Distributed fibre-optic strain sensing on cylindrical geomaterial specimens under triaxial stress, pore pressure, and temperature conditions

Characterising the mechanical behaviour of geomaterials under representative subsurface conditions requires simultaneous control of confining pressure, axial stress, pore pressure, and temperature. Conventional instrumentation for triaxial testing, comprising strain gauges and linear variable differential transformers (LVDTs), provides either localised point measurements or global average deformations, leaving gaps in capturing spatial strain heterogeneity, localisation phenomena, and end effects. Distributed strain sensing (DSS) based on fibre optics offers spatially continuous strain measurement along the fibre path, combining multipoint capability within a single sensing line and reducing wiring complexity through pressure boundaries.

We present the development and implementation of a fibre-optic DSS system integrated into a triaxial pressure cell capable of independently controlling confining pressure, pore pressure, axial stress, and temperature. Routing an optical fibre from the interrogator to the specimen surface requires passing through two critical pressure boundaries. First, a pressure-cell feedthrough carries the fibre through the vessel wall, maintaining seal integrity under confining pressure while preserving optical signal quality. This feedthrough was designed to minimize micro-bending and pinching at the sealing point and to provide mechanical decoupling, thereby preserving measurement integrity during pressure changes. Second, a specimen-sleeve feedthrough guides the fibre under the isolation sleeve that separates the specimen from the confining oil, demanding careful attention to minimum bend radii, strain relief at the sleeve edge, and avoidance of local stress concentrations to prevent fibre damage during pressurisation. Both entry points were developed to achieve leak-free and break-free operation throughout the experimental programme.

Optical fibres were bonded directly to the specimen surface and routed in both axial and circumferential orientations to capture axial and radial strain distributions, respectively.

Two specimen types were tested: an aluminium reference cylinder for calibration and validation of the DSS pipeline against known elastic properties, and a cement specimen serving as a geomaterial analogue. The test programme on the cement specimen included hydrostatic and deviatoric stress cycling at ambient and elevated temperatures (up to approximately 40 °C) with concurrent pore pressure control, enabling determination of elastic moduli, Biot's coefficient, and thermal expansion under drained conditions. DSS measurements were acquired alongside co-located strain gauges and LVDTs, with time-synchronised logging of all mechanical, hydraulic, and thermal boundary conditions.

This contribution describes the experimental design, fibre installation methodology, feedthrough development, and multi-sensor measurement strategy. Results from the experimental programme, including quantitative comparison of DSS-derived strains with conventional sensor data across the tested loading and temperature conditions, will be presented.