Strain monitoring using distributed fibre optic Sensing in a pile load test

Abstract

Distributed fibre optic sensing is a relatively new monitoring method that has been trialled in Europe for strain measurement across various engineering disciplines. The research conducted focussed on the implementation of a distributed fibre optic sensing system in a bored pile in KwaZulu Natal, South Africa. A specialised fibre optic cable, which is sensitive to changes in strain and temperature was calibrated under laboratory conditions to determine the strain sensitivity and temperature sensitivity coefficients. Following the calibration testing, the fibre optic cable was cast into the pile that was load tested using an internal jack system. The research set out to determine the suitability of the fibre optic sensing system for use in South Africa, and compare the new monitoring system to the currently accepted industry standards, i.e. vibrating wire strain gauges. The results from the laboratory testing highlights the importance of calibration of the fibre optic cable to ensure accurate results are obtained during in-field testing. The in-field testing showed that distributed fibre optic monitoring is a viable option for strain and temperature measurements, even at small strains. The strain profiles measured by the fibre optic cable showed good correlation with what was observed using vibrating wire strain gauges and allowed for the development of a strain profile across the entire installed length. The successful in field installation also illustrates that the specialised fibre optic cable, despite the preconception that the fibre optic cable is delicate and fragile, is suitable for use in various scenarios and can be installed and used without a significant deviation in standard instrumentation installation procedures.