Geotechnical research has come a long way since the formal establishment of soil mechanics in the early 1900s. With the advent of sophisticated instrumentation and advanced technologies, various innovative tools have been developed in recent years to explore soil response under complex loading conditions and its implications for geostructures. Owing to the inherent heterogeneity of soils and the complexity of their surrounding environments, conventional approaches are sometimes insufficient to fully characterise geotechnical problems. This issue of Géotechnique Letters primarily focuses on the development of innovative experimental tools for monitoring soil response, spanning scales from individual grains to field and kilometre scales (indirectly). The eight letters issue covers a broad range of geotechnical applications, from transportation geotechnics and submarine landslides to artificial ground freezing and thermally induced stresses in sand, while employing unconventional yet promising experimental approaches.
Field-scale prototype testing is among the most effective experimental approaches for understanding geotechnical phenomena and structural response. Submarine gravity flows, which play an important role in shaping submarine geology, can travel hundreds of kilometres. However, experimental modelling of such field-scale phenomena is challenging because of their enormous spatial extent and complex flow behaviour. Chen et al. (2026a) developed a centrifuge set-up to simulate underwater slurry flow over a sand bed under 100g conditions. For gravity flow, kaolin slurry was released under a ‘dam-break’ condition. The set-up enables measurement of the advancing flow front and the undrained response of the seabed. The experiments also revealed the formation of a thin water film between the seabed and the debris flow, which delays seabed deformation compared with subaerial debris flows. Qi et al. (2026) introduced a field-scale system for artificial ground freezing using carbon dioxide (CO2), which offers an efficient alternative to conventional brine-based systems. Liquid carbon dioxide is used as the circulating refrigerant, minimising refrigerant loss. The proposed system achieved faster cooling rates across different soil types, including clay, silt, and sand, despite their differences in thermal conductivity.
Laboratory-based small-scale experiments remain among the most widely used approaches for characterising geomaterials because of their relative simplicity and well-controlled conditions. This issue presents four novel experimental approaches for geomaterial characterisation that facilitate the determination of constitutive parameters and the development of predictive models. Singh et al. (2026) introduced a custom-built high-stress permeability cell for investigating creep behaviour. The set-up enables long-term assessment of creep in tailings materials. Particle breakage and the associated evolution of permeability can be monitored under sustained loading without unloading the specimen. The experimental data further enable the development of a strain-rate-dependent particle-breakage model, which is coupled with an analytical permeability model incorporating changes in porosity and particle-size distribution.
Chen et al. (2026b) proposed a large-diameter drop-weight testing apparatus to investigate the role of rubber in cushioning ballast under cyclic loading. A reduction in settlement and ballast breakage was observed with increasing rubber content under repeated drop-weight loading. Li et al. (2026a) developed a modified direct-shear apparatus to investigate soil–structure interface behaviour at elevated temperatures. The interface temperature is controlled by circulating heated liquid from a thermostatic bath, while the effects of different shear strain rates are also incorporated into the testing programme. The results indicate that interface adhesion is influenced by the duration of thermal loading, while the maximum temperature also affects the interface response. In another study, Li et al. (2026b) employed laboratory compression and tensile tests to investigate fracture development in composite-lined caverns associated with hydrogen embrittlement. Different gas-storage pressures were considered to identify the onset of crack propagation in the rock and the development of plastic strains around the lining using finite elements.
Over the past couple of decades, microstructural analysis of soils has gained considerable attention with the advancement of X-ray tomography, image processing, and numerical simulation techniques such as the discrete element method. To investigate the evolution of soil microstructure under different thermo-hydro-mechanical conditions, specialised experimental set-ups compatible with X-ray tomography are increasingly being developed. Mondal et al. (2026) developed a simple-shear apparatus that can be operated within a synchrotron during X-ray scanning. A conventional simple-shear device was modified using X-ray-transmissive confining rings and an oedometric loading frame, providing an unobstructed view of the evolving soil microstructure. The experiments capture changes in soil fabric and coordination number during simple shearing. Finally, this issue includes a numerical study by Morimoto et al. (2026), which uses discrete element simulations to investigate how thermal stresses affect particle-volume changes and, consequently, the evolution of fabric anisotropy.
In summary, this issue presents a rich blend of innovative experimental and a few associated numerical approaches spanning field-scale geotechnical applications, element-scale laboratory testing, and microstructural characterisation. These effective yet relatively simple approaches provide new opportunities to the readers for understanding and characterising a diverse range of geomaterials, including rock, tailings, submarine soils, ballast, and conventional soils. Collectively, the studies demonstrate how advances in experimental technology can bridge different length scales and provide deeper insights into the complex behaviour of geomaterials.
