Laboratory evaluation of under-ballast mat effectiveness to mitigate differential movement problem in railway transition zones

Railway transition zones between open track and rigid structures such as bridges and culverts often experience differential movements due to abrupt changes in track stiffness and complicated dynamic loading effects. Such conditions commonly lead to track geometry issues, consequently increasing the magnitude of impact loads at these areas. This in turn leads to excessive degradation of ballast and other track components. Under-ballast mats have seen increased use in the last decades especially in the area of improved vibration mitigation and track stiffness reduction at bridge approaches due to their flexibility and damping capabilities. This paper presents results from an ongoing laboratory study aimed at investigating the effectiveness of under-ballast mats on the transient deformation behavior of track sections built over stiff substructures such as bridge decks and subjected to cyclic loading. According to the study findings, under-ballast mats can provide the necessary resiliency enhancements to track structure to mitigate differential settlements at transition zones by matching the vertical transient deformations of open track with those obtained over stiff support conditions. It is important to understand that the issues at transition zones are not singular to one phenomenon or component, but constitute a system problem that requires investigation holistically. Accordingly, differential movements instigate a negative feedback loop followed by plastic deformations of the approach, increased impact loads, ballast deterioration, and additional track settlements spawning continuously accelerated deterioration loops of the track, and of other components and/or structures. The ballast is a vital component to the bearing capacity of railway tracks. As part of this composition, the ballast endures compounding effects of high impact loads stemming from heavy axle freight trains, most commonly applying heavy loads in the US, overcoming the uneven running surfaces of approach and structure and thus propagating vibrations through the track structure. Additional frictional wear of particles may also stem from the generated vibrations. There are two primary mechanisms by which ballast particles degrade. First, attrition constitutes the deterioration of the surface texture and geometry of the ballast particles removing surface texture and angular characteristics of aggregates that are critical to the sustainability of the structural skeleton providing ballast with its load bearing capabilities and resistance to permanent deformation (Tutumluer & Pan 2008, Wnek et al. 2013, Lu & McDowell 2010). Second, breakage relates to the tensile failure of the ballast particles due to exceedingly high contact stresses between individual stones, resulting in material splitting (Selig & Waters 1994). Both of the above-mentioned mechanisms contribute to ballast fouling (Qian et al. 2014, Selig & Waters 1994, Selig et al. 1988). For decades, railroads and researchers have explored the use of elastic resilient materials in the track structure. Three components have been mostly used to provide solutions as employed by railroads to manage the elastic properties of railway track; these are premium elastic fastening systems, under-sleeper pads (USP), and under-ballast mats (UBM). UBMs are elastic rubber pads – usually manufactured using natural rubber, recycled tire rubber or Ethylene Propylene Diene Monomer (EPDM) rubber – installed below the ballast layer of a ballasted track structure or under the concrete slab in a slab track design. This component has long been employed as a vibration mitigation measure for both transit and freight environments, but most notably for the former. Still, in recent years, a consistent increase in the use of UBMs in freight railroad environments has provided opportunities to explore and report their potential effectiveness for mitigating transition zone problem and/or reducing ballast stresses (Indraratna 2016, Sol-Sanchez et al. 2014, Li & Maal 2015, Indraratna et al. 2014, Sol-Sanchez et al. 2015). Published studies investigating the life cycle of UBMs are still limited (Wettschureck et al. 2002, Dold & Potocan 2013). Nevertheless, reports from tests conducted in sections of UBM retrieved from revenue service have demonstrated the capability of the component to retain its properties after many years in service (Wettschureck et al. 2002, Dold & Potocan 2013). Most of the conducted research into the topic is part of product development efforts of suppliers and results are not widely made available to the industry. The UBMs are resilient pads that can provide additional resiliency to the track structure foundation and effectively dissipate more of the produced energy – manifested in the form of vibrations that propagate through the ballast structure – from the wheel-rail and/or tie-ballast interfaces that are associated with accelerated rates of ballast degradation (Sol-Sanchez et al. 2014). Kerr & Moroney (1993) traced the transition zone problem to the sudden changes in accelerations of the wheels and vehicles at these interfaces and cited key remediation methods aimed at reducing these changes, such as the reduction of vertical stiffness on the “hard” side of the transition. Despite the good potential of UBMs for addressing the abrupt changes in track stiffness and mitigating differential movement issues of problematic track transition zones, there is little to no documentation available in the literature. 2 OBJECTIVE AND SCOPE The primary objective of this research effort has been to investigate the effectiveness of UBMs on the transient deformation behavior of track sections built over stiff substructures such as bridge decks, and subjected to cyclic loading. This study is part of a largerscope, ongoing research effort at the University of Illinois at Urbana-Champaign (UIUC) aiming at evaluating and quantifying the overall performance of UBMs and their benefits to the track structure. As part of laboratory experiments conducted on a UBM sample, ballast vertical deformations were monitored under cyclic loading. Ballast degradation trends were quantified through laboratory sieve analyses with ballast gradations compared prior and subsequent to testing. This paper presents results of the laboratory tests conducted and compares them with field measurements of transient deformations obtained in bridge approach sites in North America from previously published literature.