Applications of fast fourier transform in the integrity evaluation of deep foundations

Abstract

The widespread use of drilled shafts and the large and critical nature of loads they carry make post-construction quality control a rather important aspect of inspections. For this reason, states have implemented specific procedures to follow when performing inspections of deep foundations. The current accepted approach is to compare, at discrete points of each production shaft, the theoretical concrete volume to that actually placed. Only those shafts with a difference that exceeds a certain prespecified percentage are subjected to additional scrutiny using nondestructive testing methods. This paper reports on an innovative and comprehensive method proposed for the quality control of drilled shafts that overcomes several limitations of conventional methods. The method is developed and validated using experimental data, and is based on applying the fast Fourier transform mapping algorithm on data obtained from cross-hole sonic logging. Tomographic imaging and regression analysis are subsequently used to improve the accuracy of the integrity testing to a degree that facilitates the determination of both the diameter of the shaft and the exact location and size of anomalies.