Temporary rubble storage cells for the construction of the Toulouse Metro | France | 2025

Construction of Line C of the Toulouse Metro began in 2023. Stretching over 27 km and comprising 21 stations, this new line is expected to carry around 200,000 passengers per day from 2028, notably connecting the aeronautics hub, Matabiau railway station, and the south‑eastern part of the metropolitan area. With 80% of the project built underground, construction generates significant volumes of excavated material, up to 90% of which is recovered thanks to temporary storage cells made of reusable steel sheet piles. Located in a dense urban environment and in the immediate vicinity of existing railway infrastructure, these structures required technically demanding retaining solutions with strictly controlled nuisances.

Tisséo Ingénierie entrusted the Eiffage‑NGE consortium with the design and construction of two tunnels (3.85 km for Package 1 and 8.4 km for Package 2). In line with environmental best practices, spoil storage cells were designed for the temporary storage of millions of tonnes of material excavated by tunnel boring machines, while limiting impacts on the surrounding environment. Initially envisaged as permanent secant pile structures, the cells were ultimately built using a reversible solution based on EcoSheetPile steel sheet piles, better suited to site constraints and reuse objectives, with a reuse rate of 80 to 90%. This solution also helps reduce transport flows and optimize site logistics.

The cells were constructed using 1,720 t of hot‑rolled steel sheet piles, PU 22 profiles, in lengths ranging from 11.6 to 14.0 m. The solution offers numerous advantages: rapid installation, reversibility, watertightness, and reduced nuisance. Steel sheet piles form a continuous and homogeneous retaining wall, both above and below the natural ground level, while ensuring good geometric control of the structure.

To define the optimal profile, installation trials were carried out with three PU profiles. These trials demonstrated that the geometry and stiffness of the PU 22 profile were best suited to the encountered ground conditions, characterized by Toulouse molasse. The tests also highlighted the need for pre‑drilling using an 820 mm diameter auger, with 10 cm overlap, carried out to at least 50 cm above the theoretical final embedment depth to ensure toe restraint of the wall. In this context, the molasse exhibits limit pressures exceeding 4.5 MPa, with maximum values reaching 5.7 MPa.

Profiles with a width of 600 mm were selected to provide greater flexibility and modularity in layout, particularly given the complex geometry of the sub‑cells and their interconnections. The use of a high‑frequency, variable‑amplitude PTC vibrator was also validated during the trials, enabling compliance with the 3 mm/s vibration threshold imposed by SNCF in the vicinity of railway tracks. Continuous monitoring of vibration amplitude allows precise adjustment of the energy transmitted to the ground, with vibration‑free start‑up and shutdown phases, helping to limit nuisances, improve installation accuracy, and ensure the verticality of the sheet piles.

Manufactured in Luxembourg from 100% recycled steel using an electric arc furnace, EcoSheetPile steel sheet piles have a carbon footprint of less than 500 kg CO₂e per tonne—nearly an 80% reduction compared with blast furnace steel. Reusable up to ten times depending on conditions and fully recyclable, they will be extracted at the end of the project, allowing the site to be redeveloped as a residential area. This reversibility makes steel sheet piles a particularly suitable solution for large temporary urban projects, combining technical performance, environmental requirements, and adaptability of future land use.

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