Pressure and thermal effects on Rayleigh fiber-optic strain measurment for soil-structure interaction

Abstract

Optical strain sensing in civil engineering has been adopted for both field applications and advanced laboratory testing of structural health monitoring. Rayleigh backscatter devices (ROFDR) are presented for use with geotechnical centrifuge research since they offer distributed sensing capabilities, and through this study have been shown to have negligible interference from pressure effects, and can be made with low-cost disposable sensors. A comparison between a single channel and multi-channel fiber optic rotary joint (FORJ) is presented in the context of transmitting optical strain data across a rotating interface. The orthogonal pressure effects (eg. From soil) of a free-floating fiber under isotropic pressure was less than 0.32 με / kPa and that the pressure effect on a fiber bonded to a metal surface was below the detection limit of the instrument, 1 με, for an applied pressure of 60 kPa. The ROFDR system showed highly repeatable measurement of a constant temperature reading through the use of a water bath experiment. The system is stable to +/- 10 microstrain within 2-sigma for a >12 hr constant temperature test. An example case of a pipeline buried in a slope experiencing a landslide is presented where the optical strain sensing is used to capture strain pairs along the crownline and pipe invert to capture bending moment of the pipeline. Geotechnical centrifuge modelling in a 1 metre drum was carried out using a multi-channel FORJ coupled with an ROFDR system.