The Sizewell C nuclear power station is currently in the development phase. EDF as designer of this project oversees the cooling seawater circuit design. The cooling system is similar to the Hinkley Point C EPR project and comprises three offshore tunnels with a total length of 10kms: two intake and one outfall tunnel. Different options were assessed during the development of the Basic Design for the offshore tunnelling works, including shallow and deep tunnel alignments, and online and offline tunnel-shaft connection options. At the offshore shaft sites, the geological sequence consists of the Coralline Crag Formation at the seabed surface, under-lain by the London Clay Formation and then the Harwich Formation. The difficulties identified during the structural design of the tunnels at the Basic Design stage are linked to the ground conditions in which the tunnels are positioned, and the safety of forming the tunnel to shaft connections. Engaging the supply chain through ECIs, SZC and EDF as designer have looked at several concepts of connections, both in the sandy Coralline Crag Formation and in the deeper clayey Thames Group. This involves heavy marine ground replacement work (never done before) or driving of a TBM at depth with EPB mode in heterogeneous clayey facies. This article presents the sequential process carried out by SZC to determine a single preferred option that best addresses the project’s requirements in terms of constructability, safety, operational performance, durability, risk management and cost and schedule of construction --- Following sustained urban and economic growth in western Switzerland, several major infrastructure projects have been initiated to meet the long-term increase in mobility demand. As a key hub within the regional railway network, Geneva railway station is undergoing a major modernization program, including inter alia the construction of a new double-track railway tunnel on its eastern side. The execution of this tunnel involves significant geotechnical challenges related to the complexity of the subsurface conditions and the presence of groundwater along the alignment. The main constraints include excavation in a dense urban environment adjacent to sensitive structures, tunnelling through marly formations with swelling potential, and the crossing of two aquifer corridors composed of saturated soft soils. To address these challenges, a hybrid design approach was adopted, combining conventional tunnelling in rock, ground reinforcement techniques in soft ground (roof pipe umbrella and face bolting with grouting), and artificial ground freezing in aquifer zones. Numerical analyses based on the finite-difference method, incorporating elasto-plastic constitutive models with swelling behavior, were carried out to assess ground deformations and to design the support and lining systems. These studies aim to ensure the structural stability of the tunnel while controlling construction costs, schedule constraints, and the overall performance of the project. --- The new Lisbon circular metro line will crosses a densely urbanized part of the city, connecting Rato Station located at one of the hills of the city and Cais do Sodré Station, at the Tagus River right bank. The underground excavation intersects a wide range of materials, from rock mass to soft soils. Where the construction of the tunnel section is closer to the river, with about 10 m of cover, a Cut&Cover method is used. In this metro the tunnel intersected a pile foundation of two reinforced concrete buildings with 9 upper floors and 1 basement, determining the need to underpin the structures and change permanently its foundation system. The geotech-nical and geological conditions present in this metro, associated to highly limited access and working conditions, led to the execution of the retaining walls using jet-grouting technology. Those elements were also used as the building deep foundations, which consists of a reinforced concrete slab (length=50m, width=13m and thicknesses=1.4m and 1.8m), being also responsible for the structure underpinning. In this complex process is defined a controlled load transfer be-tween the structure and the new slab, which was executed using hydraulic jacks, limiting the building differential settlements thought gradual jacks opening and according with monitoring. This paper presents an overall description of the solutions, how they were implemented and the buildings' behaviour during the underground works. --- As part of the TELT (Tunnel Euralpin Lyon Turin) project, Herrenknecht was commissioned to design and supply 6 TBMs, among which two Gripper TBMs and two “Wurms” for Lot CO5 of the project, between Villarodin and Modane, in 2022 and 2023. This article focuses on those. On this lot, which includes two 18-kilometre-long bored tunnels, it was decided to build a concrete base slab on the tunnel boring machine and to concrete each tunnel during excavation using two systems independent of the tunnel boring machines, also known as ‘Wurms’. The constraints imposed by the various workshops and the logistics associated with these two types of specific equipment led to unusually long lengths, approximately 330 metres for each tunnel boring machine and 650 metres for each Wurm. Due to the complexity of assembling this equipment on site, entirely in tunnels, it was decided to pre-assemble and assemble as much of the equipment as possible in advance, in the factory. To date, this is the first time our company has assembled machines of this length in the factory. These unique projects required the implementation of special logistics with new concepts and block assembly phases, which had to be developed in the factory in order to meet such challenges.

In a dense and highly constrained urban setting, designing the city of tomorrow with the underground in mind offers real opportunities to make it more functional, more efficient, and more resilient to climate change and natural hazards. In cases where the underground solution is not immediately obvious, its benefits must be objectively demonstrated. However, current assessment methods are often limited to environmental impacts (carbon footprint or life cycle assessment (LCA)) and often penalize underground solutions because they consume a lot of concrete and steel. They do not reflect the positive externalities underground solutions can offer in the early stages of the decision-making process. Bibliographic research initiated as part of the Priority research program and equipment (PEPR) “Sous-sol” (projet ciblé n° 10) of the France 2030 Research Program will be continued with a PhD thesis. This work has been discussed within the Underground Space and Transition Committee of the French Association of Tunnels and Underground Space (AFTES). It is in line with a first PhD thesis defended in 2016 and conducted as part of the “Ville 10D” French research program. This study enabled first to identify, compare and analyse the implementation of general sustainable development assessment frameworks such as BREEAM infrastructure and ENVISION in real projects. It then highlighted the importance of (re)thinking underground urban planning with a view to preserving ecosystems, and therefore the well-being of human beings through the services provided. This reflection should help consolidate the development of a systemic approach to assessing the “sustainability” of tunnels and underground spaces by integrating both socio-economic factors and environmental ones. The ultimate goal is to better support underground projects, especially in urban areas where residents and users may be impacted differently depending on the spatial and temporal parameters considered. --- The article shows the design and the findings during construction related to the construction of "Venezia" tunnel, part of the new railway link connecting existing Venice-Trieste HS railway line to Venice "Marco Polo" airport, within the context of the reclamation area where the land is located at or even below sea level. In its underground portion, main civil works involve a 3.5km-long artificial tunnel in top-down method, with diaphragm walls from the surface, a bottom plug in jet grouting, pre-excavation between sheet piles and pumping systems with well points; furthermore, there is a complex groundwater equilibrium to preserve in an area already characterized by water diversion interventions put in place since the 1920s. Finally, construction works impacts the airport area, with the aim of minimizing interferences with its services and functionality. The design and construction of the works are set in a highly complex operational context due to heterogeneous geotechnical conditions, the presence of groundwater, and local environmental hazards, such as gas pockets, which required specific design insights and detailed planning of the execution phases. The proximity of the operating airport imposed additional constraints, making integrated schedule management essential. Works are financed by the NextGenerationEU National Recovery and Resilience Plan (PNRR), and they are connected with the Milan-Cortina 2026 Winter Olympics, required a particularly structured and flexible site organization, capable of guaranteeing compliance with the planned timelines and quality standards. The success of the initiative required strong resilience and synergy among the designer, contractor and Client, who worked closely together to effectively address critical issues to achieve the highly challenging goal.

The concrete spalling during fire is a complex phenomenon, and its consideration in new tunnel projects remains challenging. In order to justify structural performance whenever the risk of spalling is identified, the technical guideline IT 2000-63 (published following the Mont-Blanc tunnel fire) requires fire resistance tests. While not exhaustive, this regulatory document is supplemented in its application by normative standards (such as EN 1992-1-2) and recommendations, including those from AFTES (GT18R1F2) and CETU guidelines (2005 and 2011). However, these documents do not specify testing procedures. The European standard (EN 1363-1) govern fire resistance tests but do not address the quantification of spalling. An international framework exists based on research conducted over several years in the Netherlands (RWS R0695 2020), but this reference leaves significant room for interpretation, particularly since test results are highly sensitive to input parameters. This article presents the work carried out by CETU and the three laboratories accredited by the French Ministry of the Interior for fire resistance testing (CERIB, Efectis France, CSTB). It introduces new perspectives compared to existing documents on spalling testing under fire conditions. It focuses particularly on selecting a target stress level in the projected structure and the methodology for reproducing this stress in test specimens. Finally, the article discusses the spalling indicator to be considered in design --- Facing the climate emergency, BESSAC has been conducting research and development work aimed at designing and implementing very low- and ultra-low-carbon alternative concretes. These innovations demonstrate the technical feasibility of fully decarbonized tunnels. The latest development was notably recognized at the ITA Awards 2025, receiving a Gold Trophy in the “Innovation of the Year” category. These challenges align with international climate objectives and with the strategies of major groups such as VINCI. Several levers are being mobilized, among which the use of alternative binders is one of the most promising. Three flagship projects carried out between 2022 and 2024 illustrate these advances, achieving carbon footprint reductions by a factor of two to three compared with conventional concretes, sometimes at no additional cost to the project owner. However, scaling up these solutions requires reconciling environmental performance, technical feasibility, regulatory constraints, material availability, and, above all, project economics. BESSAC now relies on a rigorous technical protocol based on a performance-based approach. Each project thus becomes an opportunity to identify the best compromise between environmental gains and cost control, an essential condition for the large-scale deployment of decarbonized concretes. --- As part of the work carried out by the Tunnel Factory, a VINCI Group entity dedicated to the development of innovative solutions in the tunnelling sector, BESSAC, in collaboration with ACRI Ingénierie, has developed a patented robotic system for installing and connecting utility lines in tunnels behind the tunnel boring machine. The lengthening of utilities pipes remains an essentially manual operation involving the installation of heavy pipes using conventional handling equipment (motorized bridges and jib cranes). These operations require the mobilization of numerous operators, are arduous and accident-prone. They are frequently found on the critical path of tunnel boring machine production. The robotic system developed by BESSAC and ACRI, called OCTOBOT, addresses three key aspects of our business: - Safety by limiting human intervention, reducing the arduousness of the tasks performed and reducing the risk of accidents. - Reliability by automating the process, through the repeatability of the tasks performed by the system and the use of existing and proven tunnel equipment (tubes connected by clamps). - Optimization by reducing the extension cycle time, offering a more attractive activity for operators and rapid return on investment. OCTOBOT consists of a main robotic unit that unloads, stores, positions and aligns a slurry pipe with the pipe in place laid in the tunnel, and a subsystem that automatically positions the connecting collar (Victaulic type or equivalent) and its seal and tightens it onto the two previously aligned pipes.

In France, a technological and scientific demonstration program has been conducted by Andra since the early 2000s at the Meuse/Haute-Marne Underground Research Laboratory (MHM-URL), demonstrating the feasibility of radioactive waste disposal within the Callovo-Oxfordian (COx) claystone layer at a depth of approximately 500 m. One of the ongoing experiments at the URL concerns the vertical conveyance of self-compacting concrete from the surface to an underground drift at -490 m through a vertical pipe/borehole, for the casting of the lining of drifts excavated with traditional methods. A sequential experimental program was developed for this purpose first on a reduced-scaled model, then in situ in MHM-URL. The concrete is conveyed in the vertical borehole of 152 mm diameter, and ≈500 m long, equipped at its base with a deceleration device designed to dissipate the concrete’s velocity. Tests were carried out on several concrete mix designs conveyed from the surface to the URL galleries via the vertical borehole to choose a mix with a target strength class C60/75, and a SF2 slump-flow class. This mix is then used for casting the lining of an approximately 10 m diameter drift, the GRD6, consisting of a 50 cm thick layer of C60/75 self-compacting concrete conveyed through the vertical pipe/borehole. --- A study is being conducted in the geotechnical centrifuge at Gustave Eiffel University (Nantes Campus) to test, on a 1/60th scale model, the soil-structure interaction of pre-installed piles subjected to deconfinement generated by tunnel boring. An original trap door-type device is being developed to horizontally decompress a previously reconstructed dense Fontainebleau sand mass (Id=80%). The piles, positioned at different distances from the front and at different depths, are instrumented with four Bragg grating optical fibres. This makes it possible to monitor the evolution of the moment profiles induced during deconfinement (movement of the trap door). Depending on the position of the model pile in the horizontal plane relative to the tunnel, deformation mechanisms may be activated, thereby modifying the moment profile. --- The slipform is a modular, mobile, lifting system for constructing reinforced concrete walls using continuous pouring. Its main characteristic is the lifting of the formwork by small, regular hydraulic jack pulses synchronized with the concrete setting time, eliminating the need for formwork assembly and stripping cycles. This technique has the advantage of allowing the rapid construction of tall walls without the need for concrete pouring stops or through-bolts, which is highly beneficial for constructing watertight structures. Continuous pouring requires work to continue day and night until the desired height is reached. This feature provides an industrial optimization of the construction process, where the jacking of the platforms allows the work to be presented to the various operators. This process is commonly used to construct: - Silos ; Grain, cement, solid fuel… - Water, fuel, and gas storage tanks - Biogas plants - Water towers - Household waste pits for Energy Recovery Unit projects - Towers, Stairwells and elevator shafts - Marine caissons - Chimneys - Shafts, Basins In 2025, the slipform construction technique was selected to construct the counter-walls of three ventilation and drainage shafts for the Toulouse metro.

The regulatory authority now requires hydroelectric operators to carry out hazard studies for pressurized structures (EDD CF), covering not only aerial penstocks (CF) but also underground pressurized galleries and other associated structures such as shafts, surge chambers, plugs, and watertight gates. These studies aim to identify and prioritize risks, considering the diversity of configurations (linings, overburden, geology, interfaces). The approach then consists of verifying the robustness of the design and comparing it with the current condition of the structures, relying on proven criteria and operational feedback. Accident analysis thus becomes an essential lever to confirm the relevance of these criteria and detect vulnerability zones. This article seeks to draw lessons from incidents recorded in pressurized galleries and underground structures within the EDF Hydro network, attempting to identify correlations between simple parameters such as observed disorders, design criteria, or the type of lining used. In summary, the recorded incidents sometimes occurred because standard design criteria were not respected, often because of defective implementation of structures, but most frequently because they were almost systematically associated with geology presenting unfavorable physical or chemical sensitivities that were poorly anticipated. A significant proportion of disorders appeared during initial filling or in the early years of operation. Conversely, most consequences were limited to leaks, penalizing for production but rarely critical for third-party safety. Ultimately, it is observed that analyses conducted within the EDD CF framework strengthen knowledge of the network of galleries and pressurized structures, useful for optimized operation that combines long-term safety with the continuity of efficient performance. --- Opened in 1963, the Grands Montets site in Chamonix was severely damaged by a fire in 2018. To bring it back to life, the Compagnie du Mont-Blanc (CMB) launched an ambitious project to rebuild both the lift system and the cable car station, located at nearly 3,300 m above sea level. The new top station, positioned below the Aiguille des Grands Montets and designed by the internationally renowned Renzo Piano, takes the form of a 20-meter glass cube. The project also includes the construction of a 67-meter gallery connecting the top station to the Col des Grands Montets, a route highly frequented by advanced skiers. In addition, a summit walkway will be created from the station, providing safe access to the top of the Aiguille and offering an exceptional panoramic view. This technical paper presents the overall reconstruction project and focuses on the key factors that shaped the design and construction of the gallery, considering the specific constraints of working in a high-mountain environment: geological and geotechnical conditions, challenging access, and severe weather. --- In all underground transport structures (road, rail, guided transport, etc.), water ingress can pose risks to user safety and the long-term functionality of the structure (structure and equipment). Furthermore, this recurring problem is generally costly in terms of repair work and impact on the operation of the structure in service. The profession has therefore decided to adopt best practices to facilitate the specification of all these processes and their use in both new construction and repair work. To this end, in France, two interprofessional commissions related to waterproofing in underground structures, bringing together project owners, project managers, manufacturers, installers, and laboratories, were created in the 2000s: the CETU (Centre for Tunnel Studies) Technical Assessment Commission for traditional waterproofing processes and the AFTES Evaluation Commission for innovative processes. Valid for a period of five years, CETU Technical Appraisals and AFTES Evaluations provide stakeholders with information on the physical and mechanical properties and long-term performance of waterproofing processes. They also provide information on their areas of application and conditions of use. The purpose of this article is to present the specific features of France in terms of the technical evaluation of waterproofing processes used in underground structures and to highlight the main differences between the two related documents. --- The expansion of Lisbon’s Underground network through the transformation of the Green and Yellow lines into a Circular line, within the central area of the city, is taking place in a densely urbanized area of Santos and Cais do Sodré ancient quarters. The area, close to the Tagus River, combines challenging ground conditions and a large diversity of vulnerable assets, including buildings of heritage value, residential buildings and archaeological sites. The project for this new line includes two new stations and connections to existing stations, as well as two ventilation and emergency shafts and about 2 km of new tunnels. In this paper, the focus is particularly on the new Santos station and adjacent tunnel constructed using the conventional method through poor ground conditions and low overburden, including sections with half a diameter of ground cover. The Santos station is located at the base of an old neighbourhood of the city built on a hillside and was placed where a 16th century monastery once stood. The existing buildings were built after the occurrence of the massive earthquake that affected and destroyed much of the city in the 18th century, and many of them were founded in rubble fills supported by gravity walls. This situation imposed strong constraints in terms of global stability of the surrounding buildings and the slope itself, as well as from the archaeological point of view. The main access to the station as well as the technical rooms and ventilation equipment are in a central shaft, with a rectangular section and cylindrical end sides. The cavern for the station, 110 m long and 17 m wide, was excavated in vertical and horizontal phases, five in total, applying the sequential method, including previous ground reinforcement, primary support installation and concrete final lining construction. One of the critical primary structures were the reinforced shotcrete structures located at the intersections with the transverse galleries on both sides of the central shaft. The final lining was made of reinforced concrete, fitted with a waterproofing system based on a segmented PVC membrane and geotextile for protection and drainage. The most significant alteration made during the project was the redefinition of the foundation arch, reducing its curvature to avoid excessive excavation in the saturated karst limestone massif. In return, the amount of reinforcing steel was increased. Lisbon is considered a seismic zone, so the final structure was earthquake-tested in accordance with current regulations.

With climate change, Toronto has been facing increasingly frequent episodes of intense rainfall in recent years: sudden thunderstorms and record-breaking precipitation. The wastewater and stormwater system then becomes saturated, causing stormwater backflows into residential basements. In 2023, flood-related damage costs were estimated at more than 1 billion Canadian dollars in the Greater Toronto Area. To address these challenges, the City of Toronto launched the municipal Basement Flooding Protection Program. One of its flagship projects is the Fairbank–Silverthorn Storm Trunk Sewer System. The project is based on a main stormwater trunk sewer 2,400 m long and 4.50 m in diameter, excavated at depths ranging from 15 to 45 m. This tunnel functions both as a conveyance channel and as a temporary storage reservoir, capable of transporting up to 9.5 m³/s to Black Creek. This underground stormwater management system protects more than 4,600 homes across a 140-hectare drainage basin. It significantly reduces the risk of basement flooding, while also decreasing pollutant discharges into the adjacent watercourse by 40 million liters per year. The project provides a direct response to extreme weather events whose frequency is increasing as a result of climate change. Beyond its immediate function, the project illustrates the growing role of underground construction techniques in urban climate adaptation. --- Global demographic growth, urbanization, energy transition, and digital expansion are driving a surge in network infrastructure needs, particularly offshore. These networks must cross coastal zones to connect to land, requiring highly technical and environmentally compliant solutions. Among available methods, microtunneling stands out as the most suitable: it enables long crossings—often over one kilometer—beneath sensitive areas without surface impact and adapts to diverse geological conditions, offering a clear advantage over horizontal directional drilling (HDD). Offshore networks include submarine telecommunication cables (carrying 99% of global data, growing >25% annually), electrical interconnections for offshore wind (+10% per year), oil and gas pipelines (including CO₂ and hydrogen projects), and networks (wastewater, desalination, nuclear cooling). Sandy coastlines, representing one-third of global shores, pose specific challenges: erosion, protected zones, and lack of soil cohesion. The microtunneling method addresses these constraints through a dedicated site organization and advanced lubrication techniques. The INELFE project, a strategic France–Spain electrical interconnection, illustrates this relevance: three microtunnels, each 1.2–1.35 km long, were built under the Atlantic to house multiple cables while meeting strict environmental standards. This achievement demonstrates the efficiency, resilience, and sustainability of microtunneling for offshore infrastructure, confirming its role as a key solution for rapidly expanding global networks. --- The Dakar region, the capital of Senegal, is facing increasing pressure on its drinking water resources, exacerbated by climate change, rapid population growth, and the overexploitation of groundwater. In this context, the Mamelles seawater desalination plant project aims to sustainably secure the city’s drinking water supply by diversifying its sources. The project involves the design and construction of a desalination plant using reverse osmosis technology, with a capacity of up to 100,000 m³ of water per day. It includes all the necessary infrastructure for its operation: seawater intake, pretreatment, the desalination unit itself, brine management, pumping systems, and connection to the distribution network. This facility is intended to meet the growing water demand efficiently while addressing the environmental and technical constraints specific to Dakar’s coastal area. A key component of the project is the construction of two 330-meter-long tunnels beneath the Atlantic Ocean, essential for raw water intake and brine discharge. BESSAC designed and built these tunnels using a dedicated microtunneling machine in a particularly challenging coastal environment. Careful management of geotechnical risks, along with consideration of social and environmental factors, was critical to the project’s success. By enhancing access to drinking water for several hundred thousand residents, reducing pressure on underground aquifers, and improving urban resilience to climate change, the Mamelles plant represents a strategic and sustainable response to Dakar’s water crisis. --- The energy transition requires reliable, sustainable infrastructure for large-scale storage of decarbonised energy carriers. Underground hydrogen storage in salt caverns is emerging as a strategic and technologically mature solution to address key challenges in grid flexibility, energy security, and industrial decarbonisation. However, adapting existing salt caverns to high-frequency hydrogen injection and withdrawal cycles presents technical and safety challenges that remain insufficiently understood. This paper draws on the combined lessons of two complementary European projects led by Storengy with scientific support from Ineris: HyPSTER (2021–2025), a pilot demonstrator in Etrez (01), and FrHyGe (2024–2029), a more advanced demonstrator in Manosque (04). In HyPSTER, ~2.6 tonnes of hydrogen were injected in 2024–2025 into the EZ53 cavern. Advanced numerical modelling guided well integrity tests and mechanical cycling of the cavern. Risk assessment was based on Ineris’s methodology and included analysis of major accidental scenarios, notably well blowouts. FrHyGe aims to reach ~100 tonnes of H₂ stored using two interconnected caverns and 100 rapid cycles, requiring an enhanced risk evaluation covering extreme scenarios: blowouts, mechanical instabilities from seismic events, residual salt permeability, environmental impacts, and external hazards (e.g. wildfires), alongside a dynamic life-cycle environmental analysis (including CO₂ emissions). In both projects, Ineris leads the work package on safety, risk, environmental and regulatory aspects. Initial results demonstrate that when underground structures are properly designed, modelled and monitored, they offer a robust, scalable and sustainable solution for large-scale hydrogen storage. The experience gained from HyPSTER and FrHyGe provides a critical foundation for the safe, permitted and replicable deployment of hydrogen storage in salt caverns in France and across Europe.

The « full-face station crossing » technique by Tunnel Boring Machine (TBM) consists of mechanised excavation within diaphragm walls, followed by open-cut excavation of the station box and demolition of the temporary segmental lining. To prevent ring ovalisation during the open excavation phase, a continuous full-round strengthening with steel ribs is commonly installed inside the tunnel. However, the size and weight of these ribs require TBM stoppage and significant reorganisation of temporary works. To overcome these constraints, an alternative solution using Carbon Fibre Reinforced Polymer (CFRP) strips was studied and implemented at Aéroport d’Orly Station on Paris Metro Line 14. These ultra-thin, lightweight strips can be placed directly from the TBM back-up after ring erection and bonded onto the segments using an epoxy resin. The high tensile performance of carbon fibres mitigates the development of plastic hinges and the opening of longitudinal joints along the tunnel flanks. Monitoring with optical targets in the tunnel, complemented by strain gauges on the strips and segments, was carried out to verify structural stability. This paper presents the design principles of this unconventional strengthening system in tunnelling, together with production and site monitoring feedback from the Line 14 South project.

The High Speed 2 project involves the construction of a high-speed railway line between London and Birmingham. The northern section of the alignment includes the construction of the Bromford Tunnel. This tunnel construction faces several challenges, notably the TBM crossing beneath the M6 highway viaduct and the construction of the ventilation adits with unusually large dimensions The M6 viaduct, supported by deep foundations, carries the most heavily used highway in the United Kingdom. The project team develops an approach based on the general principles of prevention to demonstrate that the tunnel construction would not compromise the structural integrity of the viaduct. This paper shows how the proper definition of the TBM support pressure was fundamental in the approach. The ventilation adits are two large —9.2 metres wide and 8.7 metres high— adits that connect the intermediate shaft to the tunnel. The design and methods teams collaborated to develop a suitable solution to ensure the integrity of the tunnel lining during the construction of the adits. This solution involved installing a temporary steel frame within the tunnel. However, the frame installation was constrained by a very limited time window, requiring the method team to devise a sequence to assemble all frame elements and connections within just a few days. This paper describes the challenges related to these two works, presents the solution developed by the project team to secure the operations, and shares feedback from the implemented works. --- The project to extend Montréal's Blue Line aims to enhance accessibility and sustainability of public transit by connecting the boroughs of Villeray – Saint-Michel – Parc-Extension, Saint-Léonard, and Anjou from west to east. The Société de transport de Montréal acts as the project owner and manager, overseeing the addition of five new stations, seven auxiliary structures, and a tunnel nearly 6 km in length. The Mobilité Bleu Horizon consortium (EBC, Pomerleau, Spie Batignolles) has been awarded the main contract, which includes excavation of the tunnel as well as all stations and auxiliary structures. The station discussed in this article is located in the Saint-Léonard borough, at the intersection of Lacordaire Boulevard and Jean-Talon Street East. It reaches a total depth of approximately 25 m, with more than 20 m excavated in a highly resistant sedimentary rock formation. The open-cut excavation of the station is carried out using conventional mechanical methods (station) and drill-and-blast techniques (emergency exit), within a constrained and densely urbanized environment. First, the article presents the approach adopted to address technical challenges related to local geology and excavation methods, with particular emphasis on the use of a hydraulic rock splitter. Second, the article details the monitoring and control measures implemented to ensure the stability of structures and to preserve the integrity of existing infrastructure. --- This paper presents the outcomes of cutterhead modifications implemented on an Earth Pressure Balance Tunnel Boring Machine (EPB‑TBM) during the excavation of Shiraz Metro Line 3 (SM3). The tunnel alignment traversed mixed ground conditions comprising clayey soils (65%) with sections of coarse alluvium and bouldery zones. To address these challenges, a hybrid cutter configuration was adopted, integrating double disc cutters, intermediate precut rippers, and peripheral button rippers, with cutter spacing reduced from 100 mm to 50 mm. Tool selection was guided by geological condition (GC) using a simplified 1R classifier, which achieved 81.25% accuracy and confirmed GC as the dominant predictor. Performance evaluation demonstrated significant improvements: average penetration rate reached 39 mm/min (peak 70 mm/min), monthly advance rate achieved 215 m (79th percentile globally), and cutter life increased to 2105 m³ per Primary Cutter Type—3.2 times the reference value. Torque and thrust were reduced by 52% and 36%, respectively, while penetration per revolution increased by 18%. The study confirms that adaptive cutterhead design, supported by data-driven tool selection, can substantially enhance EPB‑TBM performance in heterogeneous urban geology.

This paper explores the application of foresight methods in the development of legal frameworks for the planning and utilization of urban underground spaces, with a particular emphasis on anticipatory and systemic approaches to the regulation of complex urban transformations. It focuses on analyzing the limitations of current legal frameworks, which often fail to address the long-term dynamics and complexities inherent in the development of underground urban areas. In the context of accelerated urbanization, technological innovation, and growing demands for sustainability and resilience in urban systems, the research highlights the need to integrate foresight methods as a foundational instrument in legal planning, rather than as an auxiliary or reactive tool. The paper develops a conceptual and methodological framework that connects legal instruments with medium- and long-term development scenarios, aiming to enable timely adaptation of regulatory mechanisms to societal, technological, and urban shifts. Rather than proposing a finalized model, the study opens space for a conscious, reflective approach to the creation of prenormative frameworks that support risk anticipation and the identification of strategic directions for managing underground spaces. Special attention is given to selecting foresight methods that align with specific temporal, cultural, and sociological contexts. The paper underscores the necessity of interdisciplinary, multisectoral, and transdisciplinary collaboration among legal experts, urban planners, policymakers, technical professionals, and end users. The findings point to the urgent need to shift from reactive to proactive regulatory approaches, positioning underground spaces as active components of a sustainable, resilient, and functional vision of the future city. --- The city of Nîmes (Gard, France), regularly affected by severe flash floods, has implemented a comprehensive protection strategy under the PAPI 3 Vistre program. This includes the construction of two underground stormwater tunnels – Uzès (980 m) and Limites (1,240 m) – excavated by TBM in a dense urban context crossing the city. With an internal diameter of 3.30 m, these tunnels aim to increase the hydraulic capacity of existing cadereaux, significantly reducing flood risk. The design and construction faced major challenges: very shallow cover (3–6 m), variable geological conditions, immediate proximity to buildings, structures and sensitive utilities, anthropogenic constraints, while ensuring hydraulic continuity of the existing flood drainage system. This project illustrates how underground solutions can be integrated into constrained urban environments to address climate resilience and adaptation to extreme events. --- Achieving decarbonization in tunnel construction is not only possible but crucial from a sustainable perspective. Currently, nearly 70% of embodied carbon in tunnels is attributed to concrete linings. To bring this down, we need to better consider the spray concrete lining as part of the permanent lining The excessive structural design using cast-in-place concrete or other lining structures used in the current practice, have a significant impact on cost, excavated volume of rock mass, construction time and CO emissions. Changing temporary spray concrete liner and permanent CIP liner to permanent shotcrete liner and improving the mix design, CO₂ emission can be reduced by 75%. Justifying the mechanical behaviour of fibre-reinforced sprayed concrete for permanent structures is a major issue for the development of this technical possibility in underground. The possibility of using permanent sprayed concrete allow to consider this preliminary lining for carrying the permanent load. In this case the possible solution is: a) to consider the preliminary sprayed FRC lining collaborating with the cast final lining b) to consider the preliminary sprayed FRC collaborating with a further sprayed in a second stage c) to consider only the preliminary FRC lining as the final stage A test method specifically adapted to sprayed concrete was standardized in Europe in November 2023 to assess flexural tensile behaviour. Ensuring the performance requirements for permanent S-SFRC tunnel linings requires reliable and representative mechanical characterization. The geometry and dimensions of the specimen match with the fabrication process (spray), which should ensure the distribution of the fibers in the matrix and being more representative of the real case of spraying job execution This contribution presents results from different recent experimental program aimed at developing an S-SFRC(Spray Steel fiber Reinforced Concrete) mix that consistently achieved the 3c strength class (Annex L, EC-2) by analyzing the strength class obtained via Method B of EN 14 488-3 . Designers have now employed and constructed tunnels and caverns utilizing S-SFRC around the globe.

The Central Kowloon Route (CKR) is a significant infrastructure project aimed at improving transportation and connectivity across Kowloon, Hong Kong. Spanning 4.7 km, the dual 3-lane road includes a 3.9 km tunnel running partly beneath Kowloon Bay’s seabed. Viaducts connect existing interchanges, forming part of Route 6 and streamlining traffic between West and East Kowloon. Bouygues TP was awarded the Central Tunnel contract (CKR-CT) in July 2019, covering 2.8 km of tunnels, 29 cross passages, a cut-and-cover section, and an 82 m ventilation shaft at Ho Man Tin. A pilot Tunnel Boring Machine (TBM) enhanced efficiency and planning. The CKR-CT was one of the largest New Engineering Contract (NEC) projects signed by the Hong Kong Government, requiring strong collaboration during the COVID-19 pandemic. Logistics were challenging due to limited access, managed via three deep shafts at Yau Ma Tei (35 m), Ho Man Tin (107 m), and Ma Tau Kok (40 m). These facilitated the transport of 3 million tonnes of spoil, 330,000 cubic meters of concrete, and daily workforce. Excavation passed under 240 buildings and potentially affecting a population of 100,000, crossing four MTR metro lines and navigating seven geological faults. Advanced site investigations, geotechnical design, sophisticated grouting, and precision blasting maintained safety and minimized environmental impacts. Alternative contractor-proposed designs optimized construction. With tunneling and permanent works completed, the CKR is set to open at the end of 2025, transforming Kowloon’s transportation and enhancing Hong Kong’s urban infrastructure. --- The "Monsanto–Santa Apolónia" (TMSA) and "Chelas–Beato" (TCB) drainage tunnel project aims to mitigate recurrent flooding in Lisbon. Awarded as a design-build contract to the consortium Mota-Engil – Spie Batignolles International (contractors) and Spie Batignolles Génie Civil – LCW – Aqualogus (designers), the project includes two tunnels: TMSA (4.4 km) and TCB (1.1 km). The TBM OLA was launched from shaft TM1 in 2023 and completed the TMSA excavation in September 2025, reaching shaft TM5. It will be reassembled in 2026 to excavate the TCB from shaft TC2. Hydraulic structures for water collection and discharge into the Tagus River were built in parallel. This article highlights the technical challenges encountered, particularly in crossing sensitive areas and managing the TBM’s exit in a dense urban setting. The chosen method combined traditional excavation of the upper half-section from TM5 with TBM excavation of the lower section. --- Balfour Beatty VINCI (BBV) is delivering a 90km section of High Speed Two (HS2), the UK’s new 225km high-speed railway connecting London and Birmingham. As the route approaches Birmingham, it runs through 5.8km of twin-bore tunnels passing beneath and alongside the 4.4km-long Bromford Viaduct, which carries the M6 motorway. An Integrated Project Team (IPT) comprising HS2 Ltd, BBV, and designer Mott MacDonald SYSTRA JV worked closely with National Highways, their consultants (Pell Frischmann and COWI), and CAT 3 checker WSP to ensure the tunnelling could proceed safely beneath this critical infrastructure. This paper outlines how the IPT applied the General Principles of Prevention (CDM 2015) to manage risks associated with tunnelling beneath the viaduct. The approach aimed to enable TBM passage within the viaduct’s zone of influence while protecting the structure, maintaining its capacity, and minimising disruption to road users. Initially risks were reduced by realigning the tunnel, shortening the affected viaduct length from 1.3km to 0.7km. while the remaining risks were assessed and mitigated through inspections, ground investigations, staged assessments, strengthening and monitoring The paper demonstrates a successful methodology for tunnelling beneath critical infrastructure, ensuring both safe tunnel delivery and continued viaduct operation, and concludes with a framework combining prevention principles with ground movement assessment and control methods for future projects --- The Lyon–Turin Euralpin Tunnel (TELT) project involves the construction of the 57.5 km Mont Cenis Base Tunnel between France and Italy. Construction sites 6 and 7 (CO6-7, Lot 2) involve excavating approximately 50 km of tunnels using three segmental tunnel boring machines and the so-called traditional method (explosives). CO6/7 comprises the construction of the two base tunnels over 23.1 km, as well as caverns, a safety site and logistical facilities. To this end, two access shafts (Saint-Martin la Porte and La Praz) have been constructed upstream of the current tunnelling site. CO6/7 will run from 2021 to 2028 with a notified budget of €1.5 billion. From the start of the construction phase, the VINCI Construction – Webuild consortium identified significant logistical challenges in coordinating the requirements of the underground works with the surface facilities via these two access shafts. At the same time, a project review with the Project Manager, the S2IP consortium (comprising SETEC, SYSTRA, ITALFER and PINI/ARX), highlighted that certain temporary logistics structures could be optimised. A ‘win-win’ contractual agreement was then drawn up and approved between the Contractor, the Project Manager and the Client (TELT), at no extra cost to the project and with a reasonable sharing of risks: - The construction of an additional logistics tunnel was approved to provide a third access point to the underground works area. This tunnel, with a cross-sectional area of 70 m² and a length of 500 metres, could then be used for logistics operations relating to conventional excavation methods. It thus facilitates ventilation, spoil removal, pumping, power supply and the management of other utility networks, the supply of concrete and support structures, as well as the movement of teams to the working faces. This new access also improves safety conditions on site. - Furthermore, the tunnel boring machine (TBM) assembly area has been moved a few metres, utilising the technical cavern already included in the project, thereby eliminating the need to construct a temporary TBM assembly cavern, a structure with a cross-sectional area of 340 m². To achieve this, the TBM will then be rotated 90° and moved several metres in order to begin tunnelling in the base tunnel, which has been excavated several metres ahead and has a wider cross-section. --- CERN, the European Organization for Particle Physics Research, is an intergovernmental organisation with 20 member states. CERN’s mission is to enable international collaboration in the field of high-energy particle physics research and to this end it designs, builds, and operates underground particle accelerators and the associated experimental areas. The accelerator complex at CERN is a succession of machines with increasingly higher energies. Each machine injects the beam into the next one, which takes over to bring the beam to an even higher energy, and so on. The LHC is currently the most powerful accelerator con-structed on CERN’s site. The Future Circular Collider (FCC) study explores the feasibility of 80 – 100 km long circular colliders for the post-LHC era, planned for construction 2032 – 2040 within the Franco - Geneva basin. The current tunnel layout is a 91 km quasi-circle which intersects the LHC in plan view, placed at an average elevation of 300 m ASL mainly in the molasse formation. It has 8 access points, and the deepest shaft is approximately 400 m. To determine the feasibility of the project, ground characterisation, through geophysics, boreholes, on-site and laboratory testing, is carried out in the areas of highest geological uncertainty, considering the various local and environmental constraints. The works have been carried out by two contractors, one working solely in France and one working in the cross-border region, mostly in Switzerland, including on Lake Geneva. This paper describes the first results from the geophysical and geotechnical investigations in France in Mandallaz, Vuache and Jura sectors, as the investigations in Switzerland have only just started --- The construction of long Alpine railway tunnels involves the creation of large underground caverns, which are essential both during the excavation phases and for the long-term operation of the infrastructure. These caverns play a central role in site logistics, the assembly, disassembly of tunnel boring machines (TBMs), as well as during the operational phase. The paper draws on several major projects carried out over the past two decades, notably the Ceneri Base Tunnel in Switzerland, the Brenner Base Tunnel in Austria, and the Mont Cenis (TELT) Tunnel between France and Italy. These tunnels are distinguished by very large caverns, with cross-sections exceeding 400 m², excavated beneath rock overburden that can exceed 1,500 meters. The design and execution of such caverns represent a major technical challenge due to the geological complexity of the Alpine massifs. They require detailed characterization of the rock mass, the use of advanced numerical modeling, and finely phased excavation methods adapted to the encountered geotechnical conditions. Supports must be able to absorb significant and time-dependent deformations while ensuring the stability of the structure despite the presence of unfavorable features such as faults or shear zones. The support solutions implemented generally combine shotcrete, steel ribs, rock bolts, and deformable systems. The proximity of other underground structures further increases the complexity, requiring careful consideration of interactions. The success of these projects relies on a multidisciplinary approach integrating geology, geotechnics, engineering, and logistics, supported by continuous monitoring and adaptive construction methods.

As part of the Lyon–Turin railway project, the Mont-Cenis base tunnel, extending over 50 km, represents a critical component of the transalpine infrastructure. A first TBM will be launched from the CO7 site, located at the base of the Saint-Martin-la-Porte access tunnel. To enable this, the prior construction of an underground technical cavern nearly 20 meters in diameter was required for the assembly and deployment of the machine. It was excavated within the Briançonnais Houiller geological formation, characterized by alternating sequences of schistose sandstones, interbedded with strongly tectonized layers of black shales, varying in carbon content. However, during the excavation of the first two benches, the deformation levels exceeding initial estimates, along with high deformation rates, were observed. This led to a temporary suspension of work to reinforce the structure. In response to these discrepancies, a back-analysis was conducted using three-dimensional finite element numerical modelling (Flac3D) to identify the geomechanical behaviours that could account for the observed phenomena. Hence the geotechnical parameters were refined based on observations and data collected during the excavation phase. The faults identified during excavation were also represented as individual discontinuities in the model. The final model provides a clear representation of the deformation mechanisms and shows good agreement between simulated and measured deformations. --- Keywords: Tunnel face stability – Liquid confinement – TBM tilting – Deep soil mixing (DSM) – Carbon footprint – High hydrostatic pressure – Shallow overburden The Hampton Road Bridge Tunnel (HRBT) is a motorway between the cities of Hampton and Norfolk in the State of Virginia, USA. The project aims to double the traffic lanes. It has resulted in the widening of 8 km of motorway, the construction of 5.6 km of bridges, and the excavation of a 2.4 km twin-bore tunnel crossing the channel leading to the harbour facility of Norfolk. The twin-bore tunnel has been excavated by a variable density tunnel boring machine (TBM) with a diameter of 14 metres. The maximum hydrostatic pressure at the tunnel axis is 5 bar. The soil cover varies between 1 and 3 excavation radii. The excavated soil is a mixture of clayey and sandy materials, which can be highly permeable. Along 520 metres long of each tunnel, ground treatment had been anticipated to ensure ground stability and prevent the risk of TBM guidance failure. This article focuses on the calculation of confinement pressures necessary to ensure the stability of the tunnel face and to prevent the tilting of the TBM in the weakest zones. Calculations point out ground treatment in the marine area were not necessary due to the TBM's capabilities. The volume of treated soil was reduced by a factor of eight, and deep soil mixing was adopted. It has not only reduced the carbon footprint of the project, but it has also avoided long-term pollution of the marine environment. --- Snowy 2.0 is a major hydro power project currently under construction in Australia, aiming to increase the capacity of the existing Snowy Mountains hydro power scheme with an additional 2200 MW and making it the largest source of renewable energy in Australia and one of the largest pumped-storage projects in the world. The Snowy 2.0 Power Station Complex is being excavated at a depth of approximately 720 meters, in conditions of high in-situ stress and fair rock mass conditions. The stability of the excavation is locally affected by pre-sheared discontinuities, particularly in the two large caverns connected by six 60-meter-long transversal galleries (IPBs) routing the electrical busbars. During the construction of the main caverns, the rock support design at the IPBs junctions is verified using the Observational Method. Inclined boreholes drilled across the rock pillars in-between the IPB galleries provide joint data, enabling the extrapolation of discontinuities crossing the rock pillars. This data, combined with LiDAR scans of the cavern sidewalls and geological mapping covering the crown of the gallery/cavern junctions, is used to develop a representative discontinuous numerical model in 3DEC used to verify the rockbolt support in both elongation and shear, enlightening the decision making on the installation of steel ribs at the junctions. The numerically estimated displacements are then compared with the output of the monitoring devices, such as Displacement Measurement Points and Multi-Rod Extensometers, enhance the reliability of the design. --- The tunnel for Lot 4 of the third line of the Toulouse metro crosses a unique geological formation characterized by soft rock, but also by the presence of lenses of coarse sand. This article will present the earth pressure balance tunnel boring machine (EPBM) containment methodology proposed in the tender documents, distinguishing between the following containment methods: a slurry containment of the tunnel face was recommended in sensitive areas, sometimes, this slurry containment at the face was combined with lateral bentonite injection around the shield, in other areas, gaseous containment of the face was planned, while retaining the option to quickly revert to slurry containment should sand enter the cutting chamber. We will discuss the predictive calculations concerning settlement and face stability, which enabled the definition of an optimal containment profile. General observations of settlement during excavation will also be presented, with particular attention paid to a specific area that underwent enhanced monitoring. We will present a back-analysis calculation using adjusted geotechnical parameters to obtain a more precise correlation between confinement and observed settlement. --- As part of the Lyon–Turin project (TELT), a particular 1.4 km-long section was excavated between 2017 and 2022 within the Saint-Martin-La-Porte (SMP4) reconnaissance site. This section crosses a heterogeneous rockmass, predominantly Carboniferous shale formations with remarkable squeezing behaviour. Significant deformations were observed, reaching up to 10% in a fault zone approximately 50 to 100 meters long, located at a depth of nearly 550 m. The crossing of this zone was carried out in two phases: first, the excavation of a pilot tunnel with a radius of 3.2 m, followed by an enlargement to a bigger section with a radius of 6.5 m. An observational method was implemented, combining in-situ instrumentation, convergence monitoring, and real-time adaptation of support measures in response to evolving ground conditions. This paper presents feedback on the excavation through the fault zone, detailing the excavation technique, the installed support system, the monitoring equipment, and the interpretation of the collected data. The results allow for an assessment of the adopted approach and provide insights into the influence of the pilot tunnel on the behaviour of the subsequent enlargement and the overall stability of the structure. --- Following several catastrophic tunnel fires, modern safety codes have established rigorous limits on concrete spalling for road and rail infrastructure. Spalling susceptibility is driven by the interaction between material properties, such as concrete mix design and thermo-mechanical variables, including heating rates, applied loads, and boundary conditions. Accurate experimental assessment requires replicating the precise stress states found in real-scale structures; however, simulating these complex boundary conditions in a laboratory is a significant technical challenge. This study utilizes numerical modelling to evaluate three distinct loading protocols: (a) the constant load approach, (b) the zero horizontal displacement criterion, and (c) an evolving load method. By analysing tunnel segments of varying thicknesses and initial stresses, the research identifies which strategies provide the most conservative safety margins. The paper details the resulting experimental setup and offers practical recommendations for future testing, ultimately improving the reliability of spalling predictions under realistic fire conditions.