Rail Trackbed and Performance Testing of Stabilised Sub-ballast in Normal and High-speed Environments

Abstract The ability of geogrids to preserve track alignment within a high-speed rail environment at close to critical velocity is somewhat uncertain; testing in a controlled environment can be problematic. This paper presents the results from a new ‘true triaxial’ test apparatus that overcomes some of these problems. In ‘normal’-speed rail environments, geogrids have been used for many years to stabilise and enhance the performance of sub-ballast to maintain both vertical and horizontal alignment and increase the interval between maintenance events. This has been reflected in controlled testing conducted in both the laboratory, in the field and under heavy loading. To look at this issue for high-speed rail and to make comparisons between track alignment preservation in normal and high-speed environments, a new ‘true triaxial’ test apparatus (GeoTT) has been developed at Heriot Watt University that can subject railway sub-ballast to forces in all 6 directions, mimicking the principle stress rotation that has been implicated in track alignment deterioration subjected to high speed train traffic. The use of this apparatus, where the rams are programmed using force-time histories developed from 3D finite element models, allows sub-ballast performance to be evaluated for the fraction of the time and cost that would be necessary for full scale testing. A comparison is made between existing testing results from ‘normal-speed’ testing and the new high speed simulations that indicate the continued potential for geogrids to continue to aid track performance in much more critical environments.