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Advances in pile integrity testingNormal access

Authors: J.-P. Ertel, E. Niederleithinger and M. Grohmann
Issue: Vol 14, No 6, December 2016 pp. 503 - 512
DOI: 10.3997/1873-0604.2016030
Language: English
Info: Article, PDF ( 1.6Mb )

Abstract:
For decades, the low-strain impact integrity testing using a hammer blow is well established as a method of quality assurance for various pile types. However, this method has its limitations. Our research and development focuses on improving the excitation signal using a shaker system in contrast to the standard hammer method. Another approach is to increase the amount of sensors used during testing. The purpose is to identify the direction of wave propagation that gives advantages under difficult conditions, such as piles below structures. Pile integrity testing using a shaker system was performed on two 11-m-long piles of 90 cm in diameter. While one pile was intact, the other one showed a flaw at approximately 3.5 m below pile top, which was confirmed by standard pile integrity testing in 2012. A logarithmic sweep between 500 Hz and 1 KHz of 0.1 s was used as the input signal, being vertically injected into the pile. Prior to that, simulations on similar pile geometries showed that the depth of the pile toe as well as flaws within the pile can be extracted by applying regularised deconvolution. The result is the impulse response in the time domain. The application of deconvolution on the measured signals shows that it is possible to identify the pile length, but it is more difficult to clearly extract the flaw’s position in the pile. Additional digital signal processing techniques and the improvement of the regularised deconvolution method and the experimental setup need to be investigated. Another way to improve the pile integrity testing method is to use a multi-channel sensor arrangement. By arranging several accelerometers vertically along the accessible part of the pile shaft, it is possible to distinguish between downward and upward travelling waves. Furthermore, it is possible to estimate the unknown wave speed, which gives the possibility of more accurate pile length calculations. The method was evaluated successfully during a measurement campaign of a slab foundation with subjacent piles. In 20 of 28 cases, the pile length could be detected accurately.


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