factor β

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, Deq and deq see Figure D.2

Figure D.3 — Pile foot shape factor (β)

NOTE This example was published in NEN 6743-1. For additional information and examples, see X.3.1.

Eurocode 7: Geotechnical design — Part 2: Ground investigation and testing