This article provides an in-depth analysis of the sustainability of underground infrastructure and highlights the need for assessment methods tailored to their specific characteristics. While existing tools such as HQE, ENVISION, and BREEAM address sustainability in a general way, they poorly account for the distinct features of underground structures, whether in terms of impacts or advantages. Underground infrastructure helps reduce land take, preserve ecological continuity, and improve urban resilience, in alignment with several Sustainable Development Goals (SDGs). It is also essential for managing natural hazards, mobility, storage, and climate change adaptation. 

The article distinguishes the functional dimension from construction related impacts. In both areas, it analyses the advantages and limitations of underground solutions, during both early-stage planning and project execution. Based on these specificities, requirements are defined for upstream and downstream project phases, combining traditional best practices with additional sustainability driven expectations: planning, stakeholder engagement, whole life costing, BIM, impact reduction, and environmental and social optimisation. 

Risk management—already central in underground works—can also support a sustainability-oriented approach: quantitative analysis can reduce overdesign and thus carbon impact, as demonstrated in fire safety design for metro systems. Similarly, accepting a higher level of settlement risk can influence construction methods and ground support design. This approach, balancing probabilistic risks with proven impacts, opens promising avenues that remain under exploration. 

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Sustainability is understood as a triad of economic, ecological, and social aspects that must be considered equally and given the same priority in evaluations. While it is increasingly becoming a dominant topic for construction in general, and for underground construction in particular, the issue of the so-called “carbon footprint” dominates all other discussions in the public perception. However, sustainability is actually much more than just balancing greenhouse gases and looking at the planning and construction period of a project. Underground infrastructure, with its service life often far exceeding 100 years and its undeniable effects on the surface provides assets far beyond the carbon footprint, which are currently underrepresented in assessment methods. With that being said: when two variants are compared, it is theoretically possible that the one with the higher CO2-emissions might be the more sustainable in the long run, if it provides a better performance regarding economic or social aspects.  

 

This article show how sustainability is currently assessed and, above all, where the weaknesses and problems of the available assessment algorithms lie. In addition, the aforementioned blueprint will be presented and an outline will be provided of the knowledge gaps that urgently need to be closed at the international level so that tunnel construction can continue to be carried out effectively.  

  

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The first metro line to be certified HQE Sustainable Infrastructures™ during the programming and design phases, the Line C project certification reflects a strong environmental commitment from the preliminary studies onward. Tisseo has thus chosen to pursue the HQE approach for its comprehensive methodology, which integrates all sustainable development challenges in a balanced manner: economic, social, and environmental.

With over 60 Sustainable Development objectives and 200 concrete actions, the environmental ambition for the project's underground spaces is embedded in the DNA of Toulouse's new metro line.

The purpose of this article is therefore to outline the essential prerequisites for successfully completing all stages of HQE Infrastructures Durables™ certification for a metro line. The final chapter presents some of the flagship initiatives implemented in Line C's underground spaces, made possible by the certification's ambitious goals. These include energy-saving measures, resource conservation, indoor air quality performance, and rigorous management of excavated soil.

 --- Underground infrastructures represent very valuable test benches for employing and developing multi-standard protocols to measure and improve the sustainability of projects and construction sites. The Mont-Cenis base tunnel project, the central structure of the Lyon-Turin line, which is a cross-border infrastructure and one of the main construction projects in Europe, is an emblematic case. From the design phase to the execution of the work, several recognised standards were used to organize and evaluate the environmental and societal performance of the project. Among them, the Envision protocols, which originated in North America, and HQE  are being used with three objectives: to measure the overall sustainability of the project or specific operational construction sites, to guide the execution project phase of the construction sites and as a basis for scoring in the evaluation phase of the tenders. During the construction phase, for example, the Sustainable Construction Site Guideline, which in 2025 became a practice recognised by the UNI under the name UNI/PdR 172:2025, developed by a working group of the Italian Sustainable Infrastructure Association, has been integrated as an operational tool to supervise practices in the field, reduce nuisance for the local area and integrate social aspects. The joint application of these standards has made it possible to implement a coherent, measurable and innovative approach to sustainability. However, protocols and standards must always be customized. This is the case, for example, of the environmental indicators during the construction phase that were developed by TELT and which were the subject of a subsequent study with the CETU, or the project carbon footprint, for which TELT has developed a dynamic monitoring tool. 

Although studies show that tunnels and stations are underground structures with high geothermal potential, their development still remains marginal. Discussions with various specialists and stakeholders involved in geothermal projects for tunnels and stations in France have allowed to point out several issues mostly organisational aspects that complicate the implementation of geothermal energy in these structures. However, favourable outcomes can be achieved if all stakeholders in the geothermal energy chain are involved from the early stages of the project.

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In the face of road congestion and ecological imperatives, rail freight development is a priority. The National Rail Freight Development Strategy (SNDFF) aims to deploy, by 2030–2050, a network of rail highways to concentrate flows and strengthen rail competitiveness versus road transport.

Existing tunnels, often over a century old, require substantial adaptation works to carry semi-trailers on wagons while maintaining operations. Several technical solutions are possible (partial or full vault widening, track lowering), adapted to each structure’s specifics (layout, condition, depth, prior works). These heavy, complex interventions on aging and sensitive structures, which must reconcile structural adaptation with continued operation, entail high costs and significant capacity impacts that discourage investment.

To address these challenges, the Tunnels and Geotechnics Division of SNCF Réseau is conducting exploratory studies as part of the Ulysse Fret project, in cooperation with the Ministry of Transport and the 4F consortium (Fret Ferroviaire Français du Futur). A dedicated methodology has been developed to assess the impact of semi-trailers trucks on wagons in existing tunnel structures, identify priority zones for intervention, and propose a phased investment plan to ensure the technical and economic feasibility of the projects. The ultimate goal is to establish a master plan for the transport of semi-trailers that will provide a long-term vision for investment.

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The French railway network includes a vast heritage of old tunnels, with an average age of around 135 years. While these structures remain robust, they no longer meet the performance standards required for a modern and competitive rail freight system-particularly for accommodating LGP400-gauge freight trains operating on European corridors designated as rail freight highways. 

Conventional tunnel enlargement methods (widening, lowering the track bed, or both) are costly, disruptive, and operationally intensive. An industrialized alternative—using a sliding gantry—has been developed to minimize disruptions. This mobile structure isolates the work area while allowing trains to pass inside, enabling continuous rail operations during construction. The solution aims to reduce service interruptions while achieving the necessary infrastructure upgrades. 

This method, inspired by practices already implemented in Germany and Spain for tunnel widening projects, aims to increase work rates, lower costs, and minimize operational impact. 

This paper compares, for five tunnels on line 070 000, the costs, timelines and operational impacts of an industrialized method versus traditional tunnel enlargement methods, in order to quantify potential performance gains and assess the relevance of large-scale deployment across the national rail network. 

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SYTRAL Mobilités is the transport authority for the city of Lyon and almost the entire Rhône department in France. SYTRAL Mobilités organizes the metro network, which consists of four lines (A, B, C, and D) with of 42 stations and 34.55 km of tracks. The structures of these four metro lines, built in the 1970s and 1980s, were not designed with fire resistance in mind. SYTRAL Mobilités wished to assess the fire resistance of its tunnel infrastructure in the event of a train fire. This paper presents the study carried out to determine the fire resistance of each homogeneous section of the metro network. 

After presenting the network, the methodology adopted to analyze the fire resistance of the existing tunnels is detailed. The first step was to gather detailed information on the structures from archives (plans, calculation notes), to select representative sections. Thermal and mechanical models were then developed for these sections. The results obtained indicate the fire resistance duration of the various sectors, enabling SYTRAL Mobilités to adapt its safety measures, particularly the management of roadways in the event of a fire. 

The calculations also provide the failure method for each homogeneous section and help identify the weaknesses of each type of structure under fire conditions. The diversity of structure types encountered (cut-and-cover, bored tunnel, tunnel boring machine, bridges, buildings, etc.) and the scale of this study provide a reference in the field of fire resistance analysis for existing structures. 

The Euralpin Lyon Turin Tunnel (TELT) company is committed to optimising the management of excavated materials from various civil engineering sites for the construction of the cross-border section of the Lyon-Turin base tunnel. Although space is limited, TELT has planned a substantial logistics operation to reuse the excavated material on site, in concrete aggregates and backfill, supplemented by the use of rail to transport surplus material to distant quarries. These circular economy practices should help to limit environmental impacts such as the depletion of natural and energy resources, climate change, land use and land use change. The aim of the study is to assess the potential environmental impacts of reusing excavated material from the Lyon-Turin tunnel and to compare them with sourcing aggregate from quarries. 

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As part of the project to build the cross-border section of the new Lyon-Turin railway line, TELT is implementing a sustainable strategy for the management of the excavated materials resulting from the excavation of the base tunnel. While the construction of the base tunnel will require the excavation of a total of 37 million tonnes of material over a period of 10 years, including 30 million tonnes on the French side and 7 million tonnes on the Italian side, most of this material (> 50%) will be used for the project.  

This is a remarkable goal that can be even more ambitious in the future. To achieve this, TELT has set itself even greater environmental challenges: ensure a binational balance and management in the use of materials.  

In 2023, with the signing of an agreement, the two governments took a political decision to go beyond the border and make the Lyon-Turin project a pilot experiment in the binational circular economy, with the aim of achieving a global balance - with a zero-waste trend - as a concrete contribution to the European Green Deal.  

A targeted implementation mechanism has been developed by TELT, under the auspices of the intergovernmental committee, to put this agreement into practice: an operational protocol will regulate the process for using surplus CL1 from Italian construction sites on French sites, and vice versa for using surplus French CL2 on Italian sites. This will enable TELT to optimize the materials balance of the single French-Italian construction site.

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Spoil management is one of the most complex challenges in the construction of the cross-border section of the Lyon–Turin railway line. The project involves the excavation of a base tunnel more than 57 km long through the Alps, including 45 km on French territory, with an estimated production of approximately 23 million tons of excavated material. The efficient, safe and sustainable management of these materials is a key factor in the success of the project. 

Sustainable development objectives are implemented through concrete actions, with more than 50% of the excavated materials intended for reuse in environmental rehabilitation works, aggregate production for concrete and the construction of embankments for the project infrastructure. Extending over more than 30 km between Saint-Jean-de-Maurienne and Villarodin-Bourget/Modane, this project is part of a complex system of construction sites, requiring the management of numerous interfaces and continuous adaptation to rescheduling linked to excavation progress. Having now exceeded two million tons of processed materials, this paper presents the key design principles of the integrated spoil management system on the French side, together with initial feedback highlighting both the opportunities and the challenges encountered at the interface between technological innovation, safety and environmental and territorial sustainability. 

 --- Building the underground infrastructures of the CERN's Future Circular Collider (FCC) in the Geneva basin beneath the Swiss and French territories, will produce approximately 6.3 million cubic meters of excavated materials (in-situ volume), mainly molasse (95%), a highly heterogeneous sedimentary rock. The feasibility study for this research infrastructure was co-financed by the European Community (EC) as part of the H2020 program. The study focused in particular on geological and environmental conditions, as well as technically feasible concepts for infrastructure, civil engineering, and detectors.
With regard to civil engineering and in accordance with the circular economy principle, the study aimed to develop approaches for the reuse and recovery of excavated materials. Uses have been identified, such as the production of fertile soil, the benefits of which were highlighted by the international challenge “Mining the future”. Beyond the economic and environmental aspects, several issues were also addressed in the deliverable: “Strategy for the management and use of excavated materials” This document, produced by a dedicated working group, describes a possible strategy based on the current state of knowledge. Anticipation, territorial analysis, compliance with French and Swiss regulations, characterization, and sorting are some of the key points for successful management to help ensure the technical and economic feasibility of the project and strengthen its societal acceptability. If the decision to build the FCC is made, the strategy will be refined as the project progresses and supplemented by operational documents.

This paper presents the design studies for the interconnection shaft between lines 15 and the future extension of line 1, located in Val de Fontenay station on line 15 East of the Grand Paris Express. This context led to the development of a complex structure that enables the two lines to cross underground, under significant geometric and urban constraints, meeting both the functional and technical requirements of the project. The structure's atypical geometry stems from the numerous constraints associated with the site (proximity to the A86 motorway and the RER E regional express railway) and the requirements of the SGP. To this end, a diaphragm wall enclosure was designed, into which a second, deeper wall was inserted, creating a framework for the M1 tunnel to pass through. In addition, the structure is divided into two areas built under separate contracts, requiring specific phasing. The design was guided by two main uses: to serve as a launch shaft for the TBM in its Line 15 section and to enable interconnection with Line 1. During the tendering process, the SGP's programme changed several times, leading to a review of the design at each iteration. The structural design was first carried out using conventional 1D approaches with subgrade model method, then using more complex 3D models based on plates supported by elastoplastic springs. All calculations were performed using in-house ‘Pythagore’ finite element software.

--- As part of the HS2 project in the UK, the excavation of the 32 km Chiltern Tunnels represented a major technical challenge and a unique opportunity to advance innovation in the field of tunnel boring machines.
Bouygues Travaux Publics deployed robotic solutions to reduce operator exposure to high-risk areas, including the KROKODYL robotic arm for the automatic removal of wedge spacers and the ATLAS system for automated ring assembly. Automated management of pumps and bypasses optimised hydraulic control, while the new digital interface of the Thrust Centre facilitated automatic TBM guidance via PYXIS and enabled the implementation of continuous excavation, allowing rings to be installed without interruption.
A third recycled water supply line and the use of an inert polymer, which processed 3 million m³ of spoil without chemicals, reduced water and energy consumption, carbon emissions and transport, while improving operator safety.
Thanks to this combination of digital, robotic and environmental innovations, the Chiltern Tunnels project showcases a new generation of smarter, safer and more sustainable underground construction.

--- TELT (Tunnel Euralpin Lyon–Turin) project is a major binational rail tunnel connecting France and Italy through the Alps. It faces unprecedented waterproofing challenges due to its extreme geological conditions, with rock overburdens exceeding 2,000 meters, groundwater temperatures above 50°C, and chemically aggressive environments. In these conditions, achieving a 120-year design life requires exceptional material performance and durability verification. To meet these demands, TELT specified a 3 mm transparent PVC-P geomembrane, exceeding the conventional 2 mm standard used in France, and tested it through a dedicated Deep-Depth Durability Protocol. The protocol was developed and run in collaboration with CETU, CEREMA, INRAE, and geomembrane manufacturers Mapei, Renolit, and Soprema ; reproduces severe thermal and chemical ageing through prolonged immersion in controlled baths, to assess the long-term resistance of these geomembranes under simulated deep-tunnel conditions. During the 24-month accelerated aging, the geomembrane samples were periodically tested for tensile strength, puncture resistance, and plasticizer retention under acidic, basic, and high-temperature exposure. Results demonstrated stable mechanical performance and limited degradation, confirming the geomembranes’ suitability for deep alpine tunnelling. This proactive, result-driven approach highlights how early collaboration among clients, researchers, and manufacturers can drive innovation and mitigate long-term risks. The Deep-Depth Durability Protocol establishes a reproducible testing framework adaptable to other large-scale tunnelling projects, providing a scientific basis for developing robust, evidence-based waterproofing strategies in extreme underground environments.

--- The upcoming updates to the AFTES recommendations introduce significant advancements in the design and structural sizing of shotcrete used for tunnel support. These updates include a more precise definition of the three key functions expected of shotcrete, improved consideration of its early-age mechanical behavior, and a harmonization of calculation methods with existing AFTES guidelines on rock bolt design and shotcrete intended for permanent use in underground structures. Additionally, the recommendations now incorporate Ultimate Limit State (ULS) design principles for fiber-reinforced shotcrete—whether using steel or synthetic fibers—and reflect the latest developments in Eurocode 2 in this area.