The growing complexity of geosystems requires novel experimental methods capable of resolving coupled hydro-mechanical processes with high spatial and temporal detail far beyond the reach of traditional approaches. Conventional element-level laboratory tests rely on point-based sensors and cannot resolve how deformation is distributed along the specimen. As a result, these tests are interpreted under representative-element-volume (REV) assumptions and struggle to capture localisation, anisotropy and heterogeneity. This study demonstrates how distributed fibre optic (DFO) sensing enables such tests to be interpreted as boundary-value problems, offering a new level of observability for geomaterials. A high-pressure triaxial device was developed at EPFL to conduct long-duration multiphase flow experiments on Opalinus Clay, the host rock selected for the Swiss radioactive waste repository. DFO sensors installed along the specimen surface delivered more than 1500 spatially distributed strain measurements with sub-millimetric spacing under high-pressure conditions. A dedicated data processing workflow, based on machine learning outlier detection, converted raw data into reliable strain profiles. During water resaturation, injected from both ends, DFO measurements captured the advance of the saturation front through symmetric swelling strains propagating toward the specimen centre. During subsequent gas injection, deformation localised near the gas entry region and evolved upward in time, capturing the interplay between porewater displacement and gas migration. Importantly, the spatially continuous strain data enabled direct observation of bedding-induced anisotropy, revealing deformation modes that would otherwise remain unseen. By extending spatial resolution far beyond traditional instrumentation, DFO transforms geomechanical element testing interpreted under REV assumptions into direct boundary-value observation. This shift provides additional constraints for constitutive, numerical and data-driven model development and offers a step change in experimental geomechanics. The approach improves the interpretation of coupled processes and opens new pathways for the analysis and modelling of complex geomaterials across a wide range of subsurface engineering applications.