Editorial: From characterising soil behaviour to engineering sustainable ground systems

Environmental geotechnics has expanded well beyond the traditional task of describing soil behaviour under load. The field now addresses ground systems whose performance is shaped by hydraulic, chemical, biological and operational factors, often under increasingly strict sustainability requirements. This broader scope has been visible in Environmental Geotechnics since its earliest years, from the journal’s inaugural framing of the discipline to later discussions on regional environmental geotechnics challenges, climate-responsive practice, decarbonisation and embodied-carbon assessment (Khabbaz and Fatahi, 2016; Paleologos et al., 2026a, 2026b; Singh, 2014; Xue et al., 2026). Within this context, the papers collected in this issue present a clear message: progress in environmental geotechnics depends not only on better material characterisation, but also on more reliable and sustainable ground intervention. The contributions in this issue speak directly to several UN Sustainable Development Goals, most notably SDG 6 (Clean water and sanitation), SDG 9 (Industry, innovation and infrastructure), SDG 11 (Sustainable cities and communities), SDG 12 (Responsible consumption and production), SDG 13 (Climate action) and, in part, SDG 15 (Life on land).The issue begins with a paper by Zhai et al. (2026), which evaluates the shear modulus of unsaturated soils estimated from different indirect methods. This is an important starting point because stiffness remains central to deformation analysis, serviceability assessment and numerical modelling, yet its estimation in unsaturated soils still carries substantial uncertainty. By comparing five existing models against an extensive set of published experimental data, the authors show that model performance depends strongly on soil type and suction range. Their results indicate that no single indirect method can be adopted without careful attention to the nature of the soil and the assumptions embedded in the model. This paper is a useful reminder that design quality in environmental geotechnics still rests on the quality of basic constitutive evaluation.A related concern appears in the study by Rattan et al. (2026), which examines the degradation of hydrophilic polymers and the resulting effect on the water retention properties of soil. Water-absorbing polymers have attracted sustained interest because of their ability to improve soil moisture retention and support vegetation establishment. However, this paper shows that their benefit is not constant with time. Repeated drying cycles progressively degrade the polymer network, reduce water absorbency and alter the soil-water characteristic response. The authors also demonstrate that a void ratio-based soil-water characteristic curve model can represent this progressive change with reasonable accuracy. The contribution is valuable for environmental geotechnics because it shifts attention from initial improvement to durability in service. This issue is especially important in bioengineered and vegetated infrastructure, where hydraulic performance over time often matters as much as short-term improvement.The next two papers move from characterisation towards intervention and multifunctionality. Wang et al. (2026) investigate the coupled effects of soil particle size and biochar on plant-soil interaction and carbon accumulation. Their results show that finer-textured soil, when combined with biochar, can enhance plant biomass, root-zone nutrient availability, soil enzyme activity and plant carbon assimilation. The paper is significant because it frames soil improvement in a broader ecological context. In environmental geotechnics, the purpose of ground treatment is increasingly linked not only to engineering stability, but also to vegetation performance, carbon storage and long-term land development. This perspective aligns closely with present efforts to integrate geotechnical practice with low-carbon and nature-supporting infrastructure strategies.Bridi et al. (2026) address another important dimension of sustainable intervention through the use of KR desulfurisation slag as a soil stabiliser. Their study shows that the stabilisation response depends strongly on fine particle content and curing time, with finer soils achieving better CBR performance after treatment. Chemical and microstructural analyses suggest that cation exchange and subsequent densification play important roles in the stabilisation mechanism. This paper is particularly relevant to SDG 9, SDG 12 and SDG 13 because it combines engineering improvement with industrial by-product reuse. It also reflects a broader direction within the discipline: sustainable stabilisation requires more than the substitution of one binder with another. It requires a mechanistic understanding of soil-additive interaction, compatibility and long-term effectiveness.The final paper, by Li et al. (2026), extends the discussion to system-scale performance in waste containment. The authors investigate leachate drawdown in a laboratory-scale vacuum-assisted pumping well model using synthetic municipal solid waste. Elevated leachate levels remain a serious concern in landfill operation because they affect hydraulic control, slope stability, contaminant management and gas behaviour. This study provides useful insight into the hydraulic response of waste during vacuum-assisted pumping, including the role of gas bubble instability and the competing effects of vacuum pressure on hydraulic conductivity and waste compression. A notable practical finding is that cyclic vacuum application can achieve nearly the same pumping capacity as constant vacuum within an optimal pressure range, while also offering an energy-saving option. The paper has clear relevance to SDG 6 and SDG 11, and it illustrates the applied side of environmental geotechnics particularly well: fundamental hydraulic processes must ultimately support safer, more efficient and more reliable waste management systems.Seen as a whole, this issue presents environmental geotechnics as a discipline that now works across several connected scales. At one end are constitutive properties and evolving material functions, such as unsaturated stiffness and polymer-assisted water retention. At the other end are engineered systems, such as vegetated soils, stabilised ground and landfill drainage control. Between these scales lies a common technical requirement: performance must be interpreted with environmental function, material durability and sustainability objectives in mind. This direction is consistent with recent contributions in Environmental Geotechnics on carbon dioxide capture, decarbonisation pathways and the carbon implications of engineering materials (Paleologos et al., 2026a, 2026b; Xue et al., 2026).A brief reflection from Norway is also relevant here. In Norway, environmental geotechnics is closely tied to climate adaptation, natural-hazard resilience, contaminated land management, landfill and leachate control, geomaterial reuse, and the safe handling of mine wastes and tailings. These priorities reflect both national engineering needs and a wider expectation that ground engineering should support environmental protection as well as infrastructure performance. This is particularly important in a country where climate adaptation for buildings, infrastructure and transport is receiving growing attention, and where floods, landslides, monitoring and early warning are treated as key elements of prevention. At NGI, this broader scope is clearly visible in environmental geotechnics work that spans contaminated ground, waste-related problems, material reuse and climate-robust solutions.From my own perspective, working in Norway has made this evolution of the field especially clear. I am currently involved in NGI’s Fibre for the Future strategic project, which aims to advance fibre optic sensing for geotechnical and environmental monitoring. The project reflects an important direction in present-day environmental geotechnics: better sensing, better interpretation and better decision support for complex ground systems. In my own work, this has meant thinking not only about soil and structure behaviour, but also about monitoring quality, material–sensor interaction and the way data can improve engineering judgement. More broadly, my research experience has developed across experimental studies, numerical modelling, field-oriented geotechnical problems and underground infrastructure. That combination has convinced me that environmental geotechnics is at its best when strong fundamentals are connected to real engineering needs.For younger researchers, this is a very good time to enter the field. Environmental geotechnics is no longer a narrow niche within geotechnical engineering. It is becoming a meeting point for soil mechanics, environmental chemistry, sensing technology, data analysis and sustainability. The future of the field will depend on researchers who are rigorous in fundamentals, but also willing to work across disciplines and engage with practical problems that are often messy, coupled and highly consequential. That challenge is exactly what makes the field so rewarding.We hope that readers will find this issue useful both scientifically and professionally. The papers offer new evidence, practical insight and a clear indication of the direction in which the field is moving. Readers are also encouraged to visit the journal’s Virtual Library homepage, where newly accepted papers are made available Ahead of Print, allowing early access to current research before formal issue publication.