Distributed strain measurement for pile foundations

A. Klar, P. J. Bennett, K. Soga, R. J. Mair, P. Tester, R. Fernie, H. D. St John and G. Torp-Peterson

Proceedings of the Institution of Civil Engineers, Geotechnical Engineering 159

July 2006 Issue GE3 Pages 135–144

Recent advances in strain measurement using optical fibres provide new opportunities for monitoring the performance of piles. Brillouin optical time-domain reflectometry (BOTDR) is an innovative technique that allows measurement of full strain profiles using standard optical fibres. Distributed optical fibre strain sensors were installed at two sites, and the details of the installation methods are presented. A theoretical analysis that compares pile behaviour evaluated from localised strain measurement at discrete locations and from distributed strain measurements is conducted for the case of a vertically loaded pile in layered soil. Two problems are considered: a load test aiming to establish the load transfer function, and changes in pile–soil interface stress due to nearby tunnelling. Advantages of such a distributed reading for monitoring the performance of pile foundations in layered soils are demonstrated from the results of both cases. Cost analysis results show that the BOTDR measurement is a cost-effective pile monitoring method compared with discrete strain measurement methods such as vibratingwire strain gauges and fibre Bragg gratings.

Keywords: foundations/geotechnical engineering/piles & piling

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