ABSTRACT
When high speed trains travel close to the wave propagation velocity of the supporting track-ground system, large amplitude track vibrations are generated, known as the ‘critical velocity’ effect—a concern on high speed lines across the world. Geogrids offer increased confinement and stability for railway subgrade materials thus increasing track performance; however benefits at critical speeds are uncertain. At critical speed, railway lines are subject to increases in both vertical and horizontal stress levels, thus causing rapid track deterioration. The ability of geogrids to reduce this deterioration is challenging to examine because testing requires that subgrade-geogrid samples are subject to rapid principal stress rotation.
A cutting edge ‘true triaxial’; testing facility (GeoTT) has been developed at Heriot Watt University with 6 independent hydraulic rams that can subject a test sample (i.e. railway subgrade) to forces in all 6 directions, mimicking the principal stress rotation. The 6 rams are programmed based upon force-time histories generated by 3D finite-element models. The material properties of track materials with and without geogrid can be tested under realistic conditions for a fraction of the time and cost required for full-scale testing.
Tests have been undertaken to demonstrate the possible benefits that might be achieved using stabilised trackbed in high speed rail scenarios over a soft clay layer (using forces generated by the GeoTT rams and deformation monitored over 500 k cycles). It is found that significant benefit can be achieved by using a hexagonal structure multi-axial geogrid with triangular apertures to stabilise the trackbed and reduce lateral and vertical movement at the sleeper level.
