The Potential of Recycling and Reusing Waste Materials in Underground Construction: A Review of Sustainable Practices and Challenges

Abstract

Underground infrastructure projects pose significant environmental risks due to resource consumption, ground stability issues, and potential ecological damage. This review explores sustainable practices for mitigating these impacts throughout the lifecycle of underground construction projects, focusing on recycling and reusing excavated tunnel materials. This review systematically analyzed a wide array of sustainable practices, including on-site reuse of excavated tunnel material as backfill, grouting, soil conditioning, and concrete production. Off-site reuses explored are road bases, refilling works, value-added materials, like aggregates and construction products, vegetation reclamation, and landscaping. Opportunities to recover and repurpose tunnel components like temporary support structures, known as “false linings”, are also reviewed. Furthermore, the potential for utilizing industrial and construction wastes in underground works are explored, such as for thermal insulation, fire protection, grouting, and tunnel lining. Incorporating green materials and energy-efficient methods in areas like grouting, lighting, and lining are also discussed. Through comprehensive analysis of numerous case studies, this review demonstrates that with optimized planning, treatment techniques, and end-use selection informed by material characterization, sustainable practices can significantly reduce the environmental footprint of underground infrastructure. However, certain approaches require further refinement and standardization, particularly in areas like the consistent assessment of recycled material properties and the development of standardized guidelines for their use in various applications. These practices contribute to broader sustainability goals by reducing resource consumption, minimizing waste generation, and promoting the use of recycled and green materials. Achieving coordinated multi-stakeholder adoption, including collaboration between contractors, suppliers, regulatory bodies, and research institutions, is crucial for maximizing the impact of these practices and accelerating the transition towards a more sustainable underground construction industry.

1. Introduction

Underground construction projects play a crucial role in infrastructure development but can also present significant environmental challenges if not properly planned and executed. These challenges include the generation of large volumes of tunnel excavated material (TEM), the potential for disruption to sensitive ecosystems, and the energy-intensive nature of some construction processes [1,2,3]. Furthermore, underground constructions, if not conducted sustainably, can negatively impact the environment in several ways. Groundwater levels and flow paths may be disrupted, potentially affecting water availability for both human and ecosystem needs. Vibrations and settlement caused by tunneling can damage ecosystems located above the alignment. Large volumes of water required for processes like slurry tunnel boring machine (TBM) operations and concrete production could strain local water supplies. Excavation activities also release airborne dust and emissions from construction equipment and vehicles. If not properly treated, excavated material also has potential to leach nutrients, salts, or chemicals into waterways [4,5]. Larger projects extracting millions of tons of material could potentially release over 1000 tons of carbon dioxide equivalents solely from on-site energy consumption [6,7].

However, these challenges also present opportunities to reduce environmental impacts and costs through innovative approaches such as increasing reuse and recycling of TEM or other industries’ waste. By embracing circular economy principles and prioritizing resource recovery, tunnel projects can minimize their environmental footprint while advancing more sustainable infrastructure systems. For instance, the Brenner Base Tunnel project in Austria demonstrates how using innovative recycling processes can transform TEM into valuable resources for other construction processes [4,8].

Therefore, understanding and mitigating these environmental impacts is crucial for ensuring the sustainability of tunnel initiatives. Key sustainability aspects for underground projects include:

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Reducing the carbon footprint: This involves minimizing energy consumption and emissions throughout the project lifecycle, from design to construction and operation [9,10].

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Optimizing material usage: This entails minimizing the use of virgin materials and maximizing the reuse and recycling of existing materials [9].

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Protecting the natural and built environments: This includes minimizing disturbance to ecosystems, preserving biodiversity, and ensuring the long-term integrity of the surrounding infrastructure [11].

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Achieving whole-life cost-effectiveness: This involves considering the long-term costs and benefits of the project, including maintenance and operational expenses [9].

Achieving these goals requires the integration of sustainable practices across all phases, from planning and design through to construction, operation, and maintenance. During planning and design, sustainable practices such as thorough geotechnical characterization of the site, optimized alignment selection to minimize excavation needs, value engineering (a systematic approach to analyze and improve the cost-effectiveness of a project without compromising its functionality), and modular construction approaches can help reduce environmental impacts. For the construction phase, mechanical excavation methods like TBMs are preferable to conventional drilling and blasting. TBMs allow for more controlled ground conditions with less vibration compared to explosive techniques. Efficient TEM removal systems are necessary to transport the large volumes of TEM with minimal disturbance to the environment. Additionally, sustainable operation and maintenance practices should focus on improving energy efficiency, implementing sustainable water management, reducing waste production, and adopting maintenance strategies that minimize long-term environmental footprint [12].

As the construction industry places increasing importance on sustainability, innovative approaches are being explored to diminish the environmental consequences of underground infrastructure development. Several case studies from various geographical contexts have demonstrated the successful implementation of sustainable practices [8,13,14,15,16,17]. This paper comprehensively reviews sustainable practices applicable throughout underground project lifecycles. As Figure 1 shows, sustainable approaches for underground projects are examined across key areas such as recycling and reuse of excavated materials, on-site or off-site reuse, utilizing other industries’ waste, and application of green construction methods. Each of these sustainable practices will be discussed in detail in subsequent sections. Through comprehensive analysis of numerous case studies from around the world, this review aims to demonstrate that with optimized planning, treatment techniques, and end-use selection informed by material characterization, sustainable practices can significantly reduce the environmental footprint of underground infrastructure projects while realizing economic benefits.

2. Recycling of Tunnel Excavation Materials

2.1. On-Site Reusing

Large volumes of TEM, commonly known as spoil, are generated during underground construction projects. It is estimated that every 1 km of tunnel construction with a diameter of 6 m produces an approximate spoil volume of 68,000 m³ [18]. A survey of 32 construction projects in the UK found that while spoil was reused on-site in some cases, most was disposed of in landfills [19]. The construction of the 33 km long twin-bore Koralm Tunnel (KAT2) in Austria generated around 5 million m³ of spoil [20]. Figure 2a shows how the expected spoil quantity (black line) over time for the KAT2 project was much higher than the maximum landfill capacity (red line).

Improper spoil management through landfilling can cause issues such as natural contamination, greenhouse gas emissions, land occupation, geological damage, and significant costs for project stakeholders [21,22]. Proper spoil management, including reuse and recycling, can provide economic and environmental benefits compared to disposal [23]. Figure 2b compares landfilling versus recycling for this project and external uses.

However, adopting sustainable practices varies due to limitations in knowledge, planning, and integration across project stages. For example, in China, as shown in Figure 3, although sand and clay are the two main recycled materials (accounting for approximately 60% of total recycled spoil), the current amount of recycled spoil is considerably lower than the amount landfilled. Several challenges to effective spoil management have been identified, including the variable physical and chemical properties of excavated material, undefined stakeholder roles and responsibilities, and regulations defining spoil as “waste”, restricting reuse options [24,25,26]. Current legal frameworks often lack consistency and uniformity in their treatment of TEM. Notably, sustainable spoil management practices have primarily been observed in some developed European countries like Austria, Switzerland, and France, with implementation so far limited to only a few underground construction projects [27]. Adoption of sustainable spoil management on a wider scale globally remains an ongoing challenge. European and Austrian waste laws (Europe Waste Framework Directive 2008/98/EC [28], Austrian Waste Management Act [29], Austrian Waste Management Plan [30]) provide criteria for tunnel spoil to cease being waste when certain quality standards are met and it is used as a substitute for raw materials [31].

Sustainable spoil management requires changes in industry attitudes as well as considering spoil planning throughout all project stages. With continuous innovation, the percentage of recycled TEM can be further increased to move towards zero waste tunneling. Recycling of TEM into reusable materials can significantly reduce greenhouse gas emissions compared to traditional material production and spoil landfilling. For example, it was estimated that between 1.1 million and 1.5 million tons of carbon dioxide equivalent emissions were avoided from 2010 and 2018 through spoil recycling in China. Recycling plants gained up to USD 22.8 million annually through recycled product sales [21].

In recent years, several tunneling projects, such as Delhi Metro Pink Line Project in India, Marmaray Rail Tunnel in Turkey, Gotthard Base Tunnel in Switzerland, and Sydney Metro Northwest in Australia, implemented advanced TBMs equipped with sensors and sorting mechanisms to classify spoil like sludge, soil, and recoverable sand–gravel. A significant percentage ranging from 70% to 96% across projects was recycled and reused [4,23,32].

However, recycling and reuse TEM depends greatly on material suitability and local context factors [15,33,34]. Based on the guidelines outlined in the Austrian Federal Waste Management Plan, four distinct potential utilization classes have been designated for excavated soil, encompassing tunnel excavation material [35]. Within these classifications, two classes demonstrated promising potential for on-site reuse, specifically for backfilling or landscaping purposes. Additionally, material from one of the classes was deemed suitable as a substitute for quarried aggregates. However, the properties of the material belonging to the remaining class imposed limitations on its potential for reuse. Therefore, comprehensive analysis of spoil properties is required due to variability from tunneling methods, additives, rock type, and excavation practices [33,34].

Various technologies have been developed for real-time analysis of spoil properties such as physical and chemical composition, minerals, and grain size. A concept for a mobile processing plant on the TBM tailskin is shown in Figure 4. An excavated material database system is proposed to facilitate recycling.

Research indicates that tunnel excavation material, or spoil, can be recycled and reused both on-site and off-site for various purposes. Potential reuse applications include as auxiliary materials to support further tunnel construction activities, as backfilling materials, in the production of construction aggregates, in recycled building materials, and as materials to support vegetation reclamation and site restoration efforts [4,36,37]. Figure 5 illustrates a general technical process for recycling and reusing spoil. Different applications for recycled spoil are discussed, including case studies and recent research.

2.1.1. Reuse as Raw Materials in Grouting

Grouting is an important process in shield tunneling used to fill gaps between the shield tail and tunnel segments, thereby preventing leakage, ground settlement, and lining displacement [38,39]. However, conventional filling materials like cement grout and mortar require significant quantities of bentonite, cement, and fine sand and have issues such as high shrinkage and consumption.

Reusing shield muck as a component in grouting material can help address these challenges by reducing waste and costs [40]. Several studies have proposed utilizing muck from tunneling as a partial replacement in grouting mixtures [40,41,42,43,44]. This approach has been implemented successfully in various tunnel projects, such as the Weisanlu River-crossing tunnel project in Nanjing [45], the S1 Vienna outer ring expressway project [17], the Nanjing Metro Line 10 tunnel project [46], and the east bound and west bound tunnel projects in Zhengzhou [47]. For instance, the S1 Vienna project involved constructing a 19 km section between Schwechat and Suessenbrunn, including an 8.3 km long tunnel under the Danube River and Lobau region. According to project reports, around 3 million m³ of TEM was excavated from this section, and approximately 1.4 million m³ of TEM was reused on-site as a component in backfill grouting [17]. Figure 6 depicts the volumes of TEM excavated and reused as a component of backfill grouting separately for different parts of this section of the S1 Vienna project.

Particle size distribution analysis demonstrated that the discharged soil closely resembles standard river sand, which is commonly used in grouting for underground applications. This similarity in particle size distribution was observed in different projects, specifically the Nanjing Metro Line 10 tunnel project and the west bound tunnel project in Zhengzhou (Figure 7a,b) [46,47,48]. Furthermore, experimental results found that discharged soil met specification requirements for properties such as compressive strength, shear strength parameters, cohesion (Figure 8a), friction angle (Figure 8b), permeability, and optimal grout microstructure when replacing up to 50% of river sand [41,43,46]. Specifically, discharged soil <5 mm directly replaced purchased sand, while clay-rich soil required processing through modification.

To ensure that the discharged soil meets the required specifications for grouting, several treatment methods can be employed. These include screening and sieving to remove oversized particles and debris, washing to eliminate fine particles, clay, and silt, chemical stabilization with additives like lime, cement, or fly ash to improve mechanical properties and durability, dewatering to reduce moisture content to an optimal level, and blending to achieve a homogeneous mixture. Quality control testing, such as particle size analysis, Atterberg limits, compressive strength tests, and permeability tests, verifies that the treated soil meets all specified requirements for grouting [33,34,49,50,51,52].

Optimization of binder/sand ratios (e.g., 0.25) and fly ash/cement ratios (e.g., 0.5) improved strength performance. The approach provides a framework for classifying and sustainably recycling discharged soil as synchronous grout (Figure 9) with economic and environmental benefits compared to conventional disposal [48]. The projected cost savings from the case studies are estimated to be between 45% and 75%. In summary, TEM shows potential as a recycled component in backfill grouting when properly processed, with benefits for performance, cost, waste reduction, and sustainability.

2.1.2. Reuse as Raw Materials in Soil Conditioning

Reusing discharged waste soil can also be applied in soil conditioning processes for mechanized tunneling. There are two main aspects of this application: (1) dewatering treatment of waste slurry in slurry shield tunneling, and (2) as a soil conditioner additive in earth pressure balance TBMs (EPB-TBMs).

Slurry shield tunneling is widely used for large underground construction projects due to its safety and efficiency. However, it generates a large amount of waste slurry containing fine soil particles that requires dewatering treatment prior to disposal. Currently, lime is commonly utilized as a filter aid to dewater waste slurry via mechanical pressure filtration. However, lime use has drawbacks such as high costs and generating strongly alkaline wastewater and mud cakes. A case study in China, the Yellow Tunnel project of Jiluo Road, Jinan City, demonstrated the feasibility of using waste sand produced from slurry shield operations as an alternative filter aid to lime for waste slurry dewatering [53]. Batch experiments using a belt filter press compared the dewatering performance waste sand and lime additions (Figure 10a), finding that with optimal additions of 2% lime and 4% waste sand, their effects on drainage rate, mud cake moisture content, and porosity were similar [53] (Figure 11). Additionally, the use of waste sand instead of lime reduced the pH of the mud cake from strongly alkaline (about 12) to near-neutral (about 8), meeting the standard for non-hazardous waste (pH < 12.5) [54,55], enabling potential reuse of the dewatered soil rather than disposal as hazardous waste.

Furthermore, EPB-TBMs generate a large volume of conditioned discharged soil during excavation [56,57]. Conventional treatment methods like open stacking are unsustainable. Another case study evaluated the utilization of discharged soil as a component in bentonite–silty clay composite slurries for soil conditioning [58]. Testing showed the particle size distributions of waste silty clay from the Shenyang Metro Line 6 project were similar to bentonite (Figure 10b). Results showed that replacing bentonite with recycled silty clay decreased shear stress and apparent viscosity non-linearly, and a 14% slurry concentration with 25% bentonite substitution met construction standards economically and sustainably. Specifically, the use of recycled silty clay in the bentonite–silty clay composite slurry met the requirements for soil conditioning specified in the Technical Code for Tunnelling [59].

Figure 10. Particle size distributions of: (a) lime, waste slurry, and waste sand from the Yellow Tunnel project of Jiluo Road [53], (b) bentonite and waste silty clay of Shenyang Metro Line 6 project [58].

Figure 10. Particle size distributions of: (a) lime, waste slurry, and waste sand from the Yellow Tunnel project of Jiluo Road [53], (b) bentonite and waste silty clay of Shenyang Metro Line 6 project [58].

Figure 11. Effects of lime and waste sand addition on dewatering properties of waste slurry, including mud cake moisture content, drainage rate, and porosity [53].

Figure 11. Effects of lime and waste sand addition on dewatering properties of waste slurry, including mud cake moisture content, drainage rate, and porosity [53].

2.1.3. Reuse as Aggregates in Concrete Production

The utilization of recycled TEM as aggregates in concrete production is gaining traction due to its environmental and economic benefits. The concept of extensively recycling excavated materials aligns with the ongoing demand for raw aggregates, offering a potential solution to the shortage of construction materials. By employing suitable techniques, the comprehensive utilization of tunnel rock can contribute to a more sustainable construction industry. This concept, illustrated in Figure 12, encompasses a multi-step process: (a) initial excavation of rock and soil from underground tunneling projects; (b) transportation of TEM for processing into aggregates; (c) utilization of the recycled aggregates in the production of various types of concrete, tailored to specific construction needs; and (d) the construction of infrastructure using concrete incorporating recycled aggregates.

Several studies have explored the feasibility of recycling TEM and utilizing it as aggregates in structural concrete, shotcrete, and tunnel lining concrete, provided rock properties and processing methods are appropriately evaluated [37,60,61,62,63,64,65]. Extensive laboratory and field testing has demonstrated the viability of using recycled TEM aggregates from various tunneling projects, such as the Götschka Tunnel, Koralm Tunnel, Brenner Base Tunnel, Torrent-La Thuile Tunnel, the Turin underground railway, Gotthard Tunnel, and Loetschberg Base Tunnel, and the Omegna Tunnel effectively recycled spoil as concrete aggregates [8,14,17,66,67]. For example, laboratory tests on quartz phyllite and schist rocks from excavation material of the Brenner Base Tunnel showed potential for recycling though variations in properties, especially for concrete mixtures, were addressed [62]. The resulting concrete demonstrated compressive strengths of 42MPa, sufficient for shotcrete and structural applications [63]. Over 35% of excavated material in the Gotthard Tunnel project was recycled and utilized as concrete aggregates [68]. Furthermore, Luo et al. evaluated the economic and environmental impacts, indicating potential cost savings of up to 9.5%, an 11% reduction in embodied energy, and a 15.5% decrease in embodied carbon compared to natural aggregates in concrete production [60].

However, TBM muck requires treatment to produce round particles and reduce shape index for use in concrete. Therefore, appropriate treatment methods are critical to produce suitable recycled aggregates. The Omegna Tunnel project evaluated innovative methods for sustainable TBM muck recycling, emphasizing proper management and reuse potential as concrete aggregates to minimize environmental impacts [67]. Testing of a mobile (Figure 13a) and fixed (Figure 13b) treatment plant showed the fixed plant optimized particle size distributions when fractions were well proportioned (Figure 14), whereas the mobile plant was insufficient [69]. In summary, the fixed plant offers a suitable product for concrete, if the different size fractions obtained in the plant are mixed appropriately [14,67].

While the use of TEM in concrete production offers significant environmental and economic benefits, scaling up this practice requires addressing several key challenges. The composition of tunnel muck can vary widely depending on the geological formations encountered, impacting the physical and chemical properties of the aggregates and leading to inconsistencies in concrete performance. For example, high mica content can negatively affect concrete strength and workability, as observed in the Koralm Tunnel project [14,25]. Additionally, the size and shape of recycled TEM can vary, affecting workability, strength, and durability of the concrete. Maintaining consistent processing of recycled aggregates is crucial for quality control, involving careful control of crushing, screening, and washing processes to ensure uniform particle size and shape. Regular testing and monitoring are essential to ensure the recycled aggregates meet the required specifications for concrete production. Blending recycled aggregates with conventional aggregates might be necessary to achieve desired concrete properties, requiring careful optimization of the mix design. Scaling up recycled aggregate use also demands sufficient processing facilities to handle increased volumes of material, including crushing, screening, and washing plants, as well as storage and transportation infrastructure. Efficient transportation and handling of the recycled aggregates are essential to minimize costs and ensure timely delivery to concrete production sites.

2.2. Off-Site Reusing

In addition to on-site reusing, TEM can be utilized for off-site applications such as embankments, road bases, refilling works, value-added materials, vegetation reclamation materials, and landscaping works. For instance, the S10 Mühlviertler Schnellstraße (S10) project in Austria adopted sustainable practices during planning and construction to balance cut-fill volumes, maximize tunnel spoil reuse, and minimize transportation. Nearly all (4.6 million m³) of the TEM was reused on-site as concrete aggregates, embankments, and road bases with less than 250,000 m³ transported off-site for deposition [66].

However, the environmental impact of transporting TEM off-site is a critical consideration. Transporting TEM can lead to increased carbon emissions, noise pollution, and traffic congestion. The process involves several steps, including the stabilization of slurry-like materials on-site, transportation to disposal or recycling sites, and subsequent processing. Each of these steps can have negative environmental effects. For example, transporting TEM over long distances can result in significant greenhouse gas emissions from vehicles. Noise pollution and increased traffic congestion are also potential issues, especially when transportation routes pass through residential or environmentally sensitive areas [70].

To mitigate these negative impacts, several measures can be implemented. These include minimizing transportation distances by selecting disposal or recycling sites close to the excavation site, optimizing transportation logistics to reduce the number of trips, and using environmentally friendly vehicles with lower emissions. Additionally, the integration of TEM recycling processes on-site can significantly reduce the need for transportation. Effective environmental auditing and monitoring programs are essential to ensure that these measures are successful and that the environmental impact is kept to an acceptable level [66,70].

2.2.1. Reuse as Sub-Base for Road Pavements

Road construction requires significant resources and impacts the environment. Several case studies demonstrated the technical feasibility of recycling and reusing TEM as road construction materials [18,37,71] such as the Deo-Ca Road Tunnel construction project in Vietnam [16], the S10 project in Austria [66], and the Sydney Metro Tunnel [72].

With appropriate processing of the muck, it could replace quarried aggregates for use in roadworks [16]. Grain composition analysis indicated that the recycled muck aggregate was suitable for use in road embankments beneath pavement layers. Modifications to the crushing process could produce aggregates meeting the standards for road bases and sub-bases as evidenced by the grain size distribution of TEM compared to the standard distribution boundary for road pavement materials shown in Figure 15.

Additionally, over 500,000 tons of sandstone extracted from Sydney Metro tunnels has been designated for utilization in constructing the Western Sydney International Airport. Sydney Metro’s objective is to achieve 100% recycling of crushed rock extracted from 15 km twin tunnels, with over 148,000 tons already delivered. The high-quality sandstone serves as a robust foundation for airport runways, taxiways, and roads. This sustainable approach reduces waste while improving efficiency and reducing carbon emissions [72,73].

Using TEM as a sub-base in construction projects presents significant environmental benefits. One major advantage is the reduction in carbon emissions. By repurposing TEM locally, the need for quarrying new materials and transporting them over long distances is minimized, thereby lowering the associated greenhouse gas emissions. For example, in the Deo-Ca Tunnel project in Vietnam, the use of recycled muck for road base construction demonstrated substantial environmental advantages by reducing the demand for new raw materials and decreasing transportation-related emissions.

Additionally, the use of TEM conserves natural resources. Recycling excavation material reduces the extraction pressure on quarries, preserving natural landscapes and decreasing habitat disruption. The environmental impact assessment for the S10 project in Austria highlighted how integrating TEM into terrain modeling and construction contributed to ecological improvements, such as noise and sight protection, and enhanced the usability and ecological value of the terrain.

2.2.2. Reuse as Refilling Materials

The recycling of the tunnel spoil from the Katzenberg Tunnel construction for Germany’s high-speed rail line between Karlsruhe and Basel saw over 2.3 million m³ of material stabilized on-site and transported to the nearby Kapf limestone quarry for reuse in refilling [70]. Hazard assessments, including a forecast of water percolation, demonstrated compliance with groundwater protection regulations.

Naturally occurring contaminants, such as heavy metals and arsenic, were determined to be immobile and not pose a significant risk to groundwater. Similarly, the presence of bitumen deposits was properly managed. Environmental audits tracked material movements and water quality throughout the process. These audits confirmed that the forecasts made in the hazard assessment were largely met. Furthermore, the low permeability of the filled spoil material in the Kapf quarry minimized the potential for groundwater contamination. The long-term stability of the refilled areas was also considered. The quarry’s location on a horst block, bounded by faults, ensured that the filled material would not impact the groundwater flow towards the Rhine Plain, where drinking water wells are situated [70].

2.2.3. Reuse as Value-Added Materials in Other Industries

TEM also exhibits potential as value-added resources for other industries when processed, such as brick, paver, and revetment production to replace more expensive materials like cement, and in glass and ceramic manufacturing. Furthermore, minerals like bentonite and lime can be extracted commercially [36,37,67,74,75].

However, appropriate treatment and strategic end use selection based on material characterization play an important role in TEM valorization efforts. In this regard, preprocessing methods such as low-temperature hydrothermal treatment show potential for recycling waste slurry into high-strength, environmentally safe building materials applicable for industrial application. The technique utilizes less energy than alternative high-temperature sintering methods, significantly reducing associated environmental impacts as evaluated through life cycle assessment (LCA) [76]. By applying the low-temperature hydrothermal technology industrially and assessing sustainability using LCA, TEM can be transformed into viable value-added materials through optimized treatment and reuse selection aligned with material properties and performance criteria.

Some studies aimed to prepare building materials directly from TEM slurry [37,74]. The results showed that TEM can be solidified into materials exhibiting a flexural strength of 25 MPa, enabling production of industrial standard bricks and non-fired bricks containing 50 to 70% slurry by mass [74]. Additionally, LCA found that the recycled slurry brick production process reduces associated environmental impacts by 13% to 152% compared to conventional bricks. Seven LCA indicators were chosen to quantitatively assess the environmental impacts of every process during brick production [77,78]. These indicators include global warming potential (GWP, kg CO₂), primary energy demand (PED, MJ), water resource usage (WU, kg), acidification potential (AP, kg SO₂), abiotic depletion potential (ADP, kg Sb), eutrophication potential (EP, kg PO4³⁻), and respiratory inorganics (RI, kg PM2.5). Figure 16 provides a comparative assessment of these seven impact categories for different brick production processes, allowing quantitative evaluation of their relative environmental impacts based on internationally recognized LCA methods and impact categories.

Figure 17 depicts the process used for preparing non-fired bricks incorporating TEM. This process demonstrates the feasibility of converting slurry, a byproduct of tunnel construction, into valuable construction materials through optimized processing. The process begins with the collection of TEM, which is then screened to remove large debris and coarse aggregates. The screened TEM is then mixed with a binder, such as cement or lime, and water to form a slurry. This slurry is then poured into molds and compacted to form bricks. The bricks are then cured for a period of time to allow the binder to harden. This process allows for the efficient and sustainable utilization of TEM, reducing waste and promoting environmental sustainability.

2.2.4. Reuse as Vegetation Reclamation Materials

Research has demonstrated the feasibility of utilizing TEM for vegetation reclamation applications after conditioning it to suit soil environments [37]. Figure 18 depicts the experimental procedure undertaken to evaluate the effects of conditioning TEM with supplementary minerals on vegetation growth and key soil environmental parameters.

The experiment involved preparing different TEM mixtures by mixing TEM with various proportions of supplementary minerals (e.g., bentonite, zeolite) to create different conditioning treatments. Selected plant species were then planted in pots containing the different TEM mixtures and a control group with standard soil. Vegetation growth parameters such as plant height, biomass, and leaf chlorophyll content were monitored over a specific period. Key soil environmental parameters, including pH, electrical conductivity, nutrient content, and water holding capacity, were analyzed in the different TEM mixtures.

The findings indicate that with appropriate processing involving additives, TEM can facilitate re-vegetation and site restoration activities. Specifically, the study revealed that conditioning TEM with supplementary minerals improved plant growth by enhancing nutrient availability, water retention, and overall soil quality, leading to improved vegetation growth compared to unconditioned TEM. The study demonstrated that TEM, when properly conditioned, can provide a suitable substrate for vegetation growth, contributing to successful site restoration [37].

While the study highlights the potential of TEM as a soil amendment to support re-establishing native plant communities, landscapes, and ecosystems impacted by sub-surface excavations, further research is needed to fully validate the effectiveness of TEM as a vegetation reclamation material. The study’s focus on the impact of conditioning on vegetation growth alone may not be sufficient to conclusively establish the suitability of TEM for vegetation reclamation without further investigation into its long-term effects on soil properties and plant health. Future research should explore the long-term implications of using TEM as a soil amendment, including its potential impact on soil nutrient cycling, microbial activity, and overall ecosystem function. Furthermore, evaluating the effectiveness of TEM in different environmental contexts and with various plant species is crucial for its successful application in vegetation reclamation.

3. Recovery and Reuse of False Segmental Tunnel Linings

False segmental linings are utilized to facilitate TBM advancement during the initial mechanized tunneling stage at station platforms. A case study from New Zealand demonstrated the feasibility of recovering and repurposing reinforced concrete tunnel lining segments [79]. At the Karanga-a-Hape Station for Auckland’s City Rail Link, false segments each weighing over 5 tons were extracted from the Canada Street site. The segments were subsequently crushed, generating over 1000 tons of high-quality graded concrete aggregates alongside extracted reusable steel rebars. The crushed concrete was screened for potential reuse as tunnel backfill

This initiative aimed to promote sustainability through materials reuse and carbon emissions reduction, showcasing circular economy potential for the construction industry [79,80]. An environmental product declaration highlighted a significant 75% reduction in the carbon footprint versus employing quarried aggregates, with minimized transportation distances critical to minimizing environmental impacts.

4. Utilization of Other Industries’ Waste in Underground Construction

Large-scale underground construction projects generate significant waste while various industries also produce high volumes requiring proper disposal. This section explores opportunities for utilizing industrial and construction byproducts in underground works to promote sustainability.

Six potential waste streams are discussed across the sub-sections. Recycled materials show potential as alternatives to conventional insulation, fire protection, and reinforcement due to their comparable engineering properties. Construction and demolition waste (CDW) is evaluated as a substitute for mining tailings in backfilling, enhancing strength, and reducing acid generation. Industrial solid wastes also demonstrate viability in grouting materials by partially replacing cement, improving flowability, strength, and durability. Crushed concrete from construction waste emerges as a technically and economically sustainable option as aggregates in tunnel linings when incorporated at optimal ratios. Lastly, recycled steel fibers sourced from tire recycling emerge as a renewable reinforcement that enhances structural performance of tunnel segments in hybrid designs requiring reduced traditional reinforcement.

In summary, by capitalizing on available industrial byproducts and construction waste, underground works can optimize waste management synergies while developing more sustainable material solutions.

4.1. Use of Recycled Materials for Thermal Insulation

Underground facilities benefit from stable sub-surface temperatures, resulting in reduced heating and cooling requirements compared to above-ground structures with proper insulation. However, effective insulation is crucial to further optimize energy demands [81]. Recycled materials such as shredded tires and waste textiles demonstrate potential for sub-surface structure (e.g., warehouses) thermal insulation across climatic zones due to due to thermal properties comparable to expanded polystyrene [82].

The results of the simulations of an underground warehouse in three climatic zones under different insulation conditions (without insulation, expanded polystyrene insulation, shredded waste tires, and waste textiles) and interior temperatures showed that waste materials provided similar insulation performance to expanded polystyrene in reducing energy demands. Specifically, waste textiles matched or surpassed expanded polystyrene in most cases [82]. Examples of temperature distributions with waste textile insulation are presented in Figure 19. The research highlights dual benefits of diverting waste from landfills while also minimizing sub-surface energy consumption through envelope insulation. Findings support recycled materials as preferred long-term thermal insulation solutions consistent with circular economy principles for underground construction.

4.2. Use of Recycled Materials for Fire Protection

Fire safety is a major concern, especially in tunnels where fires can spread rapidly and be difficult to contain, as evidenced by the Channel Tunnel fire [83]. Researchers developed fiber-reinforced concrete with enhanced fire resistance properties for use in precast tunnel segments to address this issue [84]. Studies explored recycled polyethylene terephthalate (PET) fibers and recycled waste tire polymer fibers (RTPFs) for fire resistance enhancement [85,86].

Incorporating recycled PET fibers improved resistance by mitigating explosive spalling compared to plain concrete, allowing for easy mixture distribution. Above 150 °C, melted fibers formed microchannels for vapor escape, reducing spalling risk, while fiber concretes maintained integrity up to 750 °C. An optimal 2 kg/m³ dosage proved effective, with higher dosages yielding diminishing returns [85]. Moreover, laboratory tests on specimens subjected to heating using the apparatus illustrated in Figure 20a revealed severe spalling in specimens without RTPF and those with low dosages (Figure 20b), while higher RTPF dosages (≥2 kg/m³) significantly reduced spalling, as shown in Figure 20c [86].

Utilizing recycled materials for fire protection in underground engineering promotes sustainability by reducing reliance on non-renewable resources and decreasing the environmental impact associated with their extraction and processing. This approach contributes to a more circular economy by repurposing waste materials, ultimately leading to a more sustainable construction industry.

4.3. Use of Construction and Demolition Waste as Refilling Materials in Underground Mining

Cemented paste backfill (CPB) stands out as a widely adopted technique for the disposal of mine tailings, involving the pumping of tailings back underground to form a solid mass [87]. This backfilling process plays a crucial role in facilitating subsequent excavation and ore extraction, reducing ore dilution, establishing a stable working floor, minimizing subsidence, and ensuring regional stability. Pre-exploitation conditions, as depicted in Figure 21, underscore the significance of cavity filling before commencing nearby mineral extraction activities.

Traditionally, various geomaterials, such as tailings, waste rocks, or sand, are commonly used as backfill components in either cemented or uncemented form. However, using CDW as a partial substitute for tailings in CPB can help address operational requirements, dispose of waste, and meet necessary strength criteria while reducing environmental impacts. Despite the substantial annual production of CDW from construction activities, recycling rates remain relatively low in many regions. For instance, China faced an estimated 3 billion tons of improperly disposed CDW, with an annual growth rate of 300 million tons [88]. The unsustainable disposal of large quantities of CDW poses challenges such as increased pressure on landfill capacity and significant contributions to solid waste streams in many countries [89].

Incorporating CDW into underground backfilling could sustainably reduce waste volumes and environmental impacts compared to conventional methods [90,91]. Recent studies have demonstrated the technical feasibility of partial CDW substitution in CPB mixtures through case studies in countries such as Turkey [92,93], Australia [94], and Iran [95]. These studies have shown that the long-term strength and durability of CPB mixtures containing CDW are comparable or even better than those prepared with only tailings. For instance, Yilmaz et al. [92] found that the compressive strength of CPB samples with 15% CDW substitution increased by up to 19.5% compared to control samples after 360 days of curing, even with a lower binder dosage (7.5% vs. 8.5%). Furthermore, Ercikdi et al. [93] reported that the incorporation of CDW improved the microstructural properties and durability of CPB by decreasing porosity and enhancing resistance to sulfate and acid attack over a 360-day curing period.

Preliminary evaluations show similarities between typical CDW and tailings particle size distributions, supporting the technical case for partial replacement use (Figure 22). While promising, further research is needed to fully assess the long-term performance and durability of CDW as a backfilling material. Reported replacement levels range from 15% to 70% of total solids depending on material properties and application [96]. Common CDW components like sand, concrete, brick, soil, ceramic tile, plaster, wood, plastics, metals, and rocks have been characterized for their geotechnical and leaching behavior. Concrete and brick fragments generally perform most suitably. Prepared mixtures for refilling using CDW typically utilized cement dosages ranging from 5% to 8.5% to develop sufficient strength over time [92,97]. Overall, redeploying CDW as a backfill supplement could advance both mining operations and sustainable waste management practices. Further research optimizing mixture designs and performance is still warranted.

4.4. Use of Recycled Materials as Aggregates in Tunnel Linings

As discussed previously, sustainable management of CDW has become an important issue due to the large volumes involved. Among the viable strategies, recycling and repurposing CDW as aggregates in concrete present a particularly promising avenue [98]. Figure 23 depicts the process for generating recycled concrete aggregates (RCAs) from CDW to support concrete applications. Due to tunneling projects’ considerable concrete requirements, they emerge as an ideal sector for incorporating RCA into underground structural elements. Using RCA for structural tunnel concrete applications helps address sustainability issues surrounding CDW while reducing pressures on natural aggregate sources.

Several studies have shown that while cast tunnel lining concrete containing RCA exhibits lower strengths than those with natural aggregates (NAs), the use of RCA in shotcrete may present a more viable option [99,100,101]. Fiber-reinforced shotcrete mixtures containing RCA were found to achieve comparable compressive (Figure 24a), tensile (Figure 24b), and flexural (Figure 24c) strengths to those with NA [100]. When only coarse RCAs were used, mechanical properties only were approximately 15% lower than the control mixture with NA. Despite this reduction, the concrete still met the design requirements [98].

An evaluation of the technical and economic feasibility of employing coarse RCA from CDW in shotcrete, used as primary tunnel lining support, has shown only marginal cost increases ranging from 1% to 5% per meter for replacement ratios between 20% and 50% of coarse RCA [13,64]. Data collected from Portuguese tunneling companies provided the basis for estimating the current costs to produce dry-mix shotcrete. Figure 25a illustrates the unit costs, in €/m³, of various shotcrete scenarios, considering three potential costs for coarse RCA to assess its impact: Scenario 1 (50% CNA), where the cost of RCA is assumed to be 50% of the cost of NA (7.5 €/m³); Scenario 2 (NULL), where the cost of RCA is zero, considering that CDW producers are willing to pay to dispose of it; and Scenario 3 (SUB), where a subsidy of 7.5 €/m³ is provided to promote the use of RCA. Since the cost of coarse RCA is lower than that of CNA, the unit cost of shotcrete incorporating CRCA is reduced. Figure 25a demonstrates that incorporating RCA in shotcrete leads to minor cost variations per cubic meter (<8%) across the three scenarios analyzed. For simplicity, all cases assumed rebound losses equal to 25% of the shotcrete’s theoretical volume.

In the scenario where the coarse RCA cost is 50% of the CNA cost, the cost per unit length of the tunnel lining with coarse RCA is slightly higher than that without coarse RCA for IR20 (2.3%) and IR50 (5.3%) in Case 1. For other impact ratios (IRs) and cases, the cost increase exceeds 10%. Because the recycled aggregates’ cost accounts for less than 15% of the total production costs, any increase in thickness due to the lower mechanical properties of coarse RCA leads to higher consumption of other components, thereby increasing overall costs.

Two criteria were employed to determine equivalent lining thickness: (1) similar elastic radial stiffness and (2) similar yield stress. For the first criterion, 20% to 50% coarse RCA ratios necessitated thickness increases under 10%, demonstrating technical viability (Figure 25b). For the second criterion, only 20% coarse RCA replacement was viable with a 12% increased thickness (Figure 25c). The technical assessment and economic analysis found marginal benefits for 50% coarse RCA replacement ratios, providing sustainability through limited reuse of this waste stream.

Figure 25b,c shows that the cost per unit length of the tunnel lining is relatively insensitive to the cost of CRCA. Specifically, for Case 1, mixes IR20 and IR50, which are based on similar stiffness criteria, result in marginal cost increases (between 1.2% and 5.3%), whereas the other cases studied lead to cost variations exceeding 10%. Therefore, IR20 and IR50 emerge as promising alternatives for CRCA incorporation in dry-mix shotcrete solutions.

Furthermore, an innovative tunnel project in Australia is also exploring the use of recycled crushed glass to replace fine concrete aggregates in tunnel linings, with preliminary findings showing potential reductions in embodied emissions and virgin sand consumption indicating durability benefits [102,103]. Continuing studies seek to validate the technical performance of such recycled alternative materials in underground structures.

4.5. Use of Industrial Solid Wastes in Grouting

Significant amounts of waste materials are produced annually from various industries. However, limited options are available for the reuse and recycling of these wastes, resulting in their disposal in landfills. This causes environmental pollution and waste of resources [104]. Grouting is widely used in underground construction works for ground improvement, cut-off walls, consolidation of weak layers, shield tunneling, etc. However, common grouting materials require significant amounts of natural resources and energy during production. Utilizing industrial solid wastes (ISWs) in grouting can help reduce the usage of conventional grouting materials like cement and provide an economic and environment-friendly solution for waste management. Several studies explored the feasibility of utilizing various industrial solid wastes, such as fly ash, waste clay brick powder (WCBP), and different types of slag such as ground granulated blast furnace slag (GGBS), fuming furnace slag (FFS), and crushed fuming furnace slag (CFFS), as a partial replacement for cement in grouting applications [104,105,106,107,108,109,110,111]. Overall, the recent studies demonstrated that slags can partially replace up to 50% of cement in grouting applications [104].

More specially, two-component cement-based grouts are used for backfilling the annular gap between the segment lining and surrounding soil in shield tunneling [38]. However, ordinary Portland cement production requires high energy and emits a large amount of carbon dioxide. This not only causes environmental issues but also consumes non-renewable resources. In addition, pure cement slurries are significantly more expensive than cement–fly ash slurries. According to Dun et al. [108], the preparation cost for 1 ton of pure cement slurry is approximately CNY 262, while the cost for cement–fly ash slurry is about CNY 115. More recently, Behera et al. [104] conducted studies on cost savings that could be achieved by using FFS and CFFS as fractional ordinary Portland cement replacements in paste backfilling. The authors reported that the cost of paste backfill per ton was reduced significantly with slag replacement in the binder phase. The literature shows that binder cost significantly contributes to paste backfill economy compared to other capital investment and operating costs.

Several experimental studies explored the possibility of utilizing fly ash and GGBS as solid precursors to prepare two-component backfilling grouts [106,111,112]. André et al. and Song et al. revealed that ISW-based two-component grouts exhibited superior flowability retention over time and minimal bleeding ratios, indicating excellent stability [106,112]. These grouts achieved compressive strengths comparable to or even higher than conventional two-component cement-based grouts (Figure 26). Notably, they demonstrated significantly enhanced resistance to water corrosion compared conventional grouts (Figure 26), showcasing improved performance and sustainability. Overall, these findings highlight the potential of ISW-based two-component grouts as a promising alternative to conventional two-component cement-based grouts, offering improved performance, sustainability, and resistance to water corrosion.

4.6. Use of Recycled Materials as Tunnel Lining Reinforcement

The construction of tunnels requires huge amounts of steel for tunnel lining segments. Tunnel lining segments are commonly reinforced with steel rebars or fibers to increase their strength and durability. However, the use of traditional reinforcement materials increases the cost and environmental impact of tunnel constructions. Using recycled materials as reinforcement in tunnel lining can promote sustainable construction practices [113,114,115,116]. Several studies have explored the feasibility of using recycled materials in tunnel lining, particularly in the form of recycled steel fibers (RSFs) sourced from waste tire recycling as low-cost reinforcement [115,117,118,119].

A series of laboratory tests, numerical modeling, and analytical calculations have been conducted to assess the performance of tunnel lining segments containing different dosage rates of RSF under typical TBM thrust forces [117,120,121]. Although RSF alone cannot withstand the nominal thrust force [113], it has been found to be suitable for use in hybrid designs in conjunction with rebars to sustain thrust loads in a more sustainable and cost-effective manner [120]. Notably, the hybrid design has demonstrated a reduction in rebar quantity by 40% while concurrently enhancing structural performance [117].

The outcomes of these studies revealed that RSF-reinforced concrete (RSFRC) segments and the hybrid design employing recycled steel fibers (HRSFRCS) exhibited commendable structural, environmental, and economic performance under TBM thrust loads compared to conventional reinforced concrete segments (RCSs). Figure 27a illustrates the structural performance improvements achieved with the hybrid design. The HRSFRCS design exhibits a significant increase in load-bearing capacity and a reduction in crack width compared to RCSs. For example, experimental and numerical simulations showed that the hybrid design achieved a 15% reduction in crack width compared to RCSs under the same TBM thrust force. Additionally, the implementation of RSF in hybrid designs has been shown to reduce the environmental impact and overall cost of tunnel construction due to the decreased reliance on traditional steel reinforcement (Figure 27b). In summary, the utilization of recycled waste materials, such as RSF, presents significant opportunities to enhance the sustainability of underground construction projects.

5. Application of Sustainable and Eco-Friendly Methods and Materials in Underground Construction

The promotion of sustainability in underground construction relies significantly on the incorporation of green materials and methods designed to minimize ecological footprints, which aligns with the United Nations Sustainable Development Goals (SDGs), particularly those focused on environmental protection and responsible resource management. This encompasses the utilization of environmentally friendly materials such as low-carbon-footprint cement, directly contributing to SDG 12: Responsible Consumption and Production [123]. The adoption of low-carbon materials in underground construction projects presents a significant opportunity to contribute to the mitigation of climate change (SDG 13: Climate Action [124]) and the promotion of sustainable resource use (SDG 15: Life on Land [125]). By reducing the embodied carbon footprint of materials, the construction industry can minimize its contribution to greenhouse gas emissions, thereby supporting global efforts to combat climate change.

In addition to green materials, adopting energy-efficient methods such as energy-efficient ventilation and green lighting systems supports SDG 7: Affordable and Clean Energy [126] by lowering energy usage. The collective adoption of these green materials and methods not only contributes to the overall sustainability of underground construction projects but also enhances energy efficiency and promotes the health and well-being of occupants within these structures [127], aligning with SDG 3: Good Health and Well-being [128].

An ecological tunnel refers to integrating ecological principles into various aspects of tunnel design, construction, and operation to create a harmonious balance between the tunnel infrastructure and the surrounding natural environment. Key elements of an ecological tunnel system include green lighting systems, green lining systems, green grouting, low-carbon-footprint cement, etc. [127,129,130]. This concept aligns with the ecosystem-based approach advocated by numerous scientists and engineers in recent decades [131,132,133,134], as noted by Agee and Johnson (1989) [135], Kibert (2022) [136], SDG 11: Sustainable Cities and Communities [137], and SDG 9: Industry, Innovation and Infrastructure [138], by promoting sustainable and innovative construction practices.

Green tunneling techniques focus on reducing impacts such as energy and resource usage, emissions, noise pollution, third party effects, and environmental disturbances, aligning with SDG 12 [123], SDG 13 [124], and SDG 15 [125] by minimizing environmental disruption. For example, green lining involves establishing plant species, growing medium, and irrigation systems to re-vegetate tunnels, directly supporting SDG 15 [125] (Figure 28b). Green lighting utilizes sunlight, converting it to electricity via photovoltaics and distributing light in the tunnel using optical fibers to provide energy-efficient illumination, further supporting SDG 7 [126] by promoting the use of renewable energy sources (Figure 28a).

The ecological tunnel concept presents a promising new approach for integrating ecological and sustainability principles across the entire lifecycle of a tunnel infrastructure. Its implementation has the potential to address several environmental impacts and resource demands associated with conventional tunnel construction practices [127,130,139].

Another aspect of green tunneling is related to green grouting. Grouting is commonly used to treat water-rich sand layers during underground construction projects in shallow underground excavation [140,141]. Figure 29 shows the schematic view of shallow buried underground excavation and grouting zones around tunnels.

Traditional grouting materials like cement and cement–sodium silicate have several significant issues. These materials typically have long setting times, high costs, toxicity, and cause environmental pollution, especially in vegetation protection areas [142]. Due to their strongly alkaline nature and uncontrollable diffusion, traditional grouting materials like cement slurries and cement–sodium silicate grouts are not suitable for pre-reinforcement in vegetation protection areas, as they can harm soil conditions and root systems [143,144]. Therefore, there is a need to develop eco-friendly grouting materials to addresses these concerns and promotes SDG 12 [123] by minimizing the environmental impact of construction materials.

Several studies have attempted to develop new eco-friendly grouting materials, such as cement-based grouting composites with microbial polysaccharide gum (CGMG) or with fly ash, for pre-reinforcement of shallow underground excavations located in vegetation protection areas [142,145]. It should be mentioned that this aligns with SDG 15: Life on Land [125] by minimizing the negative impact on soil conditions and root systems, thus preserving biodiversity and ecosystem health. The developed sustainable grouting materials were applied to tunnels during the construction of Qingdao Metro Line 2 [145] and Martyrs Park Station of Changsha Metro Line 6 [142] in China.

Testing of the CGMG grouting material showed it had properties well-suited for pre-reinforcement applications, including high stability, good pumpability, and sufficient early compressive strength. Moreover, pot experiments were conducted in a greenhouse to evaluate the impact of CGMG on plant germination and growth under controlled environmental conditions. The plant growth conditions in the pot experiments were similar to those depicted in Figure 18. Parameters assessed included germination rate, chlorophyll content in leaves, root activity at 30, 60, and 120 days, and final organic matter content of the soil [146].

Pot experiments demonstrated that the CGMG did not negatively impact perennial ryegrass and white clover (Trifolium repens) germination, growth, chlorophyll content, or root activity compared to the control. The root activity results, as shown in Figure 30, suggested that as the ryegrass and white clover matured over time (60–120 days), their root systems became better adapted to the soil environment, resulting in less fluctuation in root activity across developmental periods. By 120 days, both ryegrass and white clover appeared to have acclimated their root functions in response to environmental conditions, exhibiting more stable root activity independent of the growth stage. Following the successful pot experiments, CGMG was also applied in pre-reinforcing a shallow tunnel excavation (Martyrs Park Station of Changsha Metro Line 6) in a vegetation protection area. Core sample tests and field monitoring indicated the CGMG effectively reinforced the ground while the nearby vegetation and water table remained undisturbed [142].

While CGMG shows promising results as a greener alternative to traditional grouts, some limitations still need to be addressed. For example, its pumpability could potentially be improved to further enhance applicability. Additionally, long-term performance monitoring is needed to evaluate durability over time. Addressing these challenges could help establish the material as a fully viable substitute.

6. Conclusions

This review has comprehensively explored sustainable practices for mitigating environmental impacts throughout the lifecycle of underground construction projects. Key areas covered include recycling and reusing excavated tunnel materials, utilization of other industries’ waste, and adoption of green methods. The key findings and opportunities can be summarized as follows:

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On-site reuse: This review highlighted the successful implementation of on-site reuse of TEM for various applications, including backfill, grouting, soil conditioning, and concrete production. Case studies from projects like the S1 Vienna and the Nanjing Metro Line 10 Tunnel demonstrated the feasibility and benefits of these approaches.

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Off-site reuse: The potential for off-site reuse of TEM, including road bases, refilling works, and the production of value-added materials like aggregates and construction products, was explored.

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Waste utilization: Utilizing other industrial and construction wastes in underground works shows potential to substitute conventional materials. Applications such as for thermal insulation, fire protection, tunnel lining, backfilling, or grouting materials demonstrate comparable engineering properties with reduced impacts.

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Green methods: One aspect of green tunneling methods, green grouting, ensures excavation safety while preserving surrounding environments. Furthermore, the ecological tunnel concept, including green lining and green lighting, presents a promising new approach for integrating ecological and sustainability principles across the entire lifecycle of a tunnel infrastructure.

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Technological advancements: The review discussed the role of technological advancements in facilitating sustainable practices, such as real-time analysis of spoil properties, advanced TBMs with sorting mechanisms, and mobile processing plants. These technologies contribute to improved efficiency, waste reduction, and material recovery. For instance, the implementation of TBMs in the Gotthard Base Tunnel in Switzerland demonstrated a reduction in excavation time and environmental impact.

Through numerous global case studies, this review demonstrates that optimized planning, characterization-informed treatment, and end-use selection can significantly reduce underground projects’ environmental footprints while realizing economic benefits. With continuous innovation, a higher percentage of waste can be sustainably utilized to advance circular economy principles in tunneling. While research validates sustainable practices, some areas should be noted for future research:

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Standardization and assessment: Further research is needed to refine and standardize the assessment of recycled material properties and develop standardized guidelines for their use in various applications. This will enhance confidence in the performance and reliability of recycled materials.

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Multi-stakeholder collaboration: Achieving coordinated multi-stakeholder adoption, including collaboration between contractors, suppliers, regulatory bodies, and research institutions, is crucial for maximizing the impact of sustainable practices and accelerating the transition towards a more sustainable underground construction industry.

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Lifecycle assessment: Conducting comprehensive life cycle assessments of different sustainable practices is essential to quantify their environmental and economic benefits, enabling informed decision making and promoting the adoption of the most effective approaches.