3.Materials, Structures and Tunnel Durability
The Materials, Structures and Tunnel Durability Department carries out activities related to:
- the knowledge of materials
- the design of underground structures
- the monitoring and repair of underground structures.
The department is involved in engineering projects, research projects, the drafting of professional guidelines, training, standardisation and coordination or participation in professional associations and networks. Its activities address issues related to tunnel safety and the durability of structures.
The Materials, Structures and Tunnel Durability Department is involved in various stages of an underground project:
- new constructions or refurbishment works: definition, design and implementation studies;
At each stage, the department can assist the project owner or prime contractor and can also carry out expert appraisals.
Knowledge of materials
The materials used in tunnel construction can be divided into different categories:
- structural elements: reinforced/unreinforced concrete, cast concrete, shotcrete, fibre-reinforced concrete, mortar, reinforcement, welded mesh, anchor bars, etc.
- civil engineering facilities: sewerage and drainage networks, cable ducts, roadways, pavements, etc.
- waterproofing: geomembranes, segment joints, resins, coatings, etc.
- operation elements: passive fire protection, aesthetic or acoustic protection such as cladding.
To ensure the safety and proper functioning of the tunnel, these materials must meet numerous requirements imposed by regulations and best practices, including standards and guidelines. The department participates in the development of these guidelines and regulations and is committed to meeting these challenges in order to guarantee short-, medium- and long-term performance.
The Materials, Structures and Tunnel Durability Department also supports the profession in the selection of new materials and the implementation of innovative products resulting from research projects in which it participates.
Structural Design
CETU is involved in structural design calculations at various stages of a project (definition, design and implementation studies), for both normal exposure conditions and fire exposure conditions.
Design for normal exposure conditions
For new structures, the Materials, Structures and Tunnel Durability Department is involved in the design of:
- cladding (sprayed concrete, formwork or prefabricated segments);
- secondary structural elements (such as slabs and ventilation duct partitions);
- external structures (such as add-on masonry portals)
It also works on existing structures, which require re-design during diagnostics or renovation work.
Segments from the Saint-Martin-la-Porte descent tunnel construction site (TELT)
Design for fire exposure conditions
Structural design for fire events, for both new tunnels and when inspecting tunnels already in service, addresses the following issues:
- the resistance of the tunnel under high temperatures;
- the likelihood of concrete spalling.
The expected requirements are based on regulatory texts.
The Materials, Structures and Tunnel Durability Department works on the fire resistance design of structures, based on a theoretical calculation estimating the structure’s resistance time, integrating, where applicable, a thickness likely to be damaged and which is assessed by spalling resistance tests.
If it is impossible to meet the regulatory requirements, the application of a passive protection is an alternative solution. However, this has the major disadvantage of concealing the structure during future inspections, which may mask structural defects and make the management of the tunnel more difficult.
Monitoring and repair of tunnel structures
The Materials, Structures and Tunnel Durability Department is regularly called upon to assist project owners in defining their monitoring strategy. In particular, it assists cross-border tunnel managers and also acts on behalf of the French delegation to various safety committees.
With the forthcoming expiry of several motorway concession contracts, the department is helping to define the expected tunnel condition at the end of the concession and to develop the associated assessment methodology.
With regard to the monitoring of underground structures, the department’s inspection team carries out initial and periodic comprehensive inspections of tunnels on the State-managed road network and, on an ad hoc basis, tunnels belonging to third parties. At the same time, the department develops guidelines on tunnel inspection practices and identifying deteriorations.
The presence and monitoring of problems may trigger additional investigations aimed at establishing a thorough diagnosis prior to a renovation project.
Various innovative solutions are being developed to facilitate the collection of data on the condition of structural elements and limit the associated tunnel closure times. Examples include high-performance image acquisition tools used for inspection. The department is actively involved in developing technical standards that outline the best practices for using these innovative solutions.
In the field of repair works, the Materials, Structures and Tunnel Durability Department is involved in all stages of a renovation project, from preliminary studies to the control and monitoring of works, in collaboration with the Construction Methods, Contracts and Works department. While the scope and type of repair projects handled by the department vary (structural reinforcement, stabilisation of rock structures, repair of damaged concrete, etc.), many cases involve the treatment of water ingress through sealing or drainage systems, at particular locations in the tunnel or across the entire structure. The department also supports the development of innovative processes in this area.
Sustainable Development
For new tunnels or repair work in existing tunnels, the environmental issues directly related to the department’s activities are:
- limiting the environmental impact of construction materials, in particular by promoting the use of low-carbon options;
- recycling excavated materials, for example by reusing them in the manufacturing of aggregates for concrete on major tunnel projects, such as the Lyon-Turin railway line.
On these issues, Materials, Structures and Tunnel Durability Department works in collaboration with CETU’s division on Transition in Practices and Sustainable Development.
Contact us
Contact : (+33) (0)4 72 14 34 50
Courriel : Secretariat-Gc.Cetu@developpement-durable.gouv.fr