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USE OF CONE PENETRATION TEST IN PILE DESIGN

USE OF CONE PENETRATION TEST IN PILE DESIGN

Introduction

The prediction of pile capacity is complicated by the large variety of soil types and installation procedures. Many methods have been proposed to predict the axial capacity of single piles. These methods can be divided in three main groups:

  1. Full scale pile load test: This test exactly describes the piles behaviour with a load-settlement curve. At the moment this is the best method to predict the capacity of a single pile. The disadvantages of this method are: the costs of such a test are high, and it is rarely feasible in the stage of planning.
  2. Calculation methods based on results of laboratory tests: The low accuracyof these methods makes their economical use difficult.
  3. Calculation methods based on results of ‘in-situ’ tests: Among the numerous in-situ devices, the cone penetration test (CPT) and the piezocone (CPTu) represent the most versatile tools for geotechnical design. One of the earliest applications of this device was to predict a pile’s axial capacity. As a model pile it is pushed into the ground and measurements are made of the resistance to penetration of the cone. Using this test the pile capacity can be predicted time- and cost-efficiently even in the stage of planning. Nevertheless the accuracy of thismethod will not achieve that of full scale tests, but the reliability of predictions based on CPT is improving.
Test Site

Most of the tests were performed at the construction of highwayM3 in Hungary, but other tests were also performed at different sites in Hungary. The tested piles were CFA (continuous flight auger) piles. The length of the piles ranged from 6.20 m to 22.00 m, the diameter ranged from 0.60 m to 1.00 m. The pile load tests were performed in different soil conditions.

Interpretation of Pile Load Tests

The ultimate bearing capacity of the tested piles, determined by the method described in the Hungarian Code (MI-04-190), ranged from 830 kN to 3900 kN. The pile axial capacity consists of two components: end bearing load (base resistance) and side friction load (shaft resistance). At the pile load tests only total capacity of the piles is measured, so it was necessary to calculate the base resistance and the shaft resistance of the piles separately, based on the results of the performed tests. These values were determined based on detailed analysis of the test results and soil conditions.

Prediction of Pile Capacity Using CPT Data

The maximum bearing resistance of each single pile was predicted using the following methods:
• DIN 4014 (German Standard) method
• Bustamante and Giasenelli (1982) method (LCPC method)
• EUROCODE 7-3 method (process recommended by EC 7-3)
• ERTC3 method (De Cock, F. – Legrand C., 1997)

In all cases the pile capacity is derived from the known formula:
Fmax = Fmax, base + Fmax, shaft ,

where: 
  • Fmax is the maximum bearing resistance of the pile;
  • Fmax, base is the maximum base resistance;
  • Fmax, shaft is the maximum shaft resistance;
DIN 4014 Method

The German Standard provides different methods for cohesive and non-cohesive soils. In case of non-cohesive soils the unit base and shaft resistance of a bored pile can be calculated using the following tables (Table1, Table 2):





For the calculation of unit base resistance the average cone tip resistance is determined between the pile tip and depth of three times the pile diameter under the tip. In case of cohesive soils the unit base resistance and the unit skin friction can be determined with an indirect method, using the following tables (Table 3, Table 4):



Similarly to the non-cohesive case average c u is determined between the pile tip and depth of three times the pile diameter under the tip.

So the unit base and shaft resistance are calculated using the undrained shear strength (cu) of cohesive soils. There are numerous methods to predict c u based on CPT values, but the method used for this purpose has a great influence on the estimated pile capacity. So the final predicted pile capacity may be different using
different methods for undrained shear strength prediction.



Bustamante and Giasenelli (1982) Method

In this case the unit base and shaft resistances of a pile are calculated directly from the cone resistance (qc) using the following equation:



where:




EUROCODE-7-3 method

In this method the maximum unit base and shaft resistance can be derived form the following equations:


where :

qc,I, mean : is the mean of the qc,I values over the depth running from the pile base level to a level (critical depth) which is at least 0.7 times and at most 4 times the pile base diameter deeper. (Critical depth: where 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




qc,III, mean : is themean value of the qc,III values over a depth interval running from the 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 computation of qc,II, mean.



qc,z is qc at depth z
αp is the pile class factor
αs is a factor depending on the pile class and soil conditions (Table6).


ERTC3 Method

This method has the same process for calculation of unit base resistance as the EC 7-3 method, but the determination of the shaft resistance is modified. It can be calculated with the same process, but αs values are different (Table 7).

A. MAHLER



Fig. 1. DIN 4014 method


Fig. 2. LCPC method

Reliability of the Prediction Methods

In the following figures the results of the pile capacity predictions are shown. In each figure the predicted pile capacity values are plotted against the results of pile load tests (measured pile capacity). The continuous lines represent the perfect prediction ‘zone’.

Fig. 1 shows the results of the DIN 4014 method. As it was mentioned earlier it is an indirect method in case of cohesive soils, so the prediction method of undrained shear strength (cu ) has a serious effect on the calculated pile capacity. So it is obvious that the reliability of this method is varying with various c u prediction
methods.

LCPC (BUSTAMANTE and GIASENELLI, 1982) method provides a direct calculation process using solely the measured cone tip resistance (q c) values. The results of this method are shown in Fig. 2. By comparing the two figures we can say that this process gives similarly accurate pile capacity values. Fig. 3 shows the results of the EUROCODE 7-3 method. Among the investigated methods this one has the most complicated process for estimation of unit base resistance of a pile. This calculation process results in very accurate results, but the prediction of unit skin friction is less accurate in some cases. As it can be seen in Fig. 3 in these particular cases the pile capacities are extremely overestimated. These piles were of large diameter, long piles in stiff clays of high plasticity and in each case the skin friction of the piles were overestimated.

The ERTC 3 method has the same process for determination of unit base resistance, but the calculation process of unit skin friction is modified. In Fig.4 the results of this prediction method are shown. We can see that this method gives reliable results even in cases in which the pile capacities were extremely overestimated by EUROCODE 7-3 method.


Fig. 4. ERTC 3 method


Summary and Conclusion

Four pile capacity methods were evaluated using CPT data for thirteen full scale axial pile load tests. The test piles were CFA piles with various geometry and were measured in various soil conditions. DIN 4014, LCPC, EUROCODE 7-3 and ERTC 3 methods were used for pile capacity predictions.

Although the number of performed tests is not enough for statistical analysis, the results of performed tests are summarized in the following table (Table8):

In case of the EUROCODE 7-3 method the standard deviation and average estimated value are higher than the respective values of othermethods. Nevertheless these differences are caused by the overestimation of pile capacities of some piles. Without these results the EUROCODE 7-3 method gives similarly reliable results for pile capacity.


Table 8. Results of performed tests


It is necessary to note that the pile capacities were generally overestimated. To decide whether it is caused by the differing soil conditions or by differences in the pile installation process requires further studies. Nevertheless it is clear that these methods represent a useful tool in geotechnical design.

References

  1. BUSTAMANTE,M.–GIANESELLI, L., Pile Bearing Capacity Prediction by Means of Static Penetrometer CPT. Proceedings of the 2nd European Symposium on Penetration Testing, ESOPT-II, Amsterdam, 2 (1982), pp. 493–500.
  2. DE COCK,F.–LEGRAND, C. ed. Design of Axially Loaded Piles, European Practice, 1997.
  3. DE COCK,F.–LEGRAND,C.–LEHANE,B.ed. Survey Report on Design Methods for Axially Loaded Piles, European Practice, 1999.
  4. EUROCODE 7: Geotechnical Design – Part 3: Design Assisted by Fieldtesting; European Committee for Standardization, 1999.
  5. DIN 4014 Bohrpfähle – Herstellung, Bemessung und Tragverhalten; Deutsche Institut für Normen, 1990.

Source : PERIODICA POLYTECHNICA SER. CIV. ENG. VOL. 47, NO. 2, PP. 189–197 (2003)

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