Centrifuge modelling of railway embankments under static and cyclic loading

Abstract

The study comprised the modelling of embankments on a heavy haul coal export railway line between Ermelo and Richards Bay in South Africa. The design of these embankments is usually done with slope stability analysis? method of slices. In the case of railway embankments, a uniformly distributed load is applied over a central ballast width of the embankment to represent the load induced by the train. This however does not take into account the cyclic loading aspect the embankment is subjected to. Physical modelling in a centrifuge is one of the most effective tools available to a geotechnical engineer. Using the typical designed slopes currently used on the coal export line, the failure of these embankments can be modelled in a centrifuge to better understand the mechanism of failure. The main objective of the study was to investigate the influence of static and cyclic loading on railway embankments by testing models in the geotechnical centrifuge. This was done to determine the failure mechanisms and the difference in settlement between static and cyclic loading. The effect of moisture on the slope stability was also investigated. To achieve these objectives, a suitable loading system was developed that could be used for static and cyclic loading within the centrifuge. Suitable material was sourced that could be used in the centrifuge testing of embankment failures and a procedure was developed to build, compact and instrument a model of an embankment within the centrifuge. Embankment models representing a standard slope and one material type with varying moisture contents were built and tested in a geotechnical centrifuge. Clayey sand material used for the tests was sampled from one of the sites where a failure occurred on the heavy haul coal export railway line. The loading system simulated the loading created by a heavy haul coal train with a 26 t per axle load along the length of an embankment. A loading block was machined to simulate the load and a pneumatic piston was used to cycle the load on and off the embankment. The results from the centrifuge tests were finally compared to slope stability analyses done with commercial slope stability software. The results from the tests conducted in the geotechnical centrifuge indicated that cyclic loading significantly increases the magnitude of the vertical settlement which could lead to slope failures. It was observed that the moisture content had a significant effect on the slope stability. A completed slip surface failure was not observed, however significant cracks formed at the crest of the embankment along its length which could induce a failure once water from further precipitation enters those cracks. The research therefore concluded that cyclic loading on railway embankments has a significant effect on the permanent vertical settlement of the embankment compared to static loading, resulting in the development of cracks and the subsequent increase in moisture content. In the interest of safety and good practise, this should be incorporated into embankment design and slope stability analyses.