Drilling parameters as predictors of the measured full scale performance of CFA piles by using statistical analysis of CPT profiles: a case study

Abstract

The Continuous Flight Auger (CFA) piles, also known as Auger Cast In Place piles (ACIP), have a widespread use in the world market of replacement concrete piles. The installation of such piles is generally made by equipment which records several significant installation parameters, as the torque M_t and the axial force N, applied during the auger advancing step. In the auger retrieval stage other parameters that can influence the load-settlement performance of the pile are monitored. The paper presents a case study dealing with the CFA piles adopted as a foundation for a water treatment basin. In the paper first the equations governing the kinematical analysis of the augering and concreting stages of the pile construction are introduced. After the description of the test site with the subsoil conditions and the presentation of a typical sets of parameters recorded during pile installation the results of two pile loading tests to failure are reported. A new approach for determining the depth of the soil layers boundaries, based on the statistical analysis of CPT profiles with depth, is proposed and successfully applied confirming the qualitative geological analysis of the site. A further statistical analysis is carried out to find a robust correlation between the q_c of the CPT and the torque M_t needed to screw in depth the auger for pile construction. Finally, virtual CPT profiles generated from the torque M_t recorded during the tested piles installation are used for piles bearing capacity calculations which are compared with the results of pile loading tests. The successful comparison reported in the paper is a first step to validate the proposal to use parameters recorded during augering to calculate and/or to confirm design values of the bearing capacity.