Abstract:
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.