Annex B.4

(informative)

# Cone Penetration Test (CPT)

**(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 a CPT.

**(2)** The maximum bearing 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:

*F*_{max} is the maximum bearing 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;

*A*_{base} is the cross sectional area of the base, in m^{2};

*D*_{eq} is the equivalent diameter of the base, in m;

*D*_{eq} = 1,13*a/*(*b/a*)

where:

*a* is the length of the smallest side of the base area, in m;

*b* is the largest side, in m;

*p*_{max;base} is the maximum unit base resistance, in MN/m^{2};

*O*_{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;

Δ*L* is the distance from the base of the pile to the bottom of the first soil layer above the base with *q*_{c} < 2 MN/m^{2}; moreover Δ*L* ≤ the length of the enlarged part of the pile point if applied, in m;

*p*_{max;shaft} is the maximum unit shaft resistance, in MN/m^{2};

*z* is the depth or vertical direction (positive downwards).

**(3)** The maximum base resistance *p*_{max;base} can be derived from the following equation:

and

*p*_{max;base} ≤ 15 MN/m^{2}

where:

*q*_{c;l;mean} is the 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 B.2);

0,8 *D*_{eq} < *d*_{crit} < 4 *D*_{eq}

At the critical depth the calculated value of *p*_{max;base} becomes a minimum;

*q*_{c;II;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 B.2);

*q*_{c;III;mean} is the mean value of 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 higher. This procedure starts with the lowest *q*_{c;II}-value used for the computation of *q*_{c;II;mean} (see figure B.2);

For continuous flight auger piles *q*_{c;III;mean} may not exceed 2 MN/m^{2}, unless the results of CPT's which were performed at a distance from the pile > 1 m after pile fabrication are used for the calculation of the bearing resistance;

α_{p} is the pile class factor given in table B.2;

*β* is the factor which takes account of the shape of the pile point as shown in figure B.3;

*s* is the factor which accounts for the shape of the pile base as shown in figure B.4;

*OCR* is the Over Consolidation Ratio.

**(4)** The maximum shaft resistance *p*_{max;shaft} should be determined as follows:

*p*_{max;shaft} = α_{s}*q*_{c;z;a}

where:

α_{s} is the factor according to table B.2 and B.3;

*q*_{c;z;a} is *q*_{c} at depth *z*, in MN/m^{2}.

If *q*_{c;z;a} ≥ 15 MN/m^{2} over a continuous depth interval of 1 m or more,

*q*_{c;z;a} ≤ 15 MN/m^{2} over this interval.

When the depth interval with *q*_{c;z;a} > 12 MN/m^{2} is less than 1 m thick, *q*_{c} < 12 MN/m^{2} over this interval.

_{p}and α

_{s}for sands and gravely sands

Pile class or type |
α_{p} |
α_{s}^{1)} |

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. Soil replacement type piles, diameter > 150 mm — flight auger piles, — bored piles (with drilling mud). |
1,0 1,0 0,8 0,6 |
0,010 0,014 0,006 ^{2)}0,005 |

1) 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. 2) This value is used in the case of applying the results of CPT s which were carried out before pile installation. When CPT s are used that have been carried out in the vicinity of the flight auger piles, αsmay be raised to 0,01. |

_{s}-values for clay, silt and peat

Soil type |
relative depth z/d_{eq} |
α_{s} |

clay/silt (q_{c} ≤ 1 MN/m^{2})clay/silt ( q_{c} ≥ 1 MN/m^{2})clay/silt ( q_{c} > 1 MN/m^{2})peat |
5 < z/d_{eq} < 20z/d_{eq} ≥ 20not applicable not applicable |
0,025 0,055 0,035 0 |

d_{eq} equivalent pile shaft diameter |

*q*

_{c;I},

*q*

_{c;II}and

*q*

_{c;III}

*s*