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Environmental geotechnics draws on various scientific and engineering disciplines to deal with a range of negative construction, societal and climate change-related consequences. From wastes (municipal solid waste (MSW), demolition, tailings, marine sediments) and their management (by re-use, stabilisation or containment), contaminated land remediation, stability and sustainability of infrastructure, to energy and resource use. Effective protection of the geo-environment requires an understanding of these various disciplines and their deployment. However, scientific understanding and geo-environmental management are enacted at different scales. It is the application of geo-environmental engineering, in terms of both the substantive focus and the scale of investigation that will be used to contextualise the work presented here.

This issue of Environmental Geotechnics contains 5 papers, which together cover most of the substantive topics referred to above and at a range of scales necessary to translate scientific understanding into applied engineering. The topics in question are given in Table 1 together with the scale or type of investigation undertaken. Table 2 provides the definitions of the different scales of investigation adopted here.

The paper by Roque et al. (2022) provides the broadest review of environmental geotechnics and its contribution to a post-pandemic green economy. Five themes are included, highlighting (i) the importance of recycling of construction and demolition waste to minimise exploitation of virgin aggregates with CO₂ savings on extraction and processing. (ii) Marine sediments are also identified for stabilisation/solidification and using innovative binder materials such as biochar and mussel shell flour, which can wring a decrease in processing emissions to 8% that of Portland cement. (iii) MSW is next for consideration. Whilst MSW arisings have increased in US and China, the picture in EU is one of a small decrease. <br/>Management of MSW is the pressing issue. In the EU, Belgium and Denmark have excellent records in terms of landfill avoidance but the global picture is less encouraging, with a significant reliance on landfill. Bioreactor technology may offer a way out but its use is hindered by a technology gap in many parts of the world. However, landfill, through the physical properties and hydrophobicity of modified sulphur concrete, can provide secure containment for Covid-19 wastes. The paper also identifies ‘state of the art’ ways to re-use nano-plastic wastes and rubber-tyre soil mixtures. Roque et al. (2022) draw attention (iv) to soil improvement techniques such as geosynthetics, biopolymers, root reinforcement, and remediation methods such as washing, thermal treatment and soil vapour extraction. Lastly, a more proactive environmental geotechnical approach is cited – (v) thermo-active infrastructure. Acknowledging that there is little guidance beyond thermo-piles, mention is made of ‘thermo-tunnels’ and diaphragm walls. Referring to Table 1, the broad sweep of geo-environmental themes presented by Roque et al. (2022) can be seen to contribute positively to GHG emissions, raw material use, stability/sustainable and safe waste management, prevalence of nano-plastics and used tyres, and energy use.

The next paper, by Chen et al. (2022) focuses on the micro-scale but with impacts at the mesoscale – on sustainable infrastructure. Their interest is in the durability of steel fibre reinforced concrete and its resistance to chloride attack. From a laboratory scale investigation, the effects of several factors on compressive strength were assessed. The factors included water-cement ratio, chloride ion content, soaking time and permeating surfaces, by which chloride ions can attack steel fibres. A ranked order of controlling factors found water-cement ratio the most influential factor.

The third paper, by Yin et al. (2022) examines the deformation of rockfill, occupies a similar scale space to the Chen et al. (2022) paper. A laboratory scale investigation of the load-wetting-collapse behaviour of weakly weathered granite has been used to develop a constitutive model. The scale of deployment thus becomes macroscopic, enabling stability assessment of concrete dams, the failure of which might lead to consequences of global proportion – certainly regionally extensive. The constitutive model elaborates on the strain increment behaviour of rockfill on wetting and distinguishes between (i) sudden changes in hydraulic conditions, as may occur on impounding, and (ii) a time-dependent creep process, more typical of longer-term climatic impacts.

The fourth paper by Nian et al. (2022) is a laboratory scale investigation but may be considered at meso-scale. This is because the investigation seeks to simulate at reduced size, the morphology of submarine slope failure. Indeed, the Authors are careful to respect scaling laws in order to justify field-scale interpretation. There are unambiguously macro-scale impacts; GHG emissions may be negatively affected, and major submarine slope failure can trigger tsunami. The phenomenon that Nian et al. (2022) investigate is submarine slope failure due to natural gas hydrate dissociation. Methane gas is released below the sea floor separating, with sufficient pore pressure eventually uplifting, a layer of sediment thereby creating a potential plane of sliding. Observations at laboratory scale allowed for identification of two types of failure surface morphology, (i) a disc-shaped failure and (ii) a penetration failure, and for a limit equilibrium constitutive model to be put forward.

Finally, the paper by Chang et al. (2022) deals with the remediation of Pb and Cu contaminated soil by mechanical separation, a process referred to as the particle-size sieving technique (PST). In terms of scale, the investigation is laboratory based at micro-scale – a programme of PST. However, micro-scale insight and understanding allows for a meso-scale manipulation of site-wide soil particle fraction replacement. The Authors highlight the use of PST to improve cultivability, i.e. to increase food production, in other words their work has macro-scale impact. Chief amongst their experimental outcomes was the quantitative analysis of the speciation of heavy metals in Pb contaminated and Cu contaminated soil, in terms of organic matter and specific surface or particle size. Contaminated fractions were replaced by clean sand to bring about a targeted decrease in heavy metal concentration. In addition to cultivability, the Authors show a reduction in remediation cost.