Development of an optical fibre sensor system for ground displacement and pore water pressure monitoring

• System design integrates multiple sensing units with a standard optical fibre sensor. • Enhance water pressure sensitivity by embedding optical fibre in flexing diaphragms. • Shape sensing of borehole installed tubes to detect horizontal ground displacement. • Subsurface tape with fibre monitors vertical ground displacement and transfers data. • Laboratory and field tests reveal future potential for real-time ground monitoring. Monitoring ground displacements and pore water pressure is crucial for enhancing disaster resilience and ensuring a safe living environment. Optical fibre sensors are preferred over conventional ground failure monitoring methods, providing continuous and distributed measurements across large areas. In this study, a standard optical fibre was used as a distributed strain sensor to simultaneously monitor multiple subterranean parameters: pore water pressure and bi-directional ground displacements. Design, fabrication, laboratory calibration, field testing and data interpretation of the sensor system were carried out. The sensor system features three sensing units: a vertical outer tube and a horizontal flexible tape sensitive to ground displacements and a flexible diaphragm sensitive to pore water pressure. Each sensing unit was interconnected through a continuous optical fibre. A series of laboratory experiments was conducted to calibrate the horizontal ground displacement and pore water pressure sensor units. Relationships between the measured strain and the displacement and water pressure were developed. Field testing was conducted in a controlled sliding environment by inducing horizontal and vertical ground displacements and supplying pressurised water. Strain data was acquired by Optical Backscatter Reflectometry to identify the location and intensity of the resulting ground displacements and water pressure. A minimum detectable displacement up to 0.2 mm was achieved for horizontal ground displacement. Also, a minimum detectable change of 3.62 kPa was obtained for pore water pressure. This technology can be repetitively implemented in high-risk areas such as dams, roads, and runways for ground failure monitoring and early warning.