1 Introduction
The need to construct tunnels is increasing due to the expansion of cites. The new metro system in the Paris area is the Grand Paris Express (GPE) project, a fully automated transit network. It is also the largest transport project currently underway in Europe, and it represents a fundamental rethinking of the design and focus of the future public transport network on the scale of metropolitan areas. Its purpose is to provide the Grand Paris region with multimodal transport solutions and more integrated transport services, thereby supporting a model of polycentric development. The Société des Grands Projets (SGP), former Société du Grand Paris, a French public organization, is in charge of program management for the GPE project.
With its 200 km of new lines and 68 new stations, the GPE project encompasses the creation of one ring line around Paris (Line 15) and three lines to connect developing neighborhoods (Lines 16, 17, and 18). Furthermore, the GPE project involves the extension of four existing metro lines (Lines 4, 11, 12, and 14). To build these extensions, the SGP has delegated owner ship responsibility to the main transit organizations in the Ile-de-France region, namely, the Régie Autonome des Transports Parisiens (RATP) and Île-de-France Mobilités.
The GPE project circles the French capital and provides connections with Paris’ 3 airports, business districts and research clusters. It will serve 165000 companies and transport more than 2 million commuters daily. To construct such a once-in-a-century megaproject, intensive construction technology is necessary to help guarantee that the project meets its quality, cost, schedule and safety goals.
This paper provides a review of the intensive construction technology used in the GPE project and is structured as follows: after introducing the GPE project, the project’s complexity and challenges are outlined. Considering its scale and high-risk nature, the risk management plan (RMP) of the project is introduced, along with the contractual and insurance conditions, and the technical risks identified. The paper then provides a description of the major design principles and construction methods chosen to overcome these challenges, reduce the technical risks and fulfil the project aims, accompanied by monitoring methods and digital approaches. In addition, several new and innovative technologies used in the GPE project are illustrated before covering issues relating to environmental protection and providing concluding remarks.
2 Challenges of the Grand Paris Express project
The scale of the GPE project is large and its ambition is to improve connectivity across the entire Paris region by building 200 km of new railway lines and 68 new stations. Working on such an enormous project in a dense city undoubtedly poses a wide variety of complexities and challenges.
2.1 Complex project coordination and contract management
The GPE project is the biggest infrastructure project now under construction in Europe. At peak activity, the number of employees will reach nearly 15000. Specifically, turning the plans into reality involves managing approximately 180 urban projects and engaging more than 5000 companies (SGP website [
1]). As a result, coordinating these construction projects is very important for ensuring project progress, all while monitoring hazards and collaborating with multiple stakeholders. Furthermore, numerous interfaces with third parties must be coordinated, such as with the Société Nationale des Chemins de Fer, with the concession holders and operators of the existing structures and networks (Direction des routes d'Île-de-France, RATP, Syndicat interdépartemental pour l'assainissement de l'agglomération parisienne, Direction de l’Eau et de l’Assainissement, etc.), and with the operators of private sites where reinforcements, construction adaptations and/or investigation methods must be carried out.
The GPE project includes mostly underground structures and tunnel work packages but also several work packages of aboveground civil works, such as viaducts (Line 17 Lot 2 and Line 18 Lot 2). Moreover, different types of contracts are needed, including classical construction contracts (Line 15 South and Lines 16, 17, and 18) and design-build contracts (Line 15 East and Line 15 West). In addition, the GPE project poses numerous logistical challenges, for example, how to operate more than 20 tunnel boring machines (TBMs) simultaneously and how best to evacuate 45 million tonnes of excavated materials.
2.2 Difficult construction conditions and strict scheduling
It is a serious engineering challenge to tunnel beneath a crowded metropolis like Paris with a geological terrain altered by humans over millennia. In addition, because of their depth and geographical location, the tunnels of the GPE intercept geological formations rarely encountered in previous underground works. These problems were resolved with a notable lack of past experiences, especially in regard to excavation using TBMs.
Two main categories of difficulties can be observed: urban constraints and technical complexity.
First, the following are the main urban constraints.
1) Major limitations with regard to routes and the extremely constrained and limited space available for construction works.
2) Involvement of numerous stakeholders, including local inhabitants, and the existence of above-ground and occasionally subterranean structures.
3) Used subterranean areas, such as abandoned objects of anthropic origin, quarry operations, and archeological ruins.
Second, the technical complexity principally includes the following.
4) Moderate level of soil quality (loose, changing, polluted, etc.) and groundwater presence.
5) Frequent shallow overburden soil layers with a significant impact on the design and environment.
6) Considerable risk sources for neighboring structures and third parties.
There are approximately 24000 neighboring structures located around or below the tunnels across the 200 km extent of the project. Specific activities (injection, jet-grouting, freezing, etc.) must frequently be conducted to seal and consolidate the ground before earthworks can begin. Moreover, starting in 2010, the GPE project involves an ambitious schedule with major milestones planned between 2022 and 2030. One salient example is the largest station, Saint-Denis Pleyel, which needs to be finished by 2024 in time for the Olympics and Paralympics Games in Paris.
3 Management of risks
The inherent uncertainties of ground conditions are difficult issues in underground works. In recent decades, risk management has become an important component of most subterranean projects in order to limit costs and delays while guaranteeing safety and other performance parameters [
2–
8]. Due to the complexity and high-risk nature of the GPE project, a RMP has been applied since its inception [
9–
12]. The design of the RMP depends on the appropriate knowledge and competencies of designers and contractors from the preliminary design phase through the construction phase, such as
1) the execution of high-quality geotechnical investigations to minimize risks;
2) the adoption of appropriate construction methods with high-quality monitoring of the worksites;
3) the implementation of contractual process that permit parties to share risks. For this to happen, the contracting entities (client/owner, designer and contractor) and insurers must work together under the right conditions in an environment of trust and open communication.
3.1 Risk management plan
The RMP is a recommended approach to set out and account for risks in line with International Organization for Standardization (ISO) [
13] and Association Française des Tunnels et de l’Espace Souterrain (AFTES) recommendations [
9]. The project owner initiates this iterative process, which continues through the design and construction phases (Fig.1). Its objectives are to recognize possible risks, measure the relative risks, and ensure that appropriate mitigations and preventive actions are put in place. It focuses on continual improvement to prevent and minimize risks. The RMP is a contractually tender document in the French setting [
4,
14] and in compliance with Maîtrise d’ouvrage publique (MOP) law (French public works procurement law). In the event that undesirable outcomes occur, it describes technical and financial remedies as well as a balanced allocation of risks among the owner, designer and contractor.
Balanced allocation of risks and their incorporation into the RMP’s contractual framework are critical components for the success of a given project. In the GPE project, the RMP specifies the contractual terms for three distinct risk categories [
12].
1) Type 1: risks that are analyzed and preventive solutions that are integrated into the design, or low-criticality risks for which the contractor bears the financial responsibility.
2) Type 2: risks for which the unit price schedule for the contractual risks compensates the financial payment (Fig.1).
3) Type 3: unlikely risks for which mitigating actions cannot be planned ahead of time or with precision, and for which the financial payment is negotiated on a case-by-case basis.
The RMP approach must be monitored in order for a project to be insured. A contractor during the construction phase or the owner for all phases can arrange the insurance package (kind of liability, quantity, and period of coverage), which can cover different risks. However, until the risks have been reduced to as low as reasonably practicable level, the insurance approach should not be regarded as the main risk reduction strategy.
There are primarily three kinds of insurance policies in the GPE project. The purpose of these insurances is to compensate for damages and losses suffered during or following the construction of the project, whether they were brought on by the construction itself or by other parties. First, the all-risk insurance policy, taken out by the project owner and including the project designers, contractors and the subcontractors, covers material damage and losses that may take place when the project is being designed and constructed. Second, the civil liability assurance policy, taken out by the project contractors, covers their third-party civil liability during the construction phase. Third, the ten-year insurance policy, taken out by the project contractors, covers the post-acceptance risks related to the realized works’ ten-year civil liability.
3.2 Major technical risks
The principal technical risks in the GPE project were initially identified at the design phase, subsequently summarized in the residual risk register and compiled in the tender documents, and lastly contracted during contract signature and updated at the construction stage. They fall into three categories [
12]: geological/geotechnical risks, damage risk to neighboring structures or assets and environmental risks.
Anthropogenic quarries, high-level groundwater and associated gypsum dissolving, and clay swelling and shrinkage are the major geological and geotechnical challenges of the GPE project [
10–
12,
15–
22]. In the following section, the design principles and construction methods with the consideration of these major technical risks will be presented.
4 Choice of the design principles and construction methods
The Paris area is a sedimentary basin mostly composed of limestone, gypsum and clay formations that alternate throughout Tertiary layers [
23]. The Paris Basin’s Eocene sedimentary succession is where the GPE project is mostly situated. Furthermore, Paris has a complicated hydrogeology. Since aquifer units make up a large portion of the geological formations, groundwater in multilayer aquifer systems must be addressed during construction. It is possible to identify five primary aquifer groups: the Chalk, Ypresian, Lutetian, Saint Ouen/Beauchamp (or Bartonian), and Quaternary alluvium units [
24]. Therefore, it is expected that the construction of underground buildings may cause a variety of phenomena, including changes to groundwater level and groundwater flow (dam impact).
4.1 Design principles
4.1.1 Structural design
Diaphragm walls are methodically employed in the structural design to reduce groundwater level modification and to provide a waterproof enclosure that allows subterranean metro stations and structures to be excavated. Regarding sealing, the screens of diaphragm walls can descend as far as 75 m. After that, the next step will be either an injected skirt or a hydraulic sheet that is anchored in the bedrock, which is either chalk or coarse limestone. Plugs for under-raft sealing are occasionally another option for guaranteeing hydraulic stability. Concurrently, inside the structures and stations, pumping systems are installed in order to minimize the impact on external water tables while lowering the internal water tables prior to the start of earthworks. To lessen changes to groundwater flow (dam effect), a number of metro stations and cut-and-cover tunnels are constructed using specialized technical solutions, such as drainage trenches and hydraulic siphon devices.
Additionally, the swelling pressure related to expansive soils can pose a major problem to civil infrastructure. The Paris Basin’s clay formations, including plastic clays, Argenteuil marls, Luddite marls, Romainville green clays, and oyster marls, show some capacity for swelling and shrinkage [
20–
21,
25–
26]. Many underground structures in the GPE project, such as shafts, stations, diaphragm walls, tunnels, etc., intersect with these expansive geological formations. In addition to meticulous in-field geotechnical testing and laboratory testing to completely define the expansive soils’ physical properties to be taken into consideration in the design, the following engineering considerations and practices are used and implemented to handle swelling soils [
12].
1) A crawl space can be created beneath a raft to prevent contact with expansive soils.
2) A layer of expanded polystyrene can be used beneath a raft to absorb the swelling pressure.
3) A draining raft can be made by including valves to release water pressure.
4) A raft can be designed to withstand the swelling pressure including solutions such as with sufficient reinforcement, cross-vaulted slab forms, late keying, and slab anchoring.
In general, the metro stations of the GPE project have a mean depth of about 30 m. In exceptional cases, the metro station of Saint-Maur and Créteil needs be lowered to a depth of 50 m below the surface to prevent expanding soils like plastic clays (Fig.2). This design allows the station to be fully placed in the layer of chalk and enables the subsurface structures to achieve adequate coverage of high-quality ground that is also appropriate for consolidation treatment and sealing.
4.1.2 Estimating deformation
For sensitivity and vulnerability analyses, accurately estimating deformation of structures or assets is important so as to limit the damage risk to neighboring structures or assets [
2,
10–
12,
27–
28]. A wide range of factors may be concerned in estimating deformation, such as soil conditions and hydrogeology, excavation methods and tunnel geometry. A finite element model (FEM) is mostly used to estimate deformation, which allows a thorough examination of problems including complex loadings, nonlinearity of soils and structures, work phases, soil-structure interaction, and hydrogeological conditions. When estimating deformation, two-dimensional FEMs are frequently employed in conjunction with the convergence-confinement technique. Additionally, three-dimensional FEMs can be adopted using software such as ZSoil 3D and Plaxis 3D for applications where complex construction phases and three-dimensional effects need to be taken into account (Fig.3) [
12,
22]. Nevertheless, the computational time of three-dimensional FEMs can be long. In the example of the metro station of Saint-Denis Pleyel with ZSoil 3D (about 330,000 elements), it has taken about 20 h for the calculation.
The FEM analyses require properly defined hypotheses to have accurate results so as to reduce the damage risk to a manageable level. However, using a deterministic or even semi-probabilistic calculation to accurately estimate deformation is challenging due to several factors such as input data lack, soil variability, and calculation results sensitivity to assumptions. Therefore, by calculating the probability of failure, a probabilistic calculation is perhaps a better approach to account for these problems [
29–
31]. The probabilistic approach allows assessing the overall impact of the uncertainties related to the target performance or safety levels, together with the accompanying probabilities of failure or indices of reliability [
32–
35].
Furthermore, by using measurements to update a priori hypotheses, Bayesian back analysis can be used to improve estimates of probability of failure and a posteriori results [
25,
29]. Metamodels, also known as surrogate models, can be used in place of FE models to minimize the computing time using techniques like polynomial chaos kriging [
29,
36]. For example, 200 calculations of a two-dimensional probabilistic analysis (about 5000 elements) using polynomial chaos kriging techniques take about three hours. For large construction projects, this kind of analysis enables designers to optimize the project design and contractors to take into account alternative technical options. Ensuring a high-quality probabilistic method requires several conditions, including adequate and high-quality geotechnical surveys, suitable geotechnical analysis of soil characteristics, competent geotechnical engineering judgment, etc [
12].
4.2 Construction methods
4.2.1 Underground structures
In the GPE project, the stations and structures exhibit an average depth of approximately 30 m. They are constructed either in an open manner with a temporary retaining system comprising girders and metal struts or following the top-and-down method with descending construction of slabs. A hybrid solution is often used with beds of struts between the slabs, slab/raft or in the large openings of slabs (Fig.4(a)).
The main sequence of construction works established by the top-and-down method is as follows:
1) realization of diaphragm walls;
2) excavation of a few meters below the cover slab;
3) construction of the cover slab;
4) excavation of a few meters below the intermediate slabs;
5) construction of the intermediate slabs;
6) excavation to the bottom of the structure;
7) realization of waterproofing and pouring of the raft.
In regard to structures built in an open manner (bottom-and-up method), temporary supports are put in place during the construction of earthworks down to the raft, which is the first to be poured. Intermediate slabs are then constructed in ascending order before the girders and struts are removed.
In addition, specific construction methods such as shifting and underpinning were used. For example, a heavy cover slab (Line 15 South Lot T3B) was shifted by self-propelled modular transporter (SPMT) type multidirectional trailers of the Mammoet company at the Fort d’Issy Vanves Clamart station. The cover slab with its concrete protection weighs nearly 7,500 tonnes, 200 tonnes heavier than the Eiffel Tower [
37]. Five rows of SPMTs comprising a total of 240 axles were therefore used (Fig.4(b)).
4.2.2 Tunnels
TBMs are widely used for the construction of the tunnels in the GPE project. Geological circumstances and the contractor’s and manufacturer’s expertise and experience are key factors in TBM design [
38,
39]. In regard to excavation using TBMs, the Plan for Advance of Tunnel (PAT) has shown to be a useful tool. It outlines the key elements for monitoring TBM excavation (Fig.5), such as the hydrogeological longitudinal profile, the placement of singular-point along the profile, the expected surface settlement, the different pressures (water, target, stability, limit, and uplift), the vigilance zones, the calculation sections, the reinforced measurement sections, the retro analysis sections, and so on. In addition to acting as a forecasting tool, this document offers a clear, synthetic perspective of all pertinent data to help identify the pertinent sections for additional monitoring and retro analysis.
In addition, during the design process, an important geological risk was identified: high-pressure groundwater inflows that could be accompanied by fine soil particles. The hydrogeological profile indicates that the hydraulic load up to 30 m, even greater, might be applied along the tunnel axis. Therefore, adjustments to confinement pressures and mortar grouting pressures are required to guarantee the stability of the working face and to limit settlement in sensitive zones during the tunnel excavation with TBM. Overall, two types of TBMs were used [
40], namely earth-pressure-balanced TBMs and variable-density TBMs.
The variable-density confinement TBM is a hybrid TBM used to address sections with highly variable hydrogeology. It combines the advantages of the slurry TBM and the earth-pressure-balanced TBM [
41], providing both an air bubble for containment regulation and a screw conveyor to extract high-density cuttings from the cutting chamber (Fig.6).
Containment control is ensured by the following.
1) The air bubble at the rear of the excavation chamber, which allows very fine pressure control, is provided by the air regulation system. The target pressure can be specified to within 10 kPa, compared with an earth-pressure TBM allowing a precision of only 30 to 40 kPa.
2) The pressure of the hydraulic circuit through the material mixer box is located at the rear of the extraction screw. A crusher positioned just behind the auger can be used to calibrate the cuttings for evacuation and reduce the clog risk in the hydraulic mucking circuit.
The hydraulic circuit is designed to ensure a sufficient supply of water and fines for conditioning the ground at the working face as well as for hydraulic mucking. This also facilitates the prevention of confinement losses when passing through zones at risk of dissolution voids, not only because of the rheology of the slurry in the chamber but also because of its density. Bentonite forms a film (a cake) on the working face, thereby creating a waterproof fine membrane to apply a pressure gradient. Transfer between slurry and earth-pressure modes can be gradually achieved, under permanent control of the containment pressure and without intervention in the excavation chamber.
4.3 Monitoring and satellite radar interferometry
Damage from construction to neighboring structures can inhibit construction in progress and, in extreme cases, stop all work at a site. Because of this, monitoring is becoming a crucial, almost required, part of modern tunnelling and underground works, especially in urban contexts.
In general, monitoring has two main aims.
1) Aiding design by verifying whether the planned predictions match the real behavior and conditions of the ground recorded during construction.
2) Ensuring that the structures can provide the function for which they were designed.
Construction teams must use monitoring to identify deflection and subsidence early on. Timely feedback helps in adjusting operations or seeking design adjustments to mitigate such effects. Moreover, applying the observational method requires reliable field measures of monitoring. In recent years, there have been significant improvements in the quality of instrumentation. These new monitoring methods and data management systems can provide real-time monitoring and offer important new information for the observational method.
In the GPE project, in addition to conventional monitoring methods, such as automatic theodolites, inclinometers, extensometers, and topographic measurements [
10,
11], one innovative monitoring method is the interferometric synthetic aperture radar (InSAR) system [
42–
44]. InSAR technology is based on synthetic aperture radar (SAR) images. SAR satellites are equipped with active sensors that image the Earth’s surface day and night regardless of weather conditions. An electromagnetic wave is emitted toward the Earth’s surface and then reflected via permanent reflectors on the ground, such as infrastructure and buildings before backscattering toward the satellite sensor. The precise movement of the ground or structures can be inferred by measuring the backscattered signal’s return time following multiple satellite passes over the same region.
The GPE project is covered in all areas and phases by the InSAR mission, which is considered as an additional measure for identifying settlement phenomena over a large perimeter. It is designed to acquire a picture every 11 d and gives an inventory prior to the start of operations in an area matching to the zone of geotechnical influence. The InSAR mission also makes it possible to monitor the ground stability once work is finished [
42].
4.4 Digital approaches in design and construction
4.4.1 Building information modeling
Building information modeling (BIM) provides a digital representation of the physical and functional characteristics of infrastructure and buildings. More precisely, BIM involves the following:
1) a parametric digital model that contains structured data;
2) a collaborative working method;
3) The sharing of reliable information throughout the entire project lifecycle.
In the GPE project, BIM was frequently used as a method to optimize both design (Fig.7(a)) and construction (Fig.7(b)). For example, Line 16 is considered a pilot line for BIM development and application [
45]. Construction of this line allows BIM to cover the design phase of all of the works (infrastructure and buildings), promotes the harmonization of the rules for structuring models toward open BIM application and defines BIM processes during the construction phase. The BIM is systematically updated to ensure data synchronization throughout the design and construction stages, and a BIM charter is used for all contributors. At the service stage, the BIM will be updated if there are maintenance works.
In addition, Bimsync® was used as a collaborative platform to bring together all professions and enable all stakeholders to collaborate in a defined space on the platform. During the construction phase, Bimsync® allows all stakeholders to visualize the project to be built at an execution level of detail, and it also helps to settle any interface issues between all the builders and equipment manufacturers.
Furthermore, BIM could serve as a basis for the selection of proper management tools throughout the whole lifecycle of the project. To ensure suitability as a basis for digital twin establishment, the developed BIM models must follow a certain standard or schema, which sets the form of the information to be utilized for operation and maintenance.
4.4.2 Digital twin
The digital twin is defined as a digital replica of physical assets, processes and systems that can be used for various purposes. It comprises a digital representation with synchronization at an appropriate rate between the physical and digital entities. Consequently, the digital twin is defined in at least four dimensions (volume and evolution over time), thereby allowing simulation and anticipation.
In the GPE project, the digital twin is a virtual representation of a given built infrastructure that spans its lifecycle. The digital entity is systematically updated when the physical entity changes, each modification in the physical entity is integrated in the digital entity. It can use simulation and reasoning to help decision-making during operation and maintenance. Moreover, it can be employed to monitor the low-carbon performance and energy efficiency over time throughout the lifecycle of the infrastructure [
46].
The essence is the connection and synchronization between the real-world data of a physical structure and the information contained in its digital twin. Digital continuity and data traceability are therefore important factors. Once the physical structures have been built, its digital twin facilitates fine monitoring in real-time and therefore a better understanding of its behavior and uses. By managing and analyzing these measurements, it is possible to evaluate the infrastructure safety, forecast the potential damage and make timely maintenance decisions.
5 Use of new and innovative technologies
To overcome the technical, material, human and even environmental challenges of the GPE project, the SGP encouraged and integrated innovation at all levels of the project. Due to its scale, the GPE project also provided good opportunities to deploy new and innovative technologies: new construction methods, low-carbon materials, new services in the metro, etc. Through innovation, the GPE project is asserting itself as a project of ecological transition with sustainable design and construction characteristics.
5.1 Vertical shaft sinking machines
The vertical shaft sinking machine (VSM) was originally developed by Herrenknecht for mechanized construction of deep launch and reception shafts for microtunnelling. It has also been verified as an efficient solution for the safe and fast realization of shafts, especially in difficult inner-city environments, without lowering the groundwater table. The Herrenknecht VSM comprises two main components: the excavation unit and the lower unit. There is also a slurry discharge system to remove the excavated soil. All machine functions are remotely controlled without the need to view the shaft bottom or the machine [
47].
On Line 15 South Lot T3C, a Herrenknecht VSM12000 was used to construct four emergency and ventilation shafts (Fig.8). These shafts are up to 48 m deep with inner diameters between 8.3 and 11.9 m. Thus far, the corresponding value of 12.8 m is the largest outer diameter ever installed by VSM technology with prefabricated concrete segments. The VSM was applied below groundwater with a hydrostatic pressure of up to 10 bar and in heterogeneous soil and hard limestone with a compressive strength up to 100 MPa, and the peak performance reached 2.4 m/d.
5.2 Steel fiber-reinforced concrete
An innovative technique in France is tested on a large-scale in Line 16 of the GPE project: the widespread application of tunnel segments made of steel fiber-reinforced concrete (SFRC) [
49,
50].
For contractor Eiffage, the key points to ensure the success of large-scale SFRC use are as follows [
49].
1) An appropriate collaboration with Bonna Sabla and Bekaert to select the appropriate steel fibers and manufacture the precast segments in a plant.
2) The establishing of the basis of design (FIB Bulletin 83 plus Model Code 2010), as well as the class, formulation, and fiber dosage of the suitable fiber concrete and their validation.
3) The manufacturing procedure, which is accompanied by stringent quality control and customized facilities.
4) A comprehensive testing campaign comprising the design and characterization test, fire test, full-scale test (Fig.9), bending test, edge thrust segment test, pull-out test and suitability test to evaluate the precast segments.
Owing to the control of these important points, SFRC tunnel segments have been put successfully in the Line 16 tunnels, which resulted in improved performance, sustainability, and additional environmental advantages (average steel fiber of 40 kg/m3 as opposed to an average reinforcement level of 80 kg/m3).
5.3 Ultralow-carbon concrete
In the GPE project, the use of ultralow-carbon concrete for tunnel linings in metro sections was another innovation in France, even worldwide. These tunnel segments were constructed from Vinci Construction’s Exegy® Ultra Bas Carbone (UBC) with ECOCEM Ultra binder (Fig.10). Replacing conventional concrete with alkali-activated ECOCEM Ultra binder reduced worksite carbon emissions by more than 50% relative to the traditional concrete used for tunnel linings. The figures are 90 kg CO
2/m
3 for UBC concrete, 170 kg CO
2/m
3 for very-low-carbon concrete and 330 kg CO
2/m
3 for traditional concrete [
51].
Implementing definitive precast segments in ultralow-carbon concrete therefore provides an opportunity to conduct a full-scale reference study of the application of a new and more environmentally friendly construction material in tunnels. A European technical assessment study was launched in 2019 to examine the alternative alkali-activated ECOCEM Ultra binder, and its approval in October 2021 indicates that concrete formulas incorporating this binder now comply with standard NF EN 206/CN (Concrete-Specification, Performance, Production and Conformity). This approval makes it possible to generalize Exegy® UBC solutions based on the ECOCEM Ultra binder for multiple uses, thereby significantly reducing the carbon footprint of poured concrete.
6 Environmental protection
Over the last decade, the environment has come to occupy an important position in French regulations. Revised in 2021, standard NF EN 15643 [
53] relating to the contribution of construction works to development provides a methodological framework for assessing environmental, social and economic performance at the specific scales of civil engineering works and buildings [
46].
Accordingly, within the context of calls for tenders for public infrastructure work contracts, the SGP aimed to improve the way environmental issues are accounted for in its contracts. With this goal in mind, it intends to introduce new criteria for evaluating bids based on objective assessment of environmental performance and protection [
54].
6.1 Managing the excavated materials
The GPE project produces approximately 45 million tonnes of excavated materials [
55]. The management strategy comprises three main features: traceability of the excavated materials, optimization of their transport to the destination, and recycling and recovering of the excavated materials.
To be more specific, the aforementioned approach is expressed in terms of the subsequent parameters.
1) Enter the excavated materials into the T-Rex program, making sure to include daily weight data input for transportation, production sites, and excavation destinations (Fig.11).
2) Employ a novel quick characterization technique [
56] to characterize the excavated materials such as IWSF, NHWSF, HWSF and so on in order to identify the categorization type.
3) Verify the property of the extracted materials and the treatment techniques.
4) Recycle as much as 70% of the volume of the excavated materials.
5) Encourage the use of rail and river barges as alternatives to automobile transportation in order to reduce the environmental impacts.
6.2 Integrating thermoactive metro stations
The SGP is highly concerned regarding the need to develop and use renewable energy in the GPE project, including geothermal energy generated by thermoactive metro stations that could be used for heating and cooling buildings or urban infrastructures [
57,
58]. The ability to use underground geothermal energy is related to both its energy potential and to the capacity to integrate it into the global energy system.
Surface geothermal energy makes it possible to capture the subsoil geothermal energy between 0 and 200 m deep to heat or cool buildings according to seasonal needs. For example, the thermoactive foundations of the Porte de Clichy and Mairie de Saint-Ouen stations on Line 14 transmit heat from or to the ground to regulate the inside temperature. This geothermal capture can reduce CO2 emissions by 50%.
7 Concluding remarks
The need for technology in construction is driven by the industry need for increased productivity, cost-effectiveness and sustainability. This paper presents a review of the major new and innovative technologies adopted in the GPE, the biggest transport project under construction in Europe.
Its scale, location and schedule make the GPE project a once-in-a-century megaproject, so the RMP is applied throughout the whole project to address its high risks. In regard to its design and construction, it also benefits from the application of intensive construction technology.
Additionally, new monitoring and data management systems can provide important new information for the observational method. In addition to traditional design and construction methods, digital tools such as BIM can help optimize design, construction and project management. They provide a collaborative working method and disseminate reliable information relating to the entire project lifecycle.
At the time of writing, the GPE project is moving forward nicely. Several new and innovative construction technologies, such as VSMs, SFRC and ultralow-carbon concrete, have been successfully employed. Owing to the resources mobilized and tailored solutions, the tight project timeline is currently being followed.
In future development, environmental protection will become increasingly important. Effectively applying intensive construction technology and ensuring efficient collective work between the owner, designer and contractor will be the key to success for this complex megaproject.
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