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Performance Assessment of Synthetic Shock Mats and Grids in the Improvement of
Ballasted Tracks
Évaluation de la performance des nappes synthétiques à effet d’amortissement et des géogrilles
dans l'amélioration des plates-formes ferroviaires ballastées
Indraratna B., Nimbalkar S., Rujikiatkamjorn C.
Centre for Geomechnics and Railway Engineering, University of Wollongong, Wollongong City, NSW Australia; ARC
Centre of excellence in Geotechnical Science and Engineering, Australia
Neville T.
Australian Rail Track Corporation Ltd. Broadmeadow NSW Australia
Christie D.
Geotechnical Consultant, Hazelbrook, NSW Australia
ABSTRACT: In Australia, railways offer the most prominent transportation mode in terms of traffic tonnage serving the needs of bulk
freight and passenger movement. Ballast is an essential constituent of conventional rail infrastructure governing track stability and
performance. However, in recent times, higher traffic induced stresses due to dramatically increased train speeds and heavier axle loads
have caused excessive plastic deformations and degradation of ballast. This seriously hampers safety and efficiency of express tracks, for
instance, enforcing speed restrictions and effecting more frequent track maintenance. Installing layers of synthetic materials such geogrids
and rubber pads (shock mats) in rail tracks can significantly reduce ballast degradation. Field trials were conducted on rail track sections
in the towns of Bulli (near Wollongong City) and Singleton (near Newcastle) to measure track deformations associated with cyclic
stresses and impact loads. This paper describes the results of large-scale laboratory testing as well as the observations from full-scale
instrumented field trials characterising the behaviour of rail ballast improved by shock mats and synthetic grids.
RÉSUMÉ : En Australie, les chemins de fer offrent le mode de transport plus important en terme de tonnage de trafic apte à répondre aux
besoins de transport de passagers et de fret en vrac. Le ballast est un constituant essentiel de l'infrastructure ferroviaire conventionnelle
régissant les performances et la stabilité de la voie. Toutefois, dans les temps récents, les contraintes plus fortes induites par un trafic se
faisant à vitesse de plus en plus élevée et avec des charges à l’essieu plus importantes provoquent des déformations plastiques excessives
et la dégradation du ballast. Cela entrave sérieusement la sécurité et l'efficacité des voies expresses en nécessitant, par exemple, des
restrictions de vitesse et un entretien des voies plus fréquent. L’installation de couches de matériaux géosynthétiques tels que les
géogrilles et les nappes de caoutchouc dans les plates-formes ferroviaires peuvent réduire de façon significative la dégradation du ballast.
Des essais en place ont donc été réalisés sur des sections de plates-formes ferroviaires dans les villes de Bulli (près de Wollongong) et
Singleton (près de Newcastle) afin de mesurer les déformations de la voie associées à des charges cycliques et d’impacts. Cette
communication présente les résultats des essais en laboratoire à grande échelle ainsi que des observations résultant des essais en place
grandeur nature instrumentés, caractérisant le comportement du ballast ferroviaire amélioré par les renforcements en grilles
géosynthétiques.
KEYWORDS: ballast, degradation, field trial, geosynthetics, impact loads, shock mats.
1 INTRODUCTION
The rail track structure consists of rail, sleeper (crossties), ballast, sub-
ballast (capping and structural-fill) and subgrade. Ballast is one of
important track components and is used as the primary means of
distributing of the wheel loads to underlying layers, and for holding
the track in proper alignment, cross level and grade. The ballast
assembly undergoes irrecoverable deformations due to particle
breakage and cyclic densification. The breakage of ballast particles
due to wheel loading can occur due to: (a) the particle splitting, (b)
breakage of angular projections and (c) grinding of small-scale
asperities (Raymond and Diyaljee 1979). In Australia, most breakage
of latite ballast is primarily attributed to the presence of highly angular
corners of quarried aggregates (Lackenby et al. 2007).
Several previous studies focused on the laboratory testing of
the soil-geogrid interfaces (Tang et al. 2008, Liu et al. 2009)
and the ballast-geogrid interfaces (Raymond 2002, Indraratna
and Salim 2003, Brown et al. 2007, Indraratna et al. 2010a,b).
In order to reduce ballast degradation, the use of geosynthetic
grids has been recommended (Selig and Waters 1994,
Indraratna et al. 2006, 2007, Indraratna and Nimbalkar 2012).
The geosynthetic grids hinder the lateral movement of ballast
due to frictional interlock among aggregates. The grid-particle
interlock in turn increases the track stability and prolongs the
maintenance period. Wheel-rail irregularities such as wheel flats
produce high levels of impact loading (Indraratna et al. 2010).
This impact load induces high frequency vibration of the track
components (Jenkins et al. 1974, Indraratna et al. 2011a,b,c). It
has been proven that excessive impact loads aggravate ballast
degradation (Indraratna et al. 2012a,b, Nimbalkar et al. 2012). A
field trial was conducted on sections of an instrumented rail
track in the town of Bulli (near Wollongong) and Singleton
(near Newcastle) to study the effectiveness of geosynthetic grids
and shock mats. This paper describes the large-scale laboratory
studies and full-scale field trials.
2 USE OF SHOCK MATS IN MITIGATING BREAKAGE
In order to evaluate the effectiveness of shock mats, a large scale
drop-weight impact testing equipment was used.
0
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0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20
ElapsedTime, t (sec)
ImpactForcePeaksP
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ImpactForcePeakP
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Impact Force, F
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(kN)
Without Shockmat
Shockmat placedat topandbottom
A
B
B
B
A
A
Figure 1. Typical impact force responses for stiff subgrade (data
sourced from Nimbalkar et al., 2012).
