Design infrastructure according to the natural soil

Science topics November 2018 TransportInnovationEnergy

Did you know that the ground beneath a railway line has to be carefully chosen and assessed before the track is built? To guarantee the safety of the people and goods carried, its behaviour must also be predicted, monitored and maintained over time. To do this, geotechnical engineers study, among other things, the interaction between the soil and structures. In this context IFSTTAR is developing and perfecting innovative techniques and tools to enable new rail lines to be laid in what are sometimes  challenging environment

Adapting and shaping the environment to accommodate rail infrastructure

In an environment where not enough space is available, it is occasionally necessary to design a specific geotechnical structure.Retaining wall next to the track (credit SNCF)

For example, a retaining wall is being built here to allow two railway tracks to be laid side by side. The innovative "Reinforced Earth"© process is being used. This consists of inserting metal strips into a soil mass. These strips are positioned horizontally and fixed to the reinforced concrete scales that make up the facing. A mainly granular material is then spread on these strips and compacted to obtain a dense layer that will withstand the shear caused by dynamic loads in particular. The operation is repeated until a strong retaining wall is obtained. Using sensors, IFSTTAR will check that it continues to perform properly in the long term.

In some situations, the environment itself must be shaped to accommodate these infrastructures. This is the challenge posed by the tricky task of installing a railway track in an environment that is constantly moving, like sand dunes in a desert.

In the Arabian peninsula, under the action of a prevailing wind, a sand dune is formed into a crescent shape. We know from experience that the higher the dune is, the less it moves.

To build a railway line here, it is necessary to connect the existing dunes together. In the same way would nature would do it, sand fill must be designed with an aerodynamic profile that helps prevent sand grains from being blown away. In addition, semi-permeable screens can be installed on the crest to trap and fix the wind-blown sand grains as far as possible.

 

Aerodynamic profile of a sand embankment - credit Ifsttar

 

 

 

Maintaining and preserving geotechnical structures

Like bridges and tunnels, geotechnical railway structures have a certain lifespan and require regular maintenance. Also, occasionally, the foundations suffer from fatigue and settlement occurs under repeated loads (train passages) and climatic factors (rain, drought, frost).

The "Soil Mixing" technique provides an effective way of slowing down the settlement of the rail track in such cases. It consists of using an auger to incorporate a cement slurry into the upper part of the foundation. It mixes with the soil to form vertical inclusions, which can transfer loads to stronger soil below.

During an exceptional flood event, currents can weaken the bridges that cross a river. The most violent of these remove the soil from around the foundations of the piers that support the bridge. This is known as foundation scour, and it is the most frequent cause of failure in this type of structure.

In the framework of the French Research Agency (ANR) project SSHEAR (Soils, Structures and Hydraulics: Expertise and Applied Research)  in partnership notably with CEREMA and SNCF Réseau, IFSTTAR is designing and testing devices which monitor changes in the height of the riverbed, to improve our understanding of the phenomena involved and to develop warning systems to inform managers in the event of excessive scour.

 

 

 

 

 


To find out more...

Chevalier, C., Larrarte, F., Schmidt, F., Pham-Van-Bang, D., Durand, E., Gondret, P., de la Roque, S., Cheetham, M. & Hosseingholian, M. (2017) Understanding and control of scour phenomena: recent advances. GEORAIL International symposium, Nov. 23-24, Champs-sur-Marne, France, 331-339

Restitution de l’Opération de Recherche (2013-2017) «TERRANOVA » ou  Les Terrassements Novateurs