Development of a pore pressure sensor employing fibre Bragg gratings

Monitoring pore pressure is important to understand and predict the mechanical behaviour of soil, helping engineers to assess the stability of slopes and built infrastructures. The instrumentation used to monitor pore pressure should provide dense or extended spatial monitoring of the pore pressure and facilitate multiplexing with other sensors to form a multi-parameter monitoring system. The aim of this research was to develop a Fibre Bragg Grating (FBG) pore pressure sensor for soil applications, satisfying the typical measurement requirement of 1 kPa resolution over a 300 kPa measurement range with the potential for multiplexing. The technique used to develop the sensor consisted of transducing pressure into a transverse load applied to the central section of an FBG. This loading configuration induces a narrow spectral drop-out in the reflection spectrum of the FBG that tracks across its bandwidth in response to the applied load. The effect of this loading configuration on the reflection spectrum of a bare FBG was modelled with the aim of optimising the sensor range and resolution. An improvement of the sensor sensitivity to transverse load was obtained using a novel packaging technique that consisted of embedding the central section of the FBG within an epoxy cube. The deformation of the epoxy cube in response to transverse load resulted in the application of an axial strain to the embedded section of the FBG, which improved the load sensitivity. Moreover, this technique provided an efficient protection of the fibre against mechanical damage. A sensor housing was designed to allow the amplification/reduction of the load resulting from the pressure applied to a diaphragm. A pressure resolution of 0.2 kPa over a 100 kPa measurement range was obtained using a 6 mm long FBG with a 2 mm long section embedded in a epoxy cube which satisfies the sensor requisites.