Measurement Biases in the Bender Element Test
Publication: Journal of Geotechnical and Geoenvironmental Engineering
Volume 133, Issue 5
Abstract
This paper reconsiders the signals that are transmitted in bender element tests and the associated transfer functions to identify the problems that bias the velocity (travel time) determination. The findings of the numerical and physical experiments show that errors appearing in the velocity estimation can be traced to (1) the near-field effect and (2) the influence caused by the transfer function of the receiver system, . The second factor also creates distinguishing changes between the source and receiver signals. In a two-receiver measurement system, the accurate travel time (velocity) can be identified by the cross correlation of the two receiver signals only if the near field is not pronounced and the two receivers possess very similar transfer functions. The near-field effect can be effectively avoided by placing the sensors at and , where and =distances between the source and the first and second receivers, respectively, and =wavelength. In a one-receiver measurement system, the inherent time delay caused by the transfer function of the receiver system, , has to be quantified and corrected. The near-field effect also can be avoided by placing the receiver at least away from the source.
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Acknowledgments
This research was supported by the Hong Kong Research Grants Council (UNSPECIFIEDHKUST 6034/02E) and the Hong Kong University of Science and Technology (UNSPECIFIEDHIA04/05.EG02). The writers are grateful to the reviewers for their valuable comments.
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Published In
Journal of Geotechnical and Geoenvironmental Engineering
Volume 133 • Issue 5 • May 2007
Pages: 564 - 574
Copyright
© 2007 ASCE.
History
Received: Aug 18, 2005
Accepted: Nov 1, 2006
Published online: May 1, 2007
Published in print: May 2007
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