The Application of Micro-Mechanical Research on Coarse Grained Soils to Create an "Avatar" Railway Ballast

The engineering analysis of geotechnical structures such as building foundations, tunnels or embankments, is usually made assuming that the soil acts as a continuous material, as if it were, for example, a metal. The tests that we carry out to characterise soil therefore utilise samples that have a representative number of particles, often millions, within them. The problem with this approach is that the soil behaviour is extremely complex, much more so than most other materials, resulting from the soil actually consisting of discrete particles with water filled voids between, and also the fact that these particles have a natural rather than man made origin, which can affect their shape, strength and arrangement. The extreme complexity of the equations we might use to characterise soil then means that our research understanding of soil behaviour is rarely fully implemented in engineering design.

In recent years increased computing power has led to the increasing use of discrete approaches in the analysis of soils, in which individual soil particles are modelled. In such an analysis it is important to model accurately the key characteristics of the particles that will influence the overall behaviour of the soil mass; their shapes, strengths, the arrangement that they have relative to each other and how they behave when they touch. Advances have been made in all of these areas, with the exception of the particle contact behaviour, which in recent years has been the special interest of Professor Coop, who has developed a new apparatus with which to investigate this aspect.

Having understood the contact behaviour, we will be in a good position to create a virtual or "Avatar" model of soil particles, at least for sands size and above, and this is the aim of this research. The vehicle for doing this is to address the practical engineering problem of modelling particles of railway ballast used to support rail track, the ballast in this sense being regarded as a large grained sand. Collaborating with noted experts on ballast behaviour at Nottingham University (Professor McDowell) and Southampton University (Dr. Zervos), and with contributions on soil particle shape description from Dr Baudet of UCL, the research will create a virtual model of several typical ballasts, capturing their shapes, arrangement, any damage that occurs during loading by the trains as well as, most crucially, the contact behaviour between particles. Such a model could then be used both in research and engineering design, but the first step will be to validate that it is successful. This can only be done by comparing the output of discrete numerical analyses using the model particles with "real events". The most convenient real events are "triaxial" tests, which are conventional continuum type tests in which a cylindrical sample of many particles is loaded and the response monitored. These are difficult tests and ultimately the aim of this type of research would be to render them obsolete as we then rely only on the new particle scale tests and modelling.

This is blue skies research and the more direct economic and societal benefits will only be revealed over a scale of decades. The more immediate benefits will be to researchers and this in itself has an economic benefit through better focussed research. For example, there are many hundreds of researchers using DEM analyses of problems in coarse grained soils but who have little idea of the correct contact mechanics to use. The output from his work will be invaluable to them not only in choosing more realistic parameter values, but also in identifying which aspects of the mechanics it is important to model in DEM and how that should be done.

Ballast remains overwhelmingly the most common means of supporting existing rail tracks, but problems with its degradation are being exacerbated by higher axle loads, faster speeds and more frequent trains. Around 3 million tonnes of ballast are consumed annually, almost all of which has to be excavated from UK quarries, the nature of the rocks meaning that these are frequently in the areas of the country that are more sensitive to such an impact. The decision of whether to use a ballasted or concrete system for new track is often quite marginal, so while HS2 will utilise concrete slab track for phase one from London to Birmingham, phase two is likely to be ballasted (New Civil Engineer, Sept., 2016). Ballast tends to be cheaper in the construction phase but one of the major arguments against using it is the issue of long-term maintenance. This is not helped by current design being largely empirical in nature, leading to a high degree of uncertainty about future performance. At Nottingham University Kiani and Parry (2008) found that, for a full life cycle, ballast is currently actually no better environmentally than track slab in terms of global warming potential or embodied energy, while at Southampton Powrie has highlighted that the choice is also marginal in terms of whole-life cost, depending often on other construction related issues (Rail Tech. Mag., Aug/Sep, 2017). However, these conclusions might easily be changed if ballast design were more reliable, maintenance more predictable and/or easier and its longevity were improved. Network Rail currently spends about £3.5b per annum on infrastructure maintenance and renewal,. Even modest savings on ballast maintenance could be very significant and improve the resilience of our infrastructure and cause less spent ballast going to landfill. There will also be direct benefits to rail customers through improved service reliability and ride quality.

It is, however, the wider research aim of bringing more realistic contact mechanics into DEM modelling that will have the more profound benefits, but over a long time frame. The ability to model more reliably coarse soils, with the possibility that an Avatar model could be created, will have benefits in many areas of geotechnical, geological or petroleum engineering problems that require a particulate approach. For example, more realistic modelling of debris flow behaviour could lead to significant reduction in risk, which is a problem that Matthew and Béatrice are already collaborating on with researchers in Hong Kong. Other examples are the better prediction of oil well performance or design of rock fill dams. It is difficult to predict how our science will develop over the coming years. Given the progress in DEM over the last couple of decades from tens of two-dimensional disc shaped particles to tens of thousands of realistically shaped particles, there is the possibility that there will be a major shift in geotechnical engineering from continuum based approaches to discrete approaches, not just for specialised problems but for routine engineering. In this case the benefits would be incalculable.


Reference:
Kiani, M. & Parry, A. (2008) Environmental life cycle assessment of railway track beds. Engineering Sustainability, 161(2):135-142.