The surface and subsurface of the Earth are essential sources of energy and materials for modern societies. This fact alone renders subsurface geotechnologies a fundamental component in the equation that must be solved to achieve global decarbonisation targets. Yet such an ambitious goal extends far beyond purely technological matters. It is, rather, an endeavour that transcends generations, requiring societies to redefine through a long-term perspective how energy is produced, stored and utilised, as well as how the by-products of these processes are safely managed. Much like sailors navigating troubled waters who must resist the alluring call of the Sirens to reach safe harbour, advanced societies must likewise rely on science-based engineering frameworks capable of guiding the responsible development of subsurface technologies for the benefit of future generations.
The 18 letters collected in this themed issue reflect the widening role of geotechnical engineering in this challenging global context, as well as the enormous potential that stems from the interaction of fundamental advances in geotechnics with a host of related disciplines. Despite their remarkable diversity, all contributions indicate a path to foster progress in a range of low-carbon technologies, from reservoir-scale solutions for carbon dioxide storage and hydrogen extraction, to geothermal technologies able to harness the heat of the subsurface for clean energy production.
A defining feature of this emerging field is that subsurface decarbonisation technologies cannot be understood without holistic, multi-physical lenses able to merge a host of interacting mechanical, hydraulic, chemical and thermal processes in the subsurface. Carbon storage, geothermal energy, hydrogen storage, mineralisation and low-carbon ground improvement all involve geomaterials whose properties evolve during operation. Pores clog or reopen, minerals dissolve or precipitate, fractures channelise flow, thermal cycles accumulate deformation and gas–liquid mixtures alter the propagation of mechanical signals. This theme runs throughout the contributions to this issue and provides a common thread linking applications that might otherwise appear distinct.
Geological carbon storage provides one of the clearest examples of this challenge. Its success depends on the ability to predict injectivity, containment and caprock integrity over time scales that extend far beyond conventional geotechnical design horizons. The contribution by Stavropoulou et al. (2026) addresses this problem by proposing a novel metre-scale experimental methodology for geological carbon dioxide (CO2) storage. Their work targets the difficult intermediate region between element-scale laboratory testing and field-scale observations, where spatial heterogeneity, stress state and coupled hydromechanical responses collective play a crucial role. This contribution highlights that many uncertainties in carbon dioxide storage arise not from the absence of physical understanding at a single scale, but from the difficulty of transferring such understanding across scales. The long-term integrity of storage formations is also governed by chemical interactions between injected fluids and geomaterials. Vespo et al. (2026) examine this issue proposing a geochemical calibration strategy for carbonate clay caprocks exposed to acidic conditions, as the one induced by carbon dioxide storage. Their study highlights the importance of identifying model parameters able to capture the competing dissolution of carbonate minerals and the associated chemically induced strains. This work reinforces a broader message central to geological storage: the sealing capacity of a caprock is not a fixed material property, but the outcome of evolving interactions among transport, mineral reactions and deformation. Monitoring such evolution is the focus of the letter by Ciancimino et al. (2026), who investigate the use of elastic wave measurements to track the degradation of structured clay under monotonic and cyclic loading. Their results show that traditional wave velocity measurements are relatively insensitive to localised destructuration, whereas signal-based indicators such as normalised cross-correlation can capture changes associated with the formation of shear bands. The practical implication is important for underground storage systems, where early detection of degradation may require monitoring parameters that are more sensitive than conventional stiffness proxies. A complementary perspective is provided by Molina-Gómez et al. (2026), who study P-wave propagation in partially saturated sands containing either air or carbon dioxide. Their work reveals threshold-like changes in wave velocity as saturation evolves and shows that the type of pore gas can significantly influence wave propagation. Together, these contributions emphasise the growing relevance of wave-based methods for characterising gas-bearing geomaterials and for interpreting the integrity of storage systems.
As emerged from the previously mentioned contributions, this issue also highlights the relevant role played by reactive transport and mineralisation for subsurface decarbonisation strategies. These processes are in fact central to several decarbonisation pathways, from geological carbon cycling to engineered mineral carbonation and reactive ground improvement. The paper by Asem et al. (2026) investigates chemical weathering in ultramafic rocks exposed to carbon-bearing fluids, with attention to the influence of serpentinisation, microstructure and rock–water ratio on carbonate alkalinity production. Their findings show how geochemical reaction rates depend strongly on the initial state and available reactive surface area of the rock. This type of work is essential because mineral trapping and alkalinity generation are often discussed as large-scale carbon management mechanisms, yet their effectiveness ultimately depends on pore- and mineral-scale processes that regulate reaction pathways. The need to identify the characteristic scales of such processes is addressed directly by Voller et al. (2026), who introduce a first-order model for subsurface mineralisation controlled by the competition between transport, reaction and pore clogging. Their analysis shows that mineral storage is maximised when reaction and transport rates are balanced, thereby identifying a characteristic length scale for mineralisation operations. In a related contribution, Budek et al. (2026) explore mineral replacement under advective flow and show that self-blocking mechanisms can, under certain conditions, improve the spatial efficiency of replacement. Rather than allowing flow to concentrate permanently into a few dominant channels, repeated blocking and rerouting can distribute reaction fronts more uniformly through the porous medium. Both letters reveal how processes that may appear undesirable, such as clogging or localised flow diversion, can become useful if properly understood and controlled. The mechanics of mineral growth and reaction-induced deformation is further examined by Yang et al. (2026), who revisit the concept of crystallisation pressure through a poromechanical framework that accounts for the compliance of both the crystals and the surrounding porous skeleton. Their analysis challenges the direct use of classical expressions for crystallisation pressure in deformable geomaterials and shows that the stiffness of each solid phase can significantly affect the stress generated during crystal growth. Closely related issues arise in the paper by Detournay et al. (2026), who formulate a Damköhler-number criterion for reaction-driven cracking in hydration experiments. By linking reaction kinetics, hydraulic transport and tensile stress generation, their work provides a mechanistic framework for understanding when hydration reactions can trigger fracture. These contributions point to an increasingly important frontier in geomechanics: the need to treat mineral reactions not merely as chemical transformations, but as processes capable of generating stress, deformation and damage. The engineering potential of carbonation is illustrated by Gallant et al. (2026), who investigate accelerated mineral carbonation in lime-treated soils under low-pressure advective carbon dioxide flow. Their experiments show how carbonation dynamics depend on water and lime contents, with degree of saturation exerting a dominant control on gas continuity and reaction efficiency. This finding is directly relevant to low-carbon ground improvement, where the aim is not only to stabilise soils but also to promote durable carbon sequestration. A similar ambition motivates the study by Hanafi et al. (2026), who examine the use of carbon nanotube char, obtained as a by-product of hydrogen production, as a partial cement replacement in stabilised soft sensitive clay. Their results reveal both the promise and complexity of combining alternative binders with accelerated carbonation: carbon uptake may increase, but the balance between carbonate formation and cementitious hydrate development must be carefully controlled to avoid compromising mechanical performance. These two letters demonstrate that low-carbon geotechnical construction is much more profound than replacing one binder with another, in that it requires a mechanistic understanding of how new materials, curing conditions and carbonation pathways interact.
Beyond carbon storage and mineralisation, the subsurface is increasingly becoming a key component of low-carbon energy systems also at the scale of urban settings. The contribution by Arson et al. (2026) presents Cornell University’s vision for district heating through an enhanced geothermal system. This work does not only address the potential of geothermal energy for direct heat applications, but it also frames the problem as an integrated geosystem requiring the synergistic collection of geological, hydrological, thermal, mechanical and seismic data streams to be effectively optimised. This is representative of a broader transition in geoenergy research: geothermal systems are no longer considered only in terms of resource temperature or energy yield, but also in terms of coupled subsurface behaviour, operational risks and integration with energy infrastructure.
Thermal processes are also central to the performance of thermo-active geotechnical structures. Rafai et al. (2026) develop and implement a thermo-elasto-plastic constitutive model incorporating thermally accelerated creep to simulate the behaviour of energy pile systems. Their results show that irreversible pile settlements may be strongly influenced by the accumulation of volumetric contraction in the surrounding soil during thermal cycles. This contribution points out that, although energy geostructures are often promoted as efficient low-carbon technologies, their long-term serviceability depends on subtle thermo-mechanical mechanisms that are not always captured in conventional design. Within this broad theme, Nouri et al. (2026) used physical modelling to examine the effectiveness of a closed-loop steam-driven system used as a pre-construction ground thawing and improvement technique in permafrost. Their study demonstrated that the steam-driven heating probe effectively thaws permafrost soil, producing rapid radial heat transfer across its depth. These results offer important insights into the development of climate-adaptive approaches to enhance ground stability and northern infrastructure resilience, especially due to their potential to be integrated with drainage and preloading to accelerate consolidation. At the material scale, Mehraeen et al. (2026) investigate the evolution of granular assemblies subjected to thermal cycling and identify the emergence of terminal densities controlled by temperature amplitude. Their findings suggest that cyclic thermal loading can drive granular materials towards asymptotic states in ways that differ from cyclic mechanical loading. Together, these studies show that the design of geoenergy systems requires a deeper understanding of how repeated temperature variations affect both soil fabric and soil–structure interaction.
Underground gas storage, and hydrogen storage in particular, represents another rapidly developing area in which geotechnical insight is essential. Bang et al. (2026) propose a poroelastic approach for assessing hydrogen saturation during flow-through experiments by linking the Skempton coefficient to the effective bulk modulus of water–hydrogen mixtures. Their study responds to a key difficulty in hydrogen storage research: the properties of hydrogen differ significantly from those of carbon dioxide or natural gas, and methods developed for other fluids cannot always be transferred directly. López-Vizcaíno et al. (2026) address a related modelling challenge in the context of gas transport in deep geological repositories. Their explicit strategies for estimating suction under non-ideal gas conditions reduce computational cost while preserving accuracy, thereby supporting more efficient coupled thermo-hydro-mechanical simulations of engineered barrier systems. These two contributions highlight the importance of developing fluid-specific models and measurements for emerging subsurface storage technologies.
Fractured media provide a further cross-cutting challenge for several decarbonisation applications, including geothermal energy extraction, hydrogen storage and carbon sequestration. Hyman et al. (2026) discuss new opportunities for characterising flow channelisation in fractured systems across scales. Their letter draws attention to two complementary directions: the use of additive manufacturing to create controlled fracture geometries for validating direct numerical simulations, and the use of graph-based machine learning to represent flow through complex fracture networks. This contribution captures a broader methodological shift within subsurface geotechnology. As decarbonisation applications become more ambitious, progress increasingly depends on the ability to combine physical experiments, numerical simulation and data-driven tools in ways that preserve geological complexity without losing predictive efficiency.
Taken together, the letters in this themed issue show that subsurface geotechnology for decarbonisation is not a collection of isolated applications, but an emerging scientific and engineering domain built around shared questions. How can coupled processes be measured across scales? How can reactive transport be steered rather than merely tolerated? How can monitoring techniques detect early signs of degradation in storage formations? How can dissolution, precipitation and evolving flow pathways be managed to promote favourable outcomes such as mineral trapping, improved sealing or enhanced carbonation efficiency? How can constitutive models represent geomaterials whose fabric, mineralogy and saturation state evolve during operation? How can low-carbon technologies be designed for long-term performance rather than short-term functionality alone? These questions recur throughout the issue, whether the application is carbon dioxide storage, geothermal heating, hydrogen storage, mineral carbonation or sustainable ground improvement. The letters collected here also illustrate the breadth of expertise now required to address these questions. Advances in decarbonisation geotechnology depend on geomechanics, geochemistry, transport in porous media, materials science, thermodynamics, computational mechanics and data science. They also highlight the need to move across scales, from mineral surfaces and pore networks to laboratory specimens, engineered barriers, foundations, reservoirs and regional energy systems. This multiscale, multiphysical character, combined with a focus on actionable technological implementation essential to operate in subsurface settings, are in conjunction the key reasons that render the geotechnical engineering discipline increasingly crucial to a forward-looking low-carbon transition, as it is uniquely positioned to connect material behaviour with infrastructure performance and to translate subsurface processes into design, monitoring and risk-management strategies.
By improving how we characterise, model, and engineer the subsurface, geotechnics is poised to actively contribute to transformative endeavours such as carbon management, renewable energy deployment, infrastructure resilience and the responsible use of natural resources. Yet these efforts are unlikely to succeed without a long-horizon vision able to reconcile immediate societal needs with the responsibility to preserve a forward-looking trajectory for technological development. The contributions gathered in this themed issue show that progress in subsurface decarbonisation increasingly depends on the ability to integrate knowledge across scales, disciplines and applications. Advancing this field will require not only continued innovation in experimentation, modelling, monitoring and data-driven methodologies, but also stronger connections between geomechanics, geochemistry, transport physics, materials science, computational mechanics and energy systems engineering. Equally important will be the development of shared frameworks, benchmark problems and interdisciplinary communities capable of translating scientific advances into reliable technologies at the scales demanded by the global energy transition. As a result, the ideas fostered by this special collection collectively point toward a future in which the challenge ahead is not only to deepen our scientific understanding of subsurface processes, but also to provide the coherence, integration, and long-term stewardship required for subsurface geotechnologies to become a durable foundation of a sustainable energy future.
