Keywords: ballast, geosynthetics, impact forces, rail track, deformations, subsurface drainage.

Railway systems form the largest worldwide network, catering for quick and safe public and freight transportation. Rail ballast forms both the load bearing stratum and the drainage layer for the track. A proper understanding of load transfer mechanisms and their effects on track deformations are essential prerequisites for minimising maintenance costs [1,2]. The measurement of vertical displacements is an established practice in most conventional track monitoring systems. However, it is also important to monitor lateral deformations that may affect track stability arising from the loss of lateral confinement. Therefore, a field trial was conducted on a track section near the City of Wollongong, Australia, to measure the deformations and cyclic stresses. The field trial further showed that the moderately-graded recycled ballast, when used with a geocomposite (combination of biaxial geogrid and nonwoven polypropylene geotextile), could perform well in comparison with traditionally uniform fresh ballast [2]. A geocomposite, when placed below the recycled ballast layer, would function as reinforcement, drainage and separation, thereby reducing the vertical and lateral deformations.

It was demonstrated that, in the case of trains with wheel flats, extremely high stresses were transmitted to the ballast layer. Installing resilient mats, such as rubber pads (shock mats) in rail tracks, can attenuate impact forces and consequently mitigate particle degradation. In view of this, a series of laboratory tests were carried out using unique large-scale drop-weight (impact) equipment to evaluate the effectiveness of shock mats in the attenuation of high frequency impact loads, and subsequent mitigation of ballast deformations and degradation.

Stabilization of soft foundation by using prefabricated vertical drains (PVDs) is also essential for improving the overall stability of track, and to reduce the differential settlement during track operation. A rail track built on up to 30m of thick soft estuarine soil at the town of Sandgate, Australia, was stabilized with short PVDs to consolidate the soil just beneath the track. Both Class A predictions and field measurements proved that relatively short vertical drains would be sufficient to dissipate cyclically induced pore pressures, curtail the lateral movements, and increase the shear strength and bearing capacity of the subgrade.

This invited Special Paper presents the results of laboratory testing, full-scale field monitoring, theoretical modelling, and finite element analyses, which demonstrates the beneficial use of geosynthetic grids, shock mats and drains for rail infrastructure. The results highlight that particle breakage, confining pressure, soft formation: in addition to train loading patterns (cyclic and impact), have a significant influence on the engineering behaviour of ballasted rail track.

[1]

E.T. Selig, J.M. Waters, "Track Geotechnology and Substructure Management", Thomas Telford, UK, 1994.

[2]

B. Indraratna, W. Salim, C. Rujikiatkamjorn, "Advanced Rail Geotechnology - Ballasted Track", CRC Press/Balkema, 2011.

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