Carlo Viggiani

Full Scale Loading Tests on Instrumented CFA Piles

Continuous flight auger piles (CFA) procedure allows the rapid installation of piles with diameters usually ranging from 0,4 to 1,0 m and a length up to 30-35 m, free from the vibration and noise of driven piles and substantially reducing the ground loosening of bored piles. CFA are becoming increasingly popular in many countries, and sometimes they are claimed to have the advantages of both driven and bored piles, without the related shortcomings. The behaviour of CFA piles, however, is actually affected by technological factors; among them the capacity of the equipment in terms of thrust and torque and the volume of concrete pumped in during auger retrieval. In the paper the results of three failure loading tests on instrumented CFA piles are presented. The analysis of the observed behaviours confirm that the CFA piles are somewhat intermediate between bored and driven piles. Introduction Continuous Flight Auger (CFA) piles are installed by means of an auger with a hollow stem having an inner diameter of 10 to 20 cm, inserted into the soil by the combined action of an axial thrust and a torque. The stem is provided with a temporary closure plate at the bottom; after the auger has reached the desired depth, the plate is pushed out by pumping concrete or mortar through the stem, and the auger is lifted removing from the ground the soil within the screw. The sides of the hole are thus supported at all times by the soil filled auger or by the pumped concrete. The procedure allows a rapid and noiseless installation of piles with diameter of 40 to 100 cm and length up to 30-35 m, and is becoming increasingly popular all over the world. The ratio between the rate of penetration and the rate of revolution of the auger is generally less than the pitch of the screw; the penetration involves thus a lateral compression but also the removal of some soil. Further removal of soil occurs during the extraction stage. If the volume of the removed soil is less than the final volume of the pile, the net resulting effect is a compression of the soil surrounding the pile; the resulting stress state within the soil is somewhat intermediate between that of a bored pile and that of a driven one.