Revolutions at the centrifuge

The Geomaterials and Geotechnical Models Laboratory (Laboratoire Géomatériaux et Modèles Géotechniques) is going through a historical moment: the Geotechnical Centrifuge, one of IFSTTAR’s remarkable facilities, has just passed the milestone of 10,000 hours of operation, which corresponds to an astronomical number of revolutions!

The centrifuge is versatile and capable of performing many tasks in the framework of geotechnical research. It performs experiments on scale models of geotechnical structures (foundations, supporting walls, tunnels and fills etc.). The macro-gravity generated when it rotates (up to 200xg) enables it to reproduce the stress field in the full-scale structure in the scale model and thus reproduce its behaviour, satisfying the similarity rules originally stated by Edouard Phillips [1] in 1869.

Ten thousand hours of rotation represents 6 years and 3 months of continuous activity, on the basis of 1607 hours of work per year. This has been spread over 32 years, starting in 1985. It is equivalent to 52 minutes of rotation per calendar day, which in turn corresponds, at 100×g, to the consolidation time of a clayey soil during approximately one year. These figures can seem dizzying… but we should bear in mind the Russian proverb that in order to live to a hundred you have to start young!

The centrifuge is electrically powered, and during its entire period of activity has consumed the equivalent of 4 hours of production of a marine windfarm of the size of those planned off the French coast.

Technically, the team responsible for operating and improving the device (currently a permanent staff of 3 researchers and 4 technicians) has modernised the equipment (4-axis robot and a centrifuge-mounted seismic simulator) in order to satisfy new societal demands such as the energy transition and construction in seismic zones, and conduct increasingly complex tests. Today the centrifuge has a dual nature: it operates with robust technology from the 1980s i.e. analogue electronics, drive line with a clutch and eddy current braking, but it has also experienced the digital revolution, as its on-board equipment – jacks, earthquake simulator etc. – is almost all digitally controlled.

So much progress has been made, mainly by following a virtuous circle, since the facility was commissioned in 1985 by the Minister with responsibility for research Professor Hubert Curien. This includes hundreds of instrumented tests, contributions to several French standards and recommendations, managing and participating in 6 European projects, dozens of national, regional and industrial projects, intercontinental collaboration, the organisation of national, European and international congresses, the defence of dozens of doctoral theses, more than a hundred publications, and several awards for excellence.

Graph showing the cumulative number of hours of rotation against the dates of some key events: the commissioning of the centrifuge, the installation of the 4-axis robot and the centrifuge-mounted seismic simulator.

When it passed the 10,000 hour landmark, the centrifuge was revolving for a doctoral student, Léonardo, who had come from a famous Roman university to carry out tests on unsaturated soils. His interest in scale models is in the tradition of a great Italian scientist who investigated the mechanical behaviour of small objects in his “Discourses and Mathematical Demonstrations Relating to Two New Sciences”: Galileo Galilei [2]. After many difficulties (as can occur to a lesser extent to those working on centrifuges) was he not to state,

in the service of research and knowledge, E pur si muove ! “And yet it moves”.

In the current year (2017), our tests are strongly focussed on marine geotechnical engineering. This includes the design of suction caissons for oil platforms with SBM off-shore, marine slope stability in seismic zones with the Federal University of Rio de Janeiro and Petrobras, single pile foundations for fixed wind turbines with the ANR and France Energies Marines, truss pile foundations with the Pays de la Loire region, and the anchors for floating wind turbines with West Atlantic Marine Energy Centre (Weamec). Other areas of research include seismic soil liquefaction (with a number of American universities) and ore liquefaction during maritime transport (MARTEC ERA-NET project).

[1] Phillips E. (1869). De L'Equilibre Des Solides Elastiques Semblables. Comptes Rendus Hebdomadaires des Séances de L'Académie Des Sciences, vol.68, série 2, pp.75-79.
[2] Galileo G. (1638). Discorsi e Dimonstrazioni matematiche intorno a due nuoue scienze attenanti alla mecanica ed i movimenti locali. Traduit en français aux PUF, 1995

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