Soil Testing Equipment

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)

• Fmax is the maximum bearing resistance of the pile;
• Fmax, base is the maximum base resistance;
• Fmax, shaft is the maximum shaft resistance;

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.

where:

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.