Laboratory examination of ballast deformation and degradation under impact loads with synthetic inclusions
Introduction
Owing to population growth and urbanization, the demand for mobility and accessibility has been increasing, and therefore, a rail network that can cope with high speed and high capacity trains has become a necessity. Australian railways currently operate 3–5 km long heavy haul trains with 32–40-tonne axle loads and expect to increase speeds beyond 100 km/h on certain freight routes. This dramatic increase in operational frequencies, axle loads and speeds adversely affect the safety and longevity of conventional ballasted track by enforcing more frequent maintenance and associated track closures [1], [2], [3]. By weight and volume, ballast is the largest component of track structure; and it generally consists of coarse, igneous-rock aggregates with particles varying from 10 to 60 mm. Rough aggregates with high angularity, sufficient strength and hardness, and high resistance to degradation (intact grains with insignificant micro-cracks) are considered to be ideal as rail ballast [4], [5], [6]. The effects of particle size and shape on the shear strength and breakage responses of rockfill and ballast materials have been well established in the literature [7], [8], [9]. A ballast layer must be able to transmit the dynamic and static load generated by rolling stock to the subgrade at a reduced and acceptable level of stress, and also prevent excessive vertical and lateral deformation [10], [11].
The performance of granular materials such as ballast depends on the quality and properties of the aggregates, as well as the loading characteristics and the presence of artificial inclusions such as geogrids, under ballast mats, and under sleeper pads [12], [13], [14], [15], [16]. Understandably, increasing axle loads and loading frequencies progressively damage the ballast and adversely affect the longevity and safety of rail track. Impact loads are induced by wheel and/or rail irregularities and abrupt changes in track stiffness such as transition zones, railway crossings, bridges and tunnels [5], [17], [18], [19], [20], [21]. Since the magnitude and frequency of these impact forces are much higher than the cyclic load generated by moving loads, they accelerate track deterioration, which is why increasing costs and frequency of maintenance is of concern to track management [22], [23].
The inclusion of geosynthetics (geogrids, geotextiles and geocomposites) and under ballast mats (UBM) or under sleeper pads (USP) during the construction and rehabilitation of tracks is increasingly acceptable practice worldwide because of their technical, economic, and environmental benefits. The effect of geogrids on unbound and bound granular materials in railroad and other transportation sections has been well documented [24], [25], [26], [27], [28], [29], [30], [31]. In fact, a proper design approach towards adopting these inclusions will help to mitigate ballast degradation and enhance track stability. Recent experimental studies [32], [33], [34], field studies [35], [36] and numerical investigations [37], [38], [39], [40], [41], among others have revealed that the use of geogrid improves the performance of unbound granular layers by attenuating the rate of plastic deformation and particle breakage under static and cyclic loads.
Previous research has also shown that stabilising ballasted track with rubber mats also helps to reduce the deformation and degradation of ballast [42], [43], [44]. The enhanced damping properties, energy absorption, and better load distribution provided by these resilient mats have also improved track stability and reduced maintenance costs [15], [45], [46], [47], [48], [49]. However, since there is still no comprehensive understanding of how geogrids combined with UBM or USP stabilise ballast aggregates under impact loading and mitigate impact damage, these are the main aims of this current study.
Section snippets
Materials tested
The materials used in this study are fresh ballast aggregates, sub-ballast (capping), three different types of biaxial geogrids, under ballast mats (UBM) and under sleeper pads (USP). This ballast consists of sharp angular coarse aggregates of crushed volcanic basalt taken from the Bombo quarry (Wollongong, Australia). The ballast aggregates were cleaned, dried, and sieved through standard sizes to achieve the specified gradation. These aggregates were then spray painted in different colours to
Measured impact forces
Fig. 5 shows the typical impact forces for ballast assemblies with geogrid (GGR2), installed at the ballast-capping interface (test T2) and without geogrid (test T1). Data were recorded during the first 0.2 s, at the 5th drop (N = 5). Two distinct force peaks are visible under impact loads; multiple instantaneous sharp force peaks (P1) followed by a much longer gradual force with a smaller magnitude (P2). The instantaneous maximum peak P1 comes from inertia as the top-loading plate resists the
Conclusions
A series of impact tests were carried out on ballast samples with and without artificial inclusions (geogrids, UBM, and USP) using a large scale drop-weight (impact) testing facility. The influence that these inclusions have on the impact forces as well as the deformation and breakage of ballast were analysed and discussed, with respect to geogrid placement, tensile strength and type of subbase (capping, concrete). In addition, the efficiency of a combined geogrid-rubber mat (either UBM or USP)
CRediT authorship contribution statement
Buddhima Indraratna: Conceptualization, Supervision, Validation, Investigation, Project administration, Writing - original draft, Formal analysis. Trung Ngo: Writing - original draft, Formal analysis. Fernanda Bessa Ferreira: Conceptualization, Investigation, Formal analysis, Writing - review & editing. Cholachat Rujikiatkamjorn: Conceptualization, Supervision, Project administration, Writing - review & editing. Amir Shahkolahi: Project administration, Writing - review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
The authors would like to acknowledge the Rail Manufacturing CRC, Global Synthetics and Foundation QA for providing the financial support needed to undertake this research (Project R2.5.2). The authors would also like to thank the ARC-Industrial Transformation Training Centre for Advanced Technologies in Rail Track Infrastructure (IC170100006) and funded by the Australian Government. The Authors are grateful to Mr Rhys Pitchers, Dr Chamindi Jayasuriya, Mr Alan Grant, Mr. Cameron Neilson for
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