Quantifying NDT Results for Structural Piling

Rehabilitating and inspecting civil structures susceptible to damage, deterioration, and environmental effects present an array of challenges. A major focus in this area is how to reliably estimate the length of structural foundation piles. One problem may be the absence of as-built plans, or where plans do exist, the question may arise as to whether the plans actually represent what was built. Nondestructive testing (NDT) using dispersive wave propagation mechanics has been used for many years in conjunction with side-impact flexural wave and parallel seismic testing on both partially exposed and buried piling. The underlying idea is to determine the time of flight for individual frequencies inside a mechanically generated stress wave, and their speed, in order to calculate their distances of travel. These distances of travel then are used to identify the distance to the pile tip or to a location of material damage. Performing a dispersive wave analysis using mechanical stress waves often presents a difficult interpretation challenge, as such wave motion is highly dispersive in slender structural members. To conduct this analysis, Fourier theories are often coupled with cross-correlation techniques. When doing so, many true and false positives can be identified, thus presenting the engineer with many locations along a pile that can represent material damage, pile tips, or other material or geologic anomalies. An ongoing topic by the authors has been to improve these analysis methods through more automated computational interpretations and data filtering. Through years of blind tests on piles whose lengths were unknown to the investigators, a significant database of computed versus recorded (or measured) pile test results has been assembled. The results have shown that the dispersive wave method is a reliable method of estimating pile lengths in the presence of the appropriate data analyses, approaches, and result interpretations.

References
1. Abramson, N.H., “Flexural Waves in Elastic Beams,” Journal of the Acoustical Society of America, pgs. 42-46, 1957. 2. Abramson, N.H., The Propagation of Flexural Elastic Waves in Solid Circular Cylinders, PhD Thesis, University of Texas at Austin, 1956. 3. Holt, J.D., Comparing the Fourier Phase and Short Kernel Methods for Finding the Overall Lengths of Installed Timber Piles, PhD Thesis, NC State University, 1994. 4. Bendat, J. and Piersol, A., Engineering Applications of Cross Correlation and Spectral Analysis, John Wiley 7 Sons, Inc. 1980. 5. Holt, JD, Chen, S, Douglas, RA, Determining Length of Installed Timber Piles by Dispersive Wave Propagation, Transportation Research Record No. 1447, Design and Construction of Auger Cast Piles and Other Foundation Issues, National Research Council, January 1994. 6. Douglas, RA, Holt, JD, Determining Length of Installed Timber Piles by Dispersive Wave Propagation, Final Report, North Carolina Department of Transportation, June 1993. 7. Holt, JD, Douglas, RA, A Field Test Procedure for Finding the Overall Lengths of Installed Timber Piles by Dispersive Wave Propagation Methods, North Carolina Department of Transportation, UNC Institute for Transportation Research and Education (ITRE), March 1994.
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