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Sustainable Construction of Underground Transport Infrastructures

The SCOUT project develops a new environmentally friendly concept of ‘cut-and-cover’ tunnels, based on a holistic approach that combines the generalisation of the observational method, optimisation of design, breakthrough construction equipment and new applications of composite materials.

Tags: Multimodal


The development of the Trans-European Transport Network requires the construction of many new railways, highways and waterborne connections. In urban centres, burying transport infrastructures underground is the best option to avoid congestion, noise impact at the surface and, in many projects, is the only possible choice to build intermodal connections linking underground stations and airports, parking lots, pedestrian access, etc.

For such underground infrastructure, the cut-and-cover method is a cost-effective alternative to tunnels, and the best option when the tunnel is relatively shallow (maximum depth < 20 m) and the surface is free from buildings. The method is very safe, does not create subsidence, and the phasing of works allows surface traffic to be restored at a relatively early stage, when the cover slab is completed. It has a huge number of references over the world for the construction of underground roads, subways, railway lines and stations.

Cut-and-cover tunnels are therefore a vital tool for the construction of transport infrastructures which are needed for the development of the TEN-T Network. But this method is now confronted with a new set of requirements: the urgent need for safer and more cost effective techniques, and a reduction of environmental impact from the construction (use of natural resources, noise, disruption of traffic, etc).


The main objective of the project is to develop a new concept for sustainable construction of ‘cut-and-cover’ tunnels that optimises the safety and life-cycle cost of the construction, and eliminates or drastically reduces most nuisances to urban environment, which are classically associated with construction projects: noise, dust and large construction equipment causing long traffic disruption at the surface. The project concentrates on the construction of tunnel walls and, using a holistic approach, addresses the three complementary domains of construction materials, design and construction process.

The first target is to develop new composite materials, namely fibre-reinforced concrete optimised for the construction of tunnel walls. The second target is to optimise the design of the structure, firstly by using these new materials and secondly by a systematic implementation of the observational method. The third target is to develop a new and breakthrough concept of construction equipment – modular, suitable for most European soil profiles – with the capacity to install this tunnel structure with minimum environmental impact. A complementary study addresses the subject of recycling excavated materials.

The final objective is to validate the concept by the construction of a prototype of limited depth (8 metres) to be tested in real conditions.

General view of a cut and cover project in Toulouse, France
General view of a cut and cover project in Toulouse, France

Description of work

  1. New methods for design

    Starting from the analysis of a selection of reference cases, the project will develop the following:

    • new methods to optimise the engineering of cut-and-cover projects, using the concept of a ‘double-skin’ structure and aiming at reducing the cost of construction materials by 15%
    • a methodology and relevant documentation to apply the observational method to cut-and-cover tunnels on a wide scale, with the objective of eliminating the extra delays and increased costs classically related to soil heterogeneity.
  2. A breakthrough construction method

    A radically new concept for the construction of tunnel walls will be developed, where the drilling process is continuous and horizontal, with the objectives of minimal environmental impact and maximum workers’ safety. Modular architecture of the equipment will allow the adaptation of a large variety of tunnel projects in a number of different European soil types. The environmental advantages will be minimal noise and dust, and no drilling through mud. The concrete structure will be cast in situ and equivalent in quality to that of superstructures.

  3. New composite materials, material recycling

    The project will investigate how fibre-reinforced concrete materials can be used in both temporary and permanent structure members to optimise structural design. New materials are being developed and will be tested at full scale.

    The recycling of excavated soil is being analysed (soil conditions, plant equipment, quality control requirements, costs), so as to identify the current blockages and to establish a roadmap towards the implementation of soil waste recycling.


The results of the project will be:

  1. New practical tools for the deployment and integration of the observational method in the construction process as requested by the EUROCODE, with the objective of providing full control of construction safety, costs and delays.
  2. New design tools for the optimisation of structural design, with the objective of 15% savings on cost of construction materials and minimal environmental impact.
  3. New breakthrough construction equipment suitable for cut-and-cover sites of all sizes, characterised by:
    1. innovative equipment of modular architecture providing a high level of flexibility to cope with a variety of infrastructure configurations and soil profiles
    2. new type of drilling bit, specially designed for this equipment
    3. a prototype of limited depth (8 metres), validated in real conditions
    4. new fibre concrete composite materials for cut-and-cover tunnels, validated by full-size structural tests
    5. a roadmap towards material recycling in cut-and-cover tunnels.