Optical pore water pressure sensing for geotechnical applications

Abstract

Pore water pressure (PWP) monitoring is an essential aspect in geotechnical engineering. Accurate PWP measurement is necessary for correct representation of underground soil condition. Piezometer that allows simultaneous high resolution and large range measurement will be highly beneficial. In practice, conventional piezometers have shown to be reliable and robust however they have some shortcomings. The shortcomings are namely being prone to electromagnetic interference, labour intensive and time consuming. A fiber optic interferometric piezometer is designed to overcome the limitations of conventional piezometers. This piezometer is an optical fiber extrinsic Fabry-Perot interferometer (EFPI), which comprises of a fiber ferrule connector/flat contact (FC/FC) connector and an aluminium foil pasted on a thin metal plate. The thin metal plate is subjected to water pressure, causing deformation which obeys the small deflection theory. By adopting the conventional demodulation method used by FPI piezometer that is wavelength tracking method, measurement range is limited despite the high level of accuracy. Therefore, in this report, various demodulation methods were analyzed using the proposed piezometer to develop a suitable demodulation technique that allows simultaneous high resolution and large range measurement.