António José Roque
email: aroque@lnec.pt

Evan K Paleologos

Brendan C O'kelly

Anh Minh Tang

Krishna R Reddy

Claudia Vitone

Abdel-Mohsen O Mohamed

E Koda

Venkata Siva

Naga Sai Goli

Castorina S Vieira

X Fei

Francesca Sollecito

Magdalena Daria Vaverková

Michael Plötze

Rossella Petti

A Podlasek

Alexander M Puzrin

Federica Cotecchia

P Osiński

Arif Mohammad

Prithvendra Singh

Maisa El Gamal

Sherine Farouk

Moza T Al Nahyan

Slobodan B Mickovski

Narain Devendra

Singh

Sustainable Environmental Geotechnics Practices for a Green Economy

Keywords: Rehabilitation, reclamation & renovation, Renewable energy, Waste management & disposal

The revitalization of the global economy after the COVID-19 era presents Environmental Geotechnics with the opportunity to reinforce the need for a change in paradigm toward a green, circular economy and to promote aggressively the use and development of sustainable technologies and management practices. This paper aims to assist in this effort by concentrating on several thematic areas where sustainability solutions and future improvements are sought. These include the re-entry of construction and demolition waste, excavated material, industrial waste, and marine sediments into the production cycle and the reuse of existing foundations. Despite the recent trend in advanced countries toward recycling and waste-to-energy thermal treatment, landfills still constitute the most common municipal solid waste management practice, especially in low-and-middle-income countries, and technological solutions to improve their environmental footprint are presented. At the same time, remediation solutions are required to address the multitude of contaminated sites worldwide. Advanced developments that incorporate environmental, economic, and social dimensions are expounded, together with sustainable ground improvement solutions for infrastructure projects conducted in soft and weak soils. The topic of thermo-active geostructures concludes this paper, where, apart from their infrastructure utility, these structures have the potential to contribute as a renewable energy source.

Introduction

Since the beginning of 2020, the COVID-19 health crisis has been added to existing global challenges threatening the well-being of humankind. The pandemic has further exposed the relationship between our economic and social modes of operation and environmental problems. Thus, reduction in atmospheric pollution over many cities has been observed during the lockdown, environmental noise has subsided, and cleaner beaches and coastal waters have resulted from the much-reduced tourism. On the other hand, the pandemic has accentuated some environmental problems through the increase of waste production, the significant reduction in waste recycling, and the entry of disinfectants into soils and waters [START_REF] Paleologos | Post COVID-19 water and wastewater management to protect public health and geoenvironment[END_REF][START_REF] Tang | Environmental geotechnics: Challenges and opportunities in the post COVID-19 world Jour[END_REF]Vaverková et al., 2020).

The construction industry is a well-known source of negative environmental impacts due to the high consumption of natural resources and energy, and the large production of waste (Farid, 2020). The environmental performance of the construction industry remains subpar, despite the fact that, principles of sustainability and circular economy can easily find application in this industry. This has become even more critical since the rate of extraction of natural resources has become 1.6 times higher than their capacity to regenerate.

Renovation, replacement, and retrofitting of aging building stock gives rise to the reuse of existing building material and the development of new, more sustainable construction practices (European Commission, 2020b). This is assisted by the development of "green" codes and regulations regarding the design, construction, and operation of buildings (LEED, 2021;DMT, 2020). It becomes especially critical for the post COVID-19 recovery, which is seen by governments as an opportunity to "transition to a greener and more sustainable economy", as exemplified by the joint declaration of the European Union (EU) and China on September 14, 2020 (European Commission, 2020a). This paper aims to present an overview of and recommendations regarding the reuse of several types of waste in geoenvironmental projects. Foundation reuse is a topic of interest, especially for older cities that are congested by a large number of infrastructure elements in their subsurface. Management of marine sediments has become important given the expansion of ports in many countries, the need to retain the natural balance between waterways and sea, to conserve coastal ecosystems, and to accommodate tourist, transportation, and even desalination needs in an integrated manner. Waste and sediment management practices are being implemented in several countries with significant national action plans and regulations anticipated to accelerate these efforts.

Landfills remain the principal way to dispose solid waste across the world. The USA and especially the EU have moved to transition away from landfilling to recycling and wasteto-energy, but for low-and-middle-income countries (LMICs) the cost, lack of technological and organizational expertise, and waste composition make landfilling a desirable option to the alternative of open dumps. The challenges in the management of landfills are expounded here and the use of sulphur concrete as a component of landfill barrier systems is presented, as well as the emerging topic of mining landfills for the recovery of useful materials.

It has also become amply clear that improper disposal practices and accidental spills have resulted in contamination of the geoenvironment. Developers are generally hesitant to convert contaminated land into beneficial use, due to liabilities issues. Many initiatives have been undertaken and environmental laws/regulations have been promulgated over the last thirty years to assist in the clean up of brownfields, and to utilize such lands for housing and commercial needs, and for natural reclamation (USEPA, 2020a). Thus, another theme of our paper is to discuss advanced developments in sustainable land remediation, as well as sustainable ground improvement solutions in infrastructure projects conducted in soft, weak, and unstable soils.

Geotechnical engineers are routinely involved in the design and construction of underground structures, such as tunnels, base slabs, piles and diaphragm walls. Apart from the role that these elements play as infrastructure, recent studies have shown that they may also be used for the production of shallow geothermal energy. Technological improvements in the design of thermo-active geostructures together with standardization in their application will assist market penetration and increase the interest in developing countries.

Geo-structures may present an example of a situation where the synergy between a project's variable functions may be sought and this constitutes the concluding theme of our paper.

Sustainable Waste Management

Minimizing and recycling of construction and demolition waste (C&DW)

Much of the focus in environmental geotechnics has been on recycling non-hazardous industrial waste and by-products of different origins and characteristics. Examples of the application of such materials in geotechnical works include: 1) C&DW in road pavement layers, geosynthetic reinforced structures and drainage layers; 2) steel slag in road pavement layers and drainage layers; 3) waste tires in embankments, and drainage layers; 4) quarry waste in road pavement layers and landfill liners or cover systems; and 5) bottom ash from incineration of municipal waste in road pavement layers (Roque et al., 2020).

Other non-hazardous industrial waste and by-products studied for use in geotechnical works are coal fly ash and bottom ash, and foundry sand.

Prevention, reduction and reuse are top priorities in the waste management hierarchy, and are critical especially for the construction industry, which utilizes large amounts of new material and generates large volumes of waste. Two-fifths of the global raw stone, gravel, and sand have been depleted as a result of the construction industry. In Europe, the construction industry was responsible for generating about 820 Mt of C&DW in 2012, about one-third of the total waste generated for the region that year (European Commission, 2019a), while consuming 12 to 18 Mt of new construction material [START_REF] Ecorys | Resource efficiency in the building sector[END_REF].

On the positive side, several indicators suggest that sustainability considerations are becoming mainstream in construction projects, with numerous companies emphasizing green design and construction practices. Credit-bearing "green" systems that reward projects, which adapt sustainability methods in the design, construction, and operation of buildings and neighborhoods include the USA-developed LEED (Leadership in Energy and Environmental Design) (LEED, 2021). Projects certified by LEED have claimed impressive accomplishments, with water and energy savings, recycling of materials, extensive green areas, etc. Some countries have made the use of sustainability codes mandatory for the construction industry, such as the United Arab Emirates (UAE), which requires all new projects constructed in Abu Dhabi after 2010 to attain a certain rating under the Estidama Pearl Rating System (DMT, 2020).

Important benefits from reduction and reuse of C&DW include: 1) cost savings from a decrease in mining activities, reduced material transport to and from construction sites, along with reductions in transportation and disposal of waste in landfills; 2) lower carbon dioxide emissions from resource processing and transportation; 3) increased competitive advantage through differentiation; and 4) flexibility to adapt to more stringent policy requirements related to C&DW generation and disposal (European Commission, 2019a,b).

Geotechnical works can be responsible for the movement of large volumes of earth material with matching large energy consumption. Considerable quantities of excavation soil and rock are generated in transport infrastructures (e.g., roads, railways, subways), urban infrastructures (e.g., supply networks for electricity, gas, water, sewerage, telecommunications, and drainage networks for flood prevention), and multi-floor basements of large buildings. Despite the fact that excavated material often presents characteristics that makes it appropriate for reuse on-site or off-site, it is often disposed in landfills, or even dumped indiscriminately, along roads or streams. Benefits from reusing excavated soil and rock include savings of up to 14 kg of carbon dioxide per ton of soil, as well as material handling cost-savings of up to 85 % [START_REF] Magnusson | Sustainable management of excavated soil and rock in urban areas -A literature review[END_REF].

In order to encourage the reuse of excavated material, be it clean or lightly contaminated, several countries have developed regulations [START_REF] Mtes | Guide de valorisation hors site des terres excavées issues de sites et sols potentiellement pollués dans des projets d'aménagement[END_REF][START_REF] Cl:Aire | The Definition of Waste: Development Industry Code of Practice[END_REF] to support best management practices (BMP). These guidelines avoid the classification of excavated soil as waste, with commensurate savings owing to the avoidance of controlled disposal. Most regulations only allow the geotechnical improvement of excavated soil, but the French (MTES, 2017) and English-Welsh (CL:AIRE, 2011) guides also allow the chemical treatment of soil if it is contaminated.

Foundation reuse is another topic that has become progressively important in recent years because of an increased concern regarding ground congestion in urban areas, and the need to maximize material reuse and minimize resource exploitation and waste production.

Many older cities, especially in Europe, are finding that urban reconstruction below ground level has become increasingly difficult and expensive, as this space is often crowded with existing infrastructure/services that include utilities, tunnels, and shallow and/or deep foundations of buildings.

In addition to ground congestion, drivers for foundation reuse are improvements in: 1) project delays; 2) foundation costs; 3) carbon dioxide emissions; 4) waste production; and 5) archaeological preservation. [START_REF] Jones | Reuse of foundations. The opportunities for development[END_REF] presented a case where the reuse of the foundations of an old building, constructed in the mid-1970s, in a new nine-storey residential building achieved cost savings of $178k and reduced the completion time of the project by six weeks. [START_REF] Tsubakihara | Reuse of existing piles in building reconstruction and its environmental effects[END_REF] calculated that the reduction in the volume of newly built piles and in carbon dioxide emissions was about 40 % and 70 %, respectively, through the reuse of existing cast-in-place concrete piles. These authors claimed that engineering advantages with the reuse of old foundations include nondisturbance of the supporting ground, and retention of the bearing capacity of adjacent piles and of the structural integrity of neighbouring buildings or underground structures.

The main problem with foundation-reuse design is often the lack of credible information about the original design and the as-built construction of existing foundations. Thus, despite the benefits of reusing old foundations, currently such foundations tend only to be reused if the ground beneath a building is already occupied by them or if there exist at the site archaeological remains that must be preserved. Thus, in order to make foundations accessible to future interventions and reuse, detailed information about their design and construction must be retained in current projects.

Sustainable management of marine sediments

Despite the fact that the total amount of sediments dredged from marine basins in Europe reaches 100-200 million m 3 /year, current European Directives do not report about the handling of dredged sediments in a comprehensive and uniform manner. In most cases, dredged sediments are still classified as 'waste' and the disposal in large fill-in basins is the most used practice with associated high environmental risks.

In this context, stabilization/solidification (S/S) treatment of sediments with binders and additives represents a promising option for managing marine sediments, which are dredged periodically in harbour areas, either for port effectiveness or for on-site remediation (SedNet, 2011). This solution is highly flexible since the most efficient mix of binders and additives can be designed depending on both the chemo-mechanical features of the sediments at each site and the required performance of the selected recycling application.

Treatments can be specifically customized mainly to: 1) improve the hydro-mechanical properties of the sediments [START_REF] Federico | Plasticity of lime/cement-stabilised dredged clayey sediments[END_REF]; 2) prevent the release of toxic elements into the environment [START_REF] Wang | Remediation of heavy-metal-contaminated soils by biochar: a review[END_REF], and 3) reduce the interference of contaminants with binders' chemistry that may otherwise compromise the effectiveness of stabilization.

The treated sediments may be either recycled as aggregates for road construction, cemented mortars, fill material, brick production, or transformed into sustainable resources useful for port infrastructures [START_REF] Wang | Solidification/stabilization of dredged marine sediments for road construction[END_REF][START_REF] Jones | Using geosynthetics for sustainable development[END_REF].

Recent research aims to treat marine sediments by using effective and sustainable additives that comply with the lowering of carbon dioxide emissions through the minimization of hydraulic binders (e.g., 900 kg of carbon dioxide per ton of clinker are required to produce the binders). This is the case, for example, of biochar (a by-product of thermochemical industry), as an eco-friendly adsorbent for remediation of contaminated sediments, or mussel shell flour (MSF) (Papadimitriou et al., 2017). MSF is a by-product of the fish industry, made of calcium carbonates, which after specific treatment form a flour, which can be used in partial substitution of cementitious binders for chemical and hydromechanical stabilization of sediments (Petti et al., 2020).

In this context, the Politecnico di Bari, ETH Zurich and Italcementi -HeidelbergCement Group are performing experimental studies for the stabilization of dredged sediments using sustainable binders of lower environmental impact, in which part of the binder is replaced by MSF. Table 1 reports some geotechnical properties of the natural sediments from the Gulf of Taranto, in the south of Italy, both before and after treatment with either 8 % Portland cement (Sed8PC), or 6 % Portland cement with 2 % MSF (Sed6PC2MSF). All the values in Table 1 are corrected for the pore water salinity, as described in Sollecito et al.

(2019).

Table 1 shows that the addition of 8 % Portland cement (PC) to marine sediments (clay fraction: 30.4 %) causes a significant increase in liquid limit, such as to classify the amended sediments (i.e., Sed8PC) as silts of high plasticity, MH, in the Casagrande's plasticity chart. It follows that the recorded increase in consistency index (CI) is essentially due to the reduction in water content for the treated sediment. It is worth noting that, the replacement of part of the PC with 2 % MSF additive increases the CI of the produced material (i.e., Sed6PC2MF) with respect to that of the standard PC-mixture. Promising results have been obtained also from mechanical tests, both in compression and shear (Petti et al., 2020), comparing the mechanical response of the natural sediments with that of the same sediments treated with traditional cement and cement-MSF additives.

The study on the marine sediments from the Gulf of Taranto has been emblematic also in showing that the search for effective in-situ and ex-situ treatments could become extremely challenging due to the recurrent heterogeneity and contamination that characterizes natural environments close to industrial areas [START_REF] Federico | Plasticity of lime/cement-stabilised dredged clayey sediments[END_REF][START_REF] Tang | Environmental geotechnics: Challenges and opportunities in the post COVID-19 world Jour[END_REF]. Preliminary results from these sediments confirm that multi-scale testing is necessary to explore the reactions occurring during the cementation process and their effects on the treatment performance, especially when both additives and binders are used (Petti et al., 2020).

The framework defined for normally-consolidated natural clays and the most used standards (ASTM D2487-17e1 and ASTM D4318-17e1) do not focus on soils containing sources of complexity such as those typical of marine sediments: i.e., high content of organic matter, high carbonate content (e.g., fossils and shells), diatoms, and pollutants. Indeed, organic matter may absorb water and promote the aggregation of clay-size particles to form a more open fabric, and, as such, affect the soil consistency, activity and compressibility [START_REF] Levesque | Characterisation of postglacial sediments of the Saguenay Fjord, Quebec, Canada[END_REF]. Moreover, microfossils and diatoms present in marine soils may be responsible for high plasticity, activity and compressibility even within samples of low clay fraction (CF), due to their capacity to trap water into the intraskeletal pore space. Lastly, salts and pollutants alter the thickness of the diffusive double layer of clay particles, affecting the engineering properties of fine-grained soils, depending on their mineralogical composition. The presence of these contributions could have an impact on the geotechnical properties of marine sediments making them exhibit unexpected behavioural facets, with engineering implications also for the effectiveness of remediation and recycling strategies.

The activity chart of these marine sediment samples show peculiar deviations from predicted soil facets (Figure 1a) [START_REF] Tang | Environmental geotechnics: Challenges and opportunities in the post COVID-19 world Jour[END_REF]. The chaotic distribution in the chart is mainly due to some results from the most contaminated top layer (0-1.5 m below sea floor), which do not show the expected increase in soil plasticity with CF. Hence, some sediment samples exhibit high activity (A > 1.25) because they can sustain high plasticity despite their low CF. Conversely, others samples are in the low activity region (A < 0.75), due to their low plasticity and high CF.

Standard testing procedures on sediments often fail in depicting their altered physicalchemical properties due to cross contamination, salt diffusion, organic matter degradation, and fossil abundance, inducing bias in measurements and classification (Sollecito et al., 2019;[START_REF] Tang | Environmental geotechnics: Challenges and opportunities in the post COVID-19 world Jour[END_REF]. As an example, the sieving procedure at No. 40 sieve (ASTM D4318-17e1) required by the standards for the Atterberg limit determination, removes the organic components and alters the sediment plasticity [START_REF] Mitaritonna | I sedimenti del porto di Taranto: Caratterizzazione geotecnica e stabilizzazione con leganti[END_REF]. As reported in Figure 1b for the marine sediments under study, the liquid limit and the plasticity index determined without sieving the sediments (Sed_UP) are higher than those obtained according to the standard procedure (Sed_ASTM). It follows that the sediment can be classified as CL (medium to low plasticity clay, USCS; ASTM D2487-17e1) according to American Society for Testing and Materials (ASTM) and CH (high plasticity clay) if the Atterberg limits are determined with unconventional procedures taking account of the whole material.

Sustainable management of municipal solid waste (MSW)

MSW generation, incineration, and recycling

The trend in municipal solid waste (MSW) management in the USA and the EU is toward recycling, reduction of landfilling, and increase of thermal treatment tied with energy recovery. Thus, in the USA, recycling and composting increased from 10 % in 1980 to 34.5 % in 2012, while waste-to-energy treatment accounted for 11.7 % of the waste stream during the same year [START_REF] Paleologos | Waste-to-Energy: An Opportunity for a New Industrial Typology for Abu Dhabi[END_REF][START_REF] Paleologos | Bayesian analysis of air emission violations from waste incineration and co-incineration plants[END_REF]. For the 27 member states of the EU (EU-27), in 2018 recycling of the MSW had reached 47.4 % (Eurostat, 2020b), whereas 2016 data, which included the United Kingdom, showed an average of 20 % incineration across the 28 member states of the EU (Eurostat, 2020c). Per capita, MSW generation in the EU has dropped slightly from an average of 1.41 kg/day (515 kg/annum) in 2015 to 1.35 kg/day (492 kg/annum) in 2018 (Eurostat, 2020c). MSW generation in the USA is much higher and stood at 2.05 kg/day/capita in 2017. There has been a sharp increase in waste creation in the USA from the 1960s to 1990, and since then per capita production has stabilized between a minimum of 2.02 kg/capita/day, in 2010, and a maximum of 2.15 kg/capita/day, in 2000 (USEPA, 2020b).

The Organization for Economic Co-operation and Development's (OECD's) data show a sharp increase in solid waste generation in the three largest economies over the last twentyseven years. Thus, MSW generation, in thousand tonnes, was (OECD, 2020): in 1990, 227,417 in Europe, 188,939 in USA, and67,668 in China;in 2000, 277,344 in Europe, 220,854 in USA, and 118,190 in China;in 2010, 279,011 in Europe, 227,749 in USA, and 158,048 in China;and in 2017, 282,743 in Europe, 242,935 in USA, and 215,209 in China (Figure 2). This means that from 1990 to 2017, each of the three largest economies increased their waste production, albeit at different rates, and from a combined 484,021 thousand tonnes of waste in 1990 reached 740,887 thousand tonnes of waste in 2017, which represents a cumulative 53% increase. It is clear that despite technological advancements, intensification of recycling efforts, and public awareness campaigns, the continuous creation of new products and our lifestyle and consumption patterns has created a problem that can be addressed only through a radical shift in both the economic and lifestyles paradigms.

Landfills have been an essential part in the waste management system, although the trend in most developed countries is away from landfilling and towards incineration tied with energy and heat recovery. The EU's Landfill Directives specifically state that, "according to the waste management hierarchy, landfilling is the least preferable option and should be limited to the necessary minimum" (European Commission, 2019b). The progressive move away from landfilling and towards increased recycling and waste-toenergy in the EU is shown in Figure 3, where landfill use has steadily reduced during 2010-2016, composing in 2016 about 23% of the total solid waste management options (Eurostat, 2020a). Some countries in Europe have done significantly better than this figure. For example, Belgium in 2016 sent only 3.8% of waste to landfills, 17.9 % was incinerated, and 78.2 % of waste was recycled (Eurostat, 2020a). Denmark also sent about 4% of its MSWs to landfills, incinerating about 54 % of them, with the remaining being recycled (EEA, 2013). Slovenia is being quoted as a success story, in that prior to entering the EU, the country relied heavily on landfilling, but by 2018, it had almost reached the EU 2030 recycling target of 65 % for MSW.

For developing nations, mainly Asian and African countries, landfilling is mostly adopted due to the difficulty of thermal treatment of MSW that exhibits high moisture content and low calorific value in these countries. In this context, bioreactor landfill (BLF), wherein the decomposition rate of the MSW is accelerated technologically by practising leachate recirculation, air purging or a combination of both, and enhancing methane generation as an alternative energy source, can be used.

Challenges and opportunities in landfill management

The key issue in landfill management is to prevent and control harmful impacts to the environment over a period of many decades (Mohamed and Paleologos, 2018). During the COVID-19 pandemic, waste management systems had to rapidly adjust to transport and dispose safely waste potentially contaminated by SARS-CoV-2 (Vaverková et al., 2020).

The survival of pathogens in clay barrier systems highly depends on soil hydraulic properties. Changing conditions in soil parameters, such as in hydraulic conductivity (K), moisture content and ground temperature, determine the ability of the microorganisms to grow. When it comes to the unsaturated soil of the barrier system, where hydraulic properties are subject to significant changes [START_REF] Tang | Atmosphere-vegetation-soil interactions in a climate change context; impact of changing conditions on engineered transport infrastructure slopes in Europe[END_REF], development of microorganisms can become much more rapid than expected. Clay liners interact with viruses and bacteria depending on nutrient availability, soil pH, medium temperature, moisture content, and soil structure.

The complexity of virus interaction with host soils calls for new alternative barrier material to mitigate the transport of SARS-CoV-2 virus in clay liners. One example of such material may be elemental sulphur, which has found application in the construction and waste management industries [START_REF] Amo | Process for the Preparation of a Product Comprising Modified Sulfur[END_REF][START_REF] Amo | Process for the Preparation of a Product Comprising Modified Sulfur[END_REF]. The use of sulphur concrete as a basic component of landfill barrier systems, in order to prevent migration of leachate to groundwater is shown in Figure 4. This barrier is a multi-layered composite system, where an impermeable Modified Sulphur Concrete (MSC) layer is placed in between the primary and secondary leachate collection and removal systems, surrounded by sand and sand-substrate layers [START_REF] Amo | Process for the Preparation of a Product Comprising Modified Sulfur[END_REF]. The liner consists of a layer of MSC with a minimum thickness of 0.3 m. The MSC has a K of the order of about 10 -13 m/s, much lower than the value of K=10 -9 m/s specified by the US Environmental Protection Agency (USEPA). The MSC has very low leaching rate in variable-pH environments, and has good resistance to chemical and physical degradation, so it retains its strength under different environmental conditions.

Since diffusion can be a significant contributor in the migration of contaminants carrying SARS-CoV-2, the porosity of constructed barriers is important. The very low K of MSC is due to the significant low pore size distribution and the extremely low pore interconnectivity. Moreover, the use of micro fillers in the production of MSC results in high bulk density barriers, which enhance isolation properties [START_REF] Moosberg-Bustnes | The function of fillers in concrete[END_REF]. Despite the small size of viruses and bacteria (typically of a diameter of around 60-140 nm in the case of the SARS-CoV-2 virus), the combination of very low pore radius of the MSC (ranging from 38 to 110 nm) and disconnected pores -in addition to the hydrophobic properties of sulphur -restrict the fluid flow paths through the concrete bulk, thereby preventing the leakage of toxic and medical hazardous components through the MSC barrier system.

Landfill mining has emerged, lately, as a way to recover metals, construction material, refuse-derived fuel, and/or plastic from landfill sites [START_REF] Canopoli | Physico-chemical properties of excavated plastic from landfill mining and current recycling routes[END_REF]. The interest in landfill mining arose from the fact that significant quantities of useful material have been buried, with USA landfills alone holding over 589 million megatons of metals (USEPA, 2014). As an example, the amount of copper deposited in landfills is estimated to be about 393 million tons, comparable to the 330 million tons of copper in use in the techno sphere [START_REF] Krook | Landfill mining: A critical review of two decades of research[END_REF].

In the EU the total landfilled waste is estimated to be 30 to 50 billion cubic meters (Ratcliffe et al., 2012), which in the future may represent a valuable resource of metals and other materials. Recovery of plastic waste and landfill-mined, soil-like fractions (LFMSF) have become major driving forces for the economic viability of landfill mining due to the large fraction (20 to 70 %) of these in waste [START_REF] Chandana | Characterization and utilization of landfill-mined-soil-like-fractions (LFMSF) for sustainable development: A critical appraisal[END_REF].

Geoenvironmental applications plastic and rubber reuse -zero waste discharge

The extensive utilization of plastic as an everyday commodity has led to its indiscriminate disposal in the geoenvironment. It has been estimated that around 4.9 billion metric tons of plastic are discarded in landfills and the environment [START_REF] Geyer | Production, use, and fate of all plastics ever made[END_REF]. Discarded plastic degrades to micro-and nano-plastic, a critical situation because of the potential of plastic to have contaminants attached onto it, or to carry microbes, which because of the size of the micro-or nano-particles can move easily through different environmental media, and pass to the food chain. [START_REF] Ngo | Pathway, classification and removal efficiency of microplastics in wastewater treatment plants[END_REF] found micro-plastic in the influent, the sludge and the effluent that is released from wastewater treatment plants. [START_REF] Yonkos | Microplastics in four estuarine rivers in the Chesapeake Bay, USA[END_REF] found that microplastics have entered the food chain, appearing even in human placenta. This necessitates the recycling of plastic waste into secondary products and their use in industrial, infrastructure, and environmental applications. A summary of various products from plastic waste, the type of industry and method of application, together with the advantages and disadvantages of plastic waste utilization, is presented in Table 2.

Increasing demand for rubber by various industries has led to the production of numerous rubber products that at the end-of-life create a substantial environmental burden due to their non-biodegradability. The majority of rubber waste originates from discarded tires, and it is estimated that by 2030, approximately 1,200 million tires would be discarded annually [START_REF] Thomas | A comprehensive review on the applications of waste tire rubber in cement concrete[END_REF]. This constitutes a serious challenge, especially for developing countries that have insufficient landfills, and a lack of technical and practical expertise in recycling and processing of rubber waste. Unsanitary disposal of rubber waste may create a breeding ground for mosquitos and other disease vectors, in addition, to contamination of the geoenvironment [START_REF] Yung | A study of the durability properties of waste tire rubber applied to self-compacting concrete[END_REF].

Current reuse of rubber waste includes tire-derived fuel, molded and extruded products, rubber-modified asphalt and sealants, and ground-rubber applications in sport facilities surfaces and walkways. Rubber waste utilization in civil engineering applications include slope stabilization, sound barrier, road construction, vibration dampener, etc., while rubberwaste-derived aggregates have been utilized as lightweight fill material in embankments and retaining walls. For ameliorating the inferior geotechnical attributes of expansive clay, ground rubber, as additive [START_REF] Soltani | Improved geotechnical behavior of an expansive soil amended with tire-derived aggregates having different gradations[END_REF], and synthetic polymer (polyacrylamide), as binder [START_REF] Soltani | Engineering reactive clay systems by ground rubber replacement and polyacrylamide treatment[END_REF], have shown promise. Ground rubber additives also show promise for improvement in liquefaction resistance of silty sand [START_REF] Ghadr | Liquefaction resistance of silty sand with ground rubber additive[END_REF]. Wellgraded sand with nominally 10 wt% pulverized waste tire material has shown promise to act as adsorptive fill material for the removal of benzene, toluene, ethylbenzene, and xylene (BTEX) components and heavy metals, whilst providing adequate shear capacity for many load-bearing field applications (Shahrokhi-Shahraki et al., 2020). Rubber-waste-derived aggregates can also be used in leachate collection and removal systems (LCRS) due to their high K values, which are much greater than the standard recommended value of K = 10 -4 m/s for LCRS layers. Shredded rubber particles have found limited application in concrete casting as aggregate replacement [START_REF] Jalal | Waste tire rubber and pozzolans in concrete: A tradeoff between cleaner production and mechanical properties in a greener concrete[END_REF], because they may reduce the concrete strength due to the hydrophobicity and low surface energy of rubber, which prevents good adhesion to the matrix [START_REF] Nuzaimah | Sodium hydroxide treatment ofwaste rubber crumb and its effects on properties of unsaturated polyester composites[END_REF]. Finally, rubber waste applications are being explored for thermal and sound insulation [START_REF] Thai | Recycling of waste tire fibers into advanced aerogels for thermal insulation and sound absorption applications[END_REF], and as vibration dampeners to diminish/absorb the vibrational effects on structures.

More advanced reuse concepts, such as zero waste discharge that include zero-liquiddischarge (ZLD) and zero-solid-waste have become mainstream practices. ZLD is a wastewater management approach that aims to recover and reuse the entirety of the industrial wastewater generated, resulting in zero discharge in the environment. ZLD was limited by the efficiency, energy requirements and cost of the thermal and membrane technologies on which it is based, but it is now driven by new regulations on effluent limitations in several countries [START_REF] Yaqub | Zero-liquid discharge (ZLD) technology for resource recovery from wastewater: A review[END_REF]. For example, in the USA, the first federal maximum pollutant concentrations, pre-treatment standards, and best pollutant control technologies for effluent reduction from steam electric power-generating plants were mandated (USEPA, 2015). China announced a new action plan in 2015, aiming to reduce water pollution substantially by 2030. Measures in the Chinese action plan that favour ZLD are, among others, control and reduction of pollutant discharge, financial incentives to promote reuse of industrial, reclaimed and sea waters, as well as strengthening regulations, inspection and penalty structures, technological and management practices, and stakeholder participation. Finally, the EU has, since 2015, launched a Circular Economy Action Plan (EU, 2015), which has motivated several major European industrial leaders to commit to ZLD and the principles of circular economy.

Geoenvironmental Applications for Land Improvement

Green developments in ground improvement and restoration

Ground improvement works are challenging when problems of soft and weak estuarine clays, highly compressible peats, offshore marine muds and sensitive clays are encountered in infrastructure projects. Recent ground improvement solutions include recycling of C&DW in the place of quarry-based raw materials; development of adsorptive load-bearing fill from recycled by-product materials, for the removal of organic/inorganic contaminants; and new, more efficient and environmentally-friendly construction practices and technologies.

For instance, rather than importing engineering fill to a site, marginal fill material already on-site, but with nominally too high moisture content can be successfully incorporated in embankment construction operations, using geosynthetic inclusions that provide both reinforcement and drainage functions and/or amended with suitable industrial by-product materials. Limited life geosynthetics, fabricated from natural fibers, can replace conventional synthetic prefabricated vertical drains (PVDs) during the embankment foundation's consolidation stage. Lightweight fills, including recycled tire bales and expanded clay aggregate can be used in the embankment-core construction. Rather than employing surcharge preloading with PVDs to achieve acceptable residual settlements for embankments on soft compressible soil deposits, the vacuum preloading method can be used [START_REF] Griffin | Sustainability of combined vacuum and surcharge preloading[END_REF].

Major 'green' developments in the ground improvement sector include techniques and processes that are bioinspired. These include, using naturally occurring/produced polymers (biopolymers) for soil mixing and grouting applications; bio-calcification of sandy soils using the Microbial-Induced Carbonate Precipitation -MICP method [START_REF] Dejong | Biogeochemical processes and geotechnical applications: progress, opportunities and challenges[END_REF], which employs ureolytic bacteria, but more recently investigating indigenous to a given site bacteria, and the use of nanomaterials (e.g., nanosilica) for grouting treatments. Even modest amounts of bonding between soil solids can produce sufficient real cohesion and significantly increase near-surface strength for coarse-grained soil deposits. As well as reducing the carbon footprint, many of the above, though presently at bench scale, have shown promise in replacing traditional cementitious and chemical additives. Biopolymers have also been tested in the manufacturing of earthen blocks for improved load bearing and durability performance.

Wind tunnel experiments have shown that the MICP technique of applying spray treatment of the reagents (bacterial cell and urea-calcium chloride solutions) onto a loose sand surface layer has succeeded in stabilizing this layer, with the potential to protect coastal sand dunes. The MICP technique using multiple staged injections of the reagents has also been shown to reduce internal erosion, one of the main causes of failure for hydraulic earthen structures [START_REF] Amin | Reducing the hydraulic erosion of sand using microbial-induced carbonate precipitation[END_REF].

Nano-silica (aqueous silica nanoparticles) has shown promise for grouting applications, producing a viscous gel property between soil particles to improve the liquefaction resistance, geotechnical load-bearing capacity, and internal and surface layer erosion resistances of loose to medium sandy deposits [START_REF] Gallagher | Influence of colloidal silica grout on liquefaction potential and cyclic undrained behavior of loose sand[END_REF].

Foundations and linear infrastructure bearing on expansive soil deposits can experience significant expansion/contraction during the absorption-desorption of a soil, leading to foundation distortion, wall cracking, and undulation and cracking of road pavements, etc.

Rather than using traditional cementitious/calcium-based binders, addition of sulfonated oil (a group of synthetic surfactants) has been shown to mitigate the swell-shrink volume change potential of expansive soil at bench scale (Soltani et al., 2021).

Environmental geotechnics has been central to peatland management and restoration.

These dynamic eco-hydrological wetland systems (and substantial carbon sinks) are increasingly under threat due to natural and anthropogenic pressures. One solution that has proven effective in the field is the strategic construction of peat bunds (dams) at affected areas around the bog margins. These bunds can gradually re-establish the natural peatland hydrology and indigenous peat-forming vegetation through collection, storage and controlled release of runoff from the high bog. Root reinforcement and slope bioengineering stabilisation have the ability to replace classically engineered interventions (e.g., piles, soil nails, ground anchors) with natural planting for stabilisation or remediation of failures in natural and manmade slopes [START_REF] Wood | Briefing: Biological routes to improvement of geomaterials[END_REF].

The use of geosynthetics in civil engineering often contributes to reductions in the cost of imported materials and the amounts of waste generated, compared with the use of soil, concrete, or steel [START_REF] Jones | Using geosynthetics for sustainable development[END_REF]. Important cost-savings related to materials' replacement associated to geosynthetics are found in landfills, where the volume of space saved by using thin geosynthetic materials instead of thicker mineral barriers is very significant. As reinforcement material, geosynthetics can be used to improve low-quality soils or alternative fill materials, such as C&DW, dredged materials, or other industrial nonhazardous wastes.

The use of recycled C&DW as fill material of geosynthetic-reinforced structures (e.g., embankments, retaining walls) has shown very good overall performance. Geogrids and geocells have also been used to improve the performance of unpaved and paved roads made of recycled asphalt pavement [START_REF] Gongora | Influence of fill and geogrid characteristics on the performance of unpaved roads on weak subgrades[END_REF].

The use of dredged materials as backfill for geogrid-reinforced structures has been evaluated through pull-out tests [START_REF] Kondo | Evaluation of the Interaction between Dredged Materials and Geosynthetic Reinforcements[END_REF]. The coefficient of interaction between uniaxial geogrids and the dredged materials has been found to be slightly smaller than the value for clayey soils. Despite the lower performance of these to that of conventional materials, the environmental benefits can be of importance.

In mining applications, geosynthetic barriers can be used to reduce the environmental risks associated with contaminant migration and to improve the recovery of mine water [START_REF] Bouazza | Geosynthetics in mining applications[END_REF]. Geosynthetic materials, such as geopipes, geotextiles, and geodrains are also often incorporated into the drainage systems of tailing storage facilities or mining waste (waste rock) storage facilities.

The dewatering of slurry-like wastes, such as municipal waste, agriculture waste, food and food-processing waste, industrial and mining waste, and contaminated sediments can also be performed through geotextile tubes, offering cost savings and environmental benefits.

Savings regarding the amount of materials and related costs vary with project conditions.

A decrease of 30 % to 40 % in the aggregates, from the application of geosynthetics in road construction over soft soils (shear strength < 30 kPa) was reported by [START_REF] Christopher | Cost savings by using geosynthetics in the construction of civil works projects[END_REF].

The International Geosynthetics Society (IGS) Sustainability Committee has stated that geosynthetics can reduce the use of aggregate in infrastructure construction by over 50 % and up to 90 % in some cases [START_REF] Igs | Did You Know? #2: Geosynthetics can greatly reduce the dependency on aggregates in infrastructure construction[END_REF]. In drainage applications, the use of a geotextile filter wrapped around the open-graded aggregate allows a decrease in the dimensions of the drain trench typically on the order of 0.3 m in width, without a decrease in flow capacity [START_REF] Christopher | Cost savings by using geosynthetics in the construction of civil works projects[END_REF].

Sustainable land remediation

Land remediation involves site characterization, risk assessment, and remediation. Risk assessment can be complex due to incomplete knowledge of the hydrogeological conditions and of the physical, chemical, and biological processes operating at a site, the contaminants' effects, uncertainties in the toxicity data, etc. ( (Paleologos and Lerche, 1999;Mohamed et al., 2020).

Several innovative remediation technologies have been developed to reduce the risk by either removing the contaminants, transforming them into non-toxic forms, or stabilizing/immobilizing them in soil and groundwater. Such technologies for soil remediation include soil washing/flushing, soil vapour extraction, electrokinetic remediation, thermal treatment, phytoremediation, and bioremediation, while for groundwater, remediation applications include air sparging, permeable reactive barriers (PRBs), chemical oxidation/reduction, and bioremediation (Shahrokhi-Shahraki et al., 2020). The use of a particular clean-up technology is mostly site-specific, and the cost and time needed for remediation can vary significantly, spanning hundreds of millions of dollars in clean-up costs and several decades of application (National Research Council, 1999). Some regulations have stipulated specific criteria for the selection of remediation technology; for example, a nine-point criteria scheme developed by USEPA (2008) for clean-up of Superfund sites requires measures to: 1) protect human health and the environment; 2) comply applicable or relevant and appropriate requirements (ARARs); to provide: 3) long-term effectiveness and permanence; 4) reduction of waste toxicity, mobility, and/or volume; 5) short-term effectiveness; 6) implementability; 7) low cost; 8) state acceptance; and 9) community acceptance.

Conventional remediation approaches are not always sustainable, as they do not always account for broader environmental impacts, such as the extraction and use of natural resources, the wastes created, and the energy used, and related greenhouse gas (GHG) emissions for on-and off-site operations and transportation, and the value lost from the residual degradation of a resource [START_REF] Paleologos | The lost value of groundwater and its influence on environmental decision-making[END_REF].

To address these deficiencies, some green and sustainable remediation (GSR) approaches have emerged (ITRC, 2011;SURF, 2009;USEPA, 2008). USEPA proposed as a green remediation (GR) concept the practice of considering all environmental effects of clean-up during each phase of the process, and incorporating strategies to maximize the net environmental benefit of the clean-up (USEPA, 2008). The Sustainable Remediation Forum (SURF) promoted a sustainable remediation (SR) concept that uses sustainable practices during the investigation, construction, remediation, redevelopment, and monitoring of contaminated sites, with the objective of balancing economic viability, conserving natural resources and biodiversity, and enhancing the quality of life in surrounding communities (SURF, 2009). The Interstate Technology & Regulatory Council (ITRC) introduced GSR, which is defined as the site-specific employment of products, processes, technologies, and procedures that mitigate contaminant risk to receptors, while making decisions that are cognizant of balancing community goals, economic impacts, and environmental effects (ITRC, 2011).

ITRC proposed a tiered sustainable remediation framework that represents the confluence of environmental, social and economic factors in decision-making (ITRC, 2011). Specific quantitative assessment tools, such as SiteWise TM and Spreadsheets for Environmental Footprint Analysis (SEFA) have been developed by federal agencies, such as the US Navy and USEPA, to quantify the metrics for environmental impacts. However, tools to assess economic and social impacts are still being developed. Life cycle cost analysis (LCCA) and cost-benefit analysis (CBA) methods are used for broader economic impact assessments. Monetized environmental costs (e.g., social cost of carbon) are also included as indirect sources of costs in such assessments (Reddy et al., 2019). Social aspects of sustainability include consideration of interests of all stakeholders, although social impacts are difficult to quantify (Reddy et al., 2014).

Resiliency of site remediation systems under changing climatic conditions and extreme weather events are of growing concern to environmental remediation professionals. For example, inundation of a contaminated site may adversely affect the bioremediation effectiveness in hydrocarbon-contaminated soils. Similarly, salt-water intrusion into coastal aquifers from a sea-level rise or any changes in groundwater elevations may have a negative impact on groundwater remediation by PRBs or chemical oxidation.

Finally, while the environmental dimension of sustainable remediation is well addressed, the economic and social dimensions are not yet fully developed and require additional research. All stakeholders should be educated on the benefits of sustainable remediation.

Incorporating resiliency into sustainable remediation is also emerging as an urgent need to the environmental remediation professionals.

Clean Energy Production Using Thermo-active Geostructures

The use of engineering projects to accomplish supplementary goals to their original purpose has become more prevalent recently [START_REF] Burra | Synergetic effects in steam gasification of combined biomass and plastic waste mixtures[END_REF]. The final section of this article highlights a perhaps not so well known area, where traditional ground structures (e.g., pile foundations, tunnel linings, diaphragm walls) can be used as heat exchangers, as thermo-active geostructures for energy production [START_REF] Laloui | Analysis and design of energy geostructures -Theoretical essentials and practical application[END_REF]. The total number of such projects (< 200) still remains small compared to the total ground source heat pump installations worldwide.

The heat exchange in thermo-active geostructures is usually ensured by a heat pump system connected to heat exchanger pipes, embedded inside the geostructures during its construction, as a closed-loop geothermal system. A heat carrier fluid circulates inside the pipes to ensure the heat exchange between the geostructures and the heat pump. This system can be used for heating/cooling demands in residential buildings, for agricultural and industrial activities, to prevent the icing of pavements for transport infrastructures, etc.

In addition to the advantages similar to the conventional shallow geothermal energy systems, energy geostructures use underground structures, already required for structural reasons.

Between various types of energy geostructures, piles are the most common. Piles have the potential to offer reduced capital cost compared to traditional vertical ground heat exchangers, such as boreholes. In addition, as piles have a superficial resemblance to boreholes, there exist already available thermal design methods that can be adapted for use with piles. For instance, a thermal pile standard (GSHPA, 2018) has been developed from the vertical borehole standard and various methods for geotechnical design of piles subjected to thermal changes have been recently made available [START_REF] Laloui | Analysis and design of energy geostructures -Theoretical essentials and practical application[END_REF]. By contrast, for other geostructures, some demonstration projects using slabs, walls and tunnels as ground heat exchangers exist, but there are no standard design or analytical approaches, and every new project must proceed on a case-by-case basis. Sani et al. (2019) presented an extensive review of research on energy piles and identified the effects of heating and cooling on building foundations in order to successfully design and install an energy pile system that will provide a highly efficient, cost effective and long-term solution for the heating and cooling demands of a building. Bourne-Webb and Bodas Freitas (2020) confirmed that thermal activation induces stress change and deformation of the pile that depend on the boundary conditions (e.g., soil/pile/superstructure interaction). The most recent studies related to energy piles focused on their long-term thermo-mechanical behaviour and group behaviour [START_REF] Nguyen | Long-term thermo-mechanical behaviour of energy pile in clay[END_REF].

For other energy geostructures, design methods are still under development. Numerical investigations [START_REF] Barla | Energy tunnels: concept and design aspects[END_REF] have proved the feasibility of energy tunnels; the exploitable heat of the tunnel lining was estimated at 10-20 W/m 2 when no groundwater flow is present, and up to 50-60 W/m 2 , when there is significant groundwater flow. A comprehensive analysis of the energy performance and economic attractiveness of energy segmental linings for subway tunnels (Cousin et al., 2020) demonstrated that while the energy performance may be significantly enhanced, the most efficient design does not lead to the lowest LCOE (levelized cost of energy) and the earliest profitability. Observations on a real-scale experimental prototype of an energy tunnel [START_REF] Barla | A novel real-scale experimental prototype of energy tunnel[END_REF] confirmed that this technology is mature for industrial application. A preliminary study on the geothermal countries, especially developing countries, for which shallow geothermal energy applications are still in the early stages.

Concluding Remarks

The overarching objective of the current paper is to put at the forefront some significant contributions in sustainable technologies and management practices across a broad range of environmental geotechnical projects, thus highlighting the important role of the discipline for the establishment of a green economy in the post COVID-19 era.

We have concentrated on three thematic areas, and recent developments related to sustainability solutions in these areas, as well as providing recommendations to overcome technological, regulatory, and administrative obstacles. In view of the increased urbanization in developing countries, the reuse of solid waste in the construction industry, given this industry's significant material and energy consumptions, as well as the quantity of waste generated, are of critical importance. Reusing construction and demolition waste (C&DW), excavated soil and rock, and industrial waste, such as rubber and plastic, can find extensive application in road pavement layers, drainage projects, landfill cover and liners, embankments, and other infrastructure projects, thereby resulting in significant savings in resource exploitation, transportation, and waste disposal.

Stabilization/solidification (S/S) of marine sediments with binders and additives represents a promising alternative for managing marine sediments dredged periodically in harbour areas. Treated sediments may be used as aggregates for road construction, cemented mortars, fill material in brick production, or in port piers, shores, and breakwaters. Treatment with additives, such as biochar or mussel shell flour, presents a more sustainable solution since it reduces the content of cement in the current cementitious binders, and can achieve significant savings in carbon dioxide emissions during the clinker production of cementitious binders.

Despite the trend in advanced countries toward recycling and waste-to-energy, landfills constitute the most common municipal solid waste management practice. The complexity of virus interaction with host soils and the potential for transport of viruses through clay liners requires a rethinking of the material in landfill barriers. One example of such material may be elemental sulphur, through the use of modified sulphur concrete (MSC) as a component of landfill barrier systems, in order to prevent migration of leachate to groundwater. The MSC has a lower K than that specified by USEPA for landfill barriers. Furthermore, it is an inert material, and has very low leaching rate in variable-pH environments, and good resistance to chemical and physical degradation. Its very low K and extremely low pore interconnectivity may prove beneficial in hindering the migration of medical contaminants, such as those carrying SARS-CoV-2, through landfill liners.

Sustainable ground improvement solutions include recycling of C&DW instead of using quarry-based raw materials; development of adsorptive load-bearing fill from recycled byproduct materials, for the removal of organic/inorganic contaminants; novel foundation systems instead of conventional concrete foundations; and the embedment of new, more efficient and environmentally-friendly construction practices and technologies. Instead of using traditional cementitious/calcium-based binders, foundations and linear infrastructure constructed on expansive soil deposits may use sulfonated oil or ground rubber, recycled from scrap tires, and synthetic polymer, as binder, to mitigate the swell-shrink volume change potential.

Geosynthetics play a major role in drainage, barrier and final cover systems of landfills, and in mining applications, and their use contributes to significant reductions in the cost of imported materials, and quantity of waste generated, thus providing a more efficient use of natural resources compared to traditional approaches that use soil, concrete, or steel.

At the same time, remediation solutions are required to address the multitude of contaminated sites worldwide. Advanced developments that incorporate environmental, economic, and social dimensions are presented herein.

Finally, the topic of thermo-active geostructures concludes this paper, where, apart from their infrastructure utility, these structures have the potential to contribute to the energy mix as a renewable energy source.

Thus, the benefits of adapting the above technologies and practices include, among others, waste minimization and promotion of reuse and recycling; preservation of natural resources; improvement of waste management systems; reduction of the threats to soil and groundwater contamination; and reduction of the carbon footprint and of greenhouse gas emissions. Altogether, the presented topics may provide a blueprint for a path forward to environmental geotechnics in achieving more sustainable and cost-effective solutions, and the development of more resilient infrastructures and greener societies. Reddy KR, Kumar G and Chetri JK (2019) Green and sustainable site remediation:

Incorporating broader economic impacts. In Proceedings of 17 th African Regional

  detected micro-plastic in estuarine rivers, whereas O' Connor et al. (2019) and Panno et al. (2019) found it present in both the unsaturated and saturated zones. Recently, Ragusa et al. (2021)

  Paleologos E K and Lerche I (1999) Multiple decision-making criteria in the transport and burial of hazardous and radioactive wastes. Stochastic Environmental Research & Risk Assessment 13(6): 381-395. Panno SV, Kelly WR, Scott J et al. (2019) Microplastic contamination in karst groundwater systems. Groundwater 57(2): 189-196, https://doi.org/10.1111/gwat.12862. Papadimitriou CA, Krey G, Stamatis N et al. (2017) The use of waste mussel shells for the adsorption of dyes and heavy metals. Journal of Chemical Technology & Biotechnology, https://doi.org/10.1002/jctb.5247. Petti R, Marchi M, Ploetze M et al. (2020) Additivi a basso impatto ambientale per la stabilizzazione di sedimenti di dragaggio: effetti geotecnici. In XXVII Convegno nazionale di Geotecnica, Reggio Calabria. Ragusa A, Svelato A, Santacroce C et al. (2021) Plasticenta: First evidence of doi.org/10.1016/j.envint.2020.106274. Rammal D, Mroueh H and Burlon S (2020) Thermal behaviour of geothermal diaphragm walls: Evaluation of exchanged thermal power. Renewable Energy 147: 2643 -2653. Ratcliffe A, Prent OJ and van Vossen W (2012) Feasibility of material recovery from landfills (MFL) in the European Union. In Proceedings of the ISWA World Waste Congress, Florence, Italy.

Figure captions Figure 1 .

 captions1 Figure captions

Figure 2 .Figure 3 .

 23 Figure 2. MSW generation (in thousand tonnes) in the three largest economies (OECD,

Figure 4 .

 4 Figure 4. Design of MSC-containment construction barrier (modified after Mohamed and

Figure 1 .Figure 3 .

 13 Figure 1. a) Activity chart of the Gulf of Taranto marine sediments (Italy) collected down to 39 m depth (Vitone et al., 2020); b) Casagrande plasticity chart of a sediment sample from Gulf of Taranto. Sed_1ASTM: standard procedure, Sed_1UP: unconventional procedure (Petti et al., 2020), A: activity; bsf: below sea floor.

Figure 4 .

 4 Figure 4. Design of MSC-containment construction barrier (modified after Mohamed and El Gamal 2017, 2014).

Table 1 .

 1 Physical and state properties of natural and treated sediments with 8% Portland cement (Sed8PC), or 6% Portland cement with 2% MSF (Sed6PC2MSF) (curing time: 28 days).

	Tables

Table 2 .

 2 State-of-the-art on plastics waste utilization for geoenvironmental applications.

Table 1 .

 1 Physical and state properties of natural and treated sediments with 8% Portland cement (Sed8PC), or 6% Portland cement with 2% MSF (Sed6PC2MSF) (curing time: 28 days).

	Mix	w L (%) PI (%) w 0 (%) CI (-)
	Sed_UP	53.59	28.61	74.0	-0.72
	Sed8PC	86.50	32.20	66.95	0.61
	Sed6PC2MF 85.17	29.83	66.47	0.63
	w

L : Liquid limit, PI: Plasticity Index, w 0 : Water content and CI: Consistency index.

Table 2 .

 2 State-of-the-art on plastics waste utilization for geoenvironmental applications.

	1120
	1121
	Industry	Technique/method /field of application	Product s	Advantages	Disadvantages	Referen ces

Acknowledgements

The second author would like to acknowledge the support of the Office of the Associate Provost for Research and Academic Development at Abu Dhabi University, UAE through grant 19300540.

potential assessment and thermal influences of planned tunnel infrastructure at the cityscale of Basel (Switzerland) illustrated the applicability of the developed methods to evaluate the general feasibility of the energetic use of tunnel structures (Epting et al., 2020).

For energy diaphragm walls, both energy and geotechnical aspects are still being investigated. Several numerical tools have been recently developed to maximise the energy performance of a diaphragm wall through the design of piping layout [START_REF] Sterpi | Energy performance of ground heat exchangers embedded in diaphragm walls: Field observations and optimization by numerical modelling[END_REF], or to evaluate the conductive and advective exchanged heat by the wall and the surrounding soil (Rammal et al., 2020). From the mechanical point of view, thermal activation of a diaphragm wall would induce additional bending moment and horizontal displacement of the wall. However, the computed stress variations are largely compatible with strength limits.

Although the scientific literature on the design and operation of shallow geothermal energy systems (including energy geostructures) has been increasing over the past years, this technology can be considered as a decentralized source of energy without being fully competitive yet [START_REF] Tsagarakis | Shallow geothermal energy under microscope: social, economic, and institutional aspects[END_REF]. What hinders the wide penetration of these systems in most countries are issues related to high installation costs, administration, stakeholders' awareness, and marketing. A recent review of the legal framework in shallow geothermal energy in several European countries [START_REF] Tsagarakis | A review of the legal framework in shallow geothermal energy in selected European countries: need for guidelines[END_REF] showed that a high diversity exists on legislation provisions, regulations, standards, and institutional support. These authors suggested that this discrepancy acts as a barrier for further development of the shallow geothermal energy market. They concluded that issuing appropriate technical rules and standardization among EU countries would result in an increase of these energy systems' market. A common legislative EU framework will also benefit and affect other