Guided wave interpretation of surface reflection techniques for deep foundations



Nondestructive testing techniques that utilize surface reflections, including sonic echo and impulse response methods, have been used for many years to verify the length or to identify gross defects of drilled shafts and concrete piles. To interpret results of these tests, one assumes conditions of one-dimensional wave propagation along a pile, Consequently, the wavelength of the propagating stress wave is large — at least larger than the diameter of the pile — and the wave pulse is considered non-dispersive, i.e., the wave velocity does not vary with frequency. If the wavelength of the stress wave becomes less than the diameter of the pile, one-dimensional conditions no longer exist, and the waves become dispersive, i.e., the propagation velocity becomes a function of its frequency. By treating the pile as a wave guide, the frequency equation can be derived for cylindrical piles embedded in soil. The solution to this equation describes the relation between frequency and propagation velocity for various modes ofwave propagation. The guided wave approach to wave propagation in cylindrical concrete piles and drilled shafts is summarized herein. The solution to the dispersion equation and its implications are presented, and the conventional surface reflection methods are evaluated in light of the theory. The theoretical results concerning propagation velocity, attenuation, and displacements are presented, and compared with impulse response test results. The results show that the frequencies that are consistent with a one-dimensional interpretation, and hence that will provide reliable information in conventional tests, primarily depend on the diameter ofthe pile. The theory indicates that for drilled shafts with diameters of in or greater, one can expect reliable information up to 1500 Hz. For drilled shafts or piles with diameters of 1 mor less, one can expect reliahle information up to 1000 Hz.

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AUTORI: Finno R.J., Popovics J.S., Hanifah A.A., Kath W.L., Chao H.-C., Hu Y.-H.
Numero di pagine: 76

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