Laboratory examination of ballast deformation and degradation under impact loads with synthetic inclusions

Abstract

This paper presents a laboratory study on alleviating the deformation and degradation (breakage) of ballast subjected to impact loads using geogrids and rubber mats. A series of drop hammer impact tests are carried out to determine how well the geogrid, under-ballast mat (UBM) or under-sleeper pad (USP) can attenuate impact loads and mitigate ballast degradation. Geogrids to be placed at different locations in a ballast assembly, in combination either with a UBM or a USP are tested. Laboratory test results prove that the inclusion of rubber mats and geogrids decrease the dynamic impact loads transferred to the ballast aggregates and subsequently decrease the degradation (breakage) and deformation of ballast. The tensile strength of geogrids and subgrade stiffness are found to considerably influence the performance of geogrid-reinforced ballast under impact loading conditions. The measured impact forces and accelerations of ballast with and without an artificial inclusion show that rubber mats definitely reduce track vibration (acceleration) and the subsequent deformation and breakage of ballast. These inclusions will not only increase safety and passenger comfort they will also lead to more economical and efficient track designs.

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.