Comparing the Fourier phase and short kernel methods for finding the overall lengths of installed timber piles

Timber piles are widely used as the primary means of support for many structures. Determining their in-place lengths represents a major problem. Only by knowing a pile's length can its embedment be determined and remaining load bearing capacity evaluated. A research project was conducted in the Civil Engineering Department of North Carolina State University to determine the overall lengths of installed timber piles by dispersive wave propagation methods. Dispersive time records were analyzed by the Short Kernel Method (SKM) of digital signal analysis. Another signal processing technique is available known as the Fourier phase method. Since both techniques exist, they were compared to determine whether one better approximated the wave speeds and locations of frequencies inside dispersive time records by offering a closer solution to the overall length problem.; Forming a comparison also required an experimental investigation be conducted to determine the effects of a pile's tapered geometry upon the wave speeds of a dispersive flexural wave. Computations indicated that a frequency's phase velocity decreases in value as a signal moves to areas of lesser diameter, and increases as a signal moves through areas of increasing diameter. Mathematical expressions were derived to relate taper to wave speeds. These expressions were then incorporated into the algorithms and programs used for computing a pile's length.; Laboratory pile models and installed field piles were used to conduct overall length experiments for comparing the Fourier phase method and the SKM. These specimens offered a way of comparing the two methods using known solutions: the true measured lengths. The SKM method was found to offer a better approximation to a frequency's wave speed and location inside a dispersive time record by giving a closer solution to the length problem. Errors between the computed and measured overall lengths from the Fourier phase method ranged from -55% to +75.4%. Errors from the SKM method ranged from -36.3% to +13.4%.