Centrifuge modelling of train passage over clay subgrades

The modelling of large geotechnical structures on the centrifuge is now well established, but there remain technical challenges with each new application. This work arose from a larger project investigating alternative improved embankment designs. The project required full scale modelling of new embankment designs using the GRAFT II simulator at Heriot Watt University together with consideration of the large scale settlement problem over a wider area. To model foundation soil beneath the embankment required the design of a rail simulator at centrifuge scale, together with appropriate measurement techniques and appropriate modelling of useful foundation soil types. This paper therefore describes the design decisions taken to produce a centrifuge mounted rail simulator including details of modelling a translating cycling load. To measure total stress at the base of the embankment during a train passage was a key challenge faced, overcome by use of thin Singletact stress transducers. These transducers are low deflection capacitance-based force sensors that will be further discussed in the paper. In addition to the translating load and the instrumentation, the third aspect presented will be the formation and characterization of a soft clay subgrade, required due to the nature of soils that are most of concern to UK rail industry. To accelerate consolidation without compromising the ability to achieve a clay-type mechanical response, a hybrid clay-sand-silt mixture known as KSS 541 is installed in the model container as slurry and consolidated in-flight. Experience with this material is discussed.