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 qc-values from a CPT.
(2) The maximum bearing resistance of a pile follows from:
Fmax = Fmax;base + Fmax;shaft
Fmax;base = Abase × pmax;base
Fmax is the maximum bearing resistance of the pile, in MN;
Fmax;base is the maximum base resistance, in MN;
Fmax;shaft is the maximum shaft resistance, in MN;
Abase is the cross sectional area of the base, in m2;
Deq is the equivalent diameter of the base, in m;
Deq = 1,13a/(b/a)
a is the length of the smallest side of the base area, in m;
b is the largest side, in m;
pmax;base is the maximum unit base resistance, in MN/m2;
Op is the circumference of the part of the pile shaft in the layer in which the base of the pile is placed, in m;
ΔL is the distance from the base of the pile to the bottom of the first soil layer above the base with qc < 2 MN/m2; moreover ΔL ≤ the length of the enlarged part of the pile point if applied, in m;
pmax;shaft is the maximum unit shaft resistance, in MN/m2;
z is the depth or vertical direction (positive downwards).
(3) The maximum base resistance pmax;base can be derived from the following equation:
pmax;base ≤ 15 MN/m2
qc;l;mean is the 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 B.2);
0,8 Deq < dcrit < 4 Deq
At the critical depth the calculated value of pmax;base becomes a minimum;
qc;II;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 B.2);
qc;III;mean is the mean value of of the qc;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 qc;II-value used for the computation of qc;II;mean (see figure B.2);
For continuous flight auger piles qc;III;mean may not exceed 2 MN/m2, 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;
αp 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 pmax;shaft should be determined as follows:
pmax;shaft = αsqc;z;a
αs is the factor according to table B.2 and B.3;
qc;z;a is qc at depth z, in MN/m2.
If qc;z;a ≥ 15 MN/m2 over a continuous depth interval of 1 m or more,
qc;z;a ≤ 15 MN/m2 over this interval.
When the depth interval with qc;z;a > 12 MN/m2 is less than 1 m thick, qc < 12 MN/m2 over this interval.
|Pile class or type||αp||αs1)|
|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) 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.
|Soil type||relative depth z/deq||αs|
|clay/silt (qc ≤ 1 MN/m2)
clay/silt (qc ≥ 1 MN/m2)
clay/silt (qc > 1 MN/m2)
|5 < z/deq < 20
z/deq ≥ 20
|deq equivalent pile shaft diameter|