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TRACK MAINTENANCE

ALPINE CORRIDORSIN FOCUS

Life-cycle costs
Understanding andaddressing the mechanismsof track deterioration

Gotthard Base Tunnel
SBB prepares for thetransformation oftransalpine traffic flows

Heavy Haul
IHHA conference debates
wheel-rail interaction andcontinuous monitoring

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PAGE 46

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August 2015

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TRACK MAINTENANCE Methodology

Managing track stiffness
in transition zones

Photo: Regis Chessum

Even in an era of highly mechanised maintenance, transition zones still require particular attention
to manage variations in track parameters caused by the change in substructure. Tailoring the resilient
elastic support to specific locations through finite element modelling can improve track quality andreduce costs for infrastructure managers.

SNCF Réseau has been investigating the degradation of track geometry in the transition zone from slab to ballasted track on LGV Est.

DIPL-ING MARTIN QUIRCHMAIRDR HARALD LOY
System Development
Getzner Werkstoffe GmbHA
Fig 1. Changes
in parametersat the transitionzone create a
discontinuity in thetrack.

s the proverb goes, a chain
is only as strong as its
weakest link. This applies
equally to a rail network,as the track maintenance interval is determined by the condition of the mostsensitive section. Typically this will be atransition zone, where interruptions tothe continuous support provided by thesubstructure and trackbed alter the stiffness of the track (Fig 1).
Transition zones occur at the interfaces between slab and ballasted track, or
where plain line traverses a built structure such as a bridge, tunnel or culvert.
Discontinuities may also occur in trackforms of the same type. For example, ifhigher demands for vibration protection in residential areas require the useof a ballasted track with soft, vibrationisolated sub-ballast mats, this will createtransition zones at the interfaces withthe standard track, posing a track design
and maintenance challenge.
The transition zone problem
Due to the varying degrees of stiffness and the associated deflection differences, an abrupt change in trackparameters from one type of superstructure to another can result in increased
dynamic stress. A rail vehicle has tocross a step, which, depending on its
height, can lead to sudden increases in
the wheel-rail forces (Fig 2).
Across a transition from ballasted toslab track, ballast settlement as a resultof movement and wear is unavoidable.
It is therefore necessary to tamp thetrack at regular intervals to prevent theemergence of voids and hollow areas
underneath the sleepers. Maintenanceintervals will depend on train speeds
and the dynamic stress that is exerted.1
Because of its solid design, slab track
exhibits much less or sometimes no settlement, resulting in the running surfaceof the ballasted track becoming lower
than that of the adjacent slab track. When
combined with a local change in stiffness,the resulting height difference places aRailway Gazette International | August 2015

35

Methodology TRACK MAINTENANCE

significantly increased dynamic stress on
the track structure. This can generate excessive loads around the rail seats.
Indications of wear intensify as timegoes by. These include white spots due
to excessive abrasion of the ballast, settlement caused by high specific loads
and short-pitch corrugation on the rail
surface (Fig 3). Hollowness and voids
underneath the sleepers can result inoverloading, with potentially serious
consequences such as broken rail clips,
bolts and sleepers, or even rail fractures.
Any short pitch corrugation thatforms in the transition zone can be seen
as a phenomenon arising from the widely differing natural frequencies of thedifferent superstructures. These are theresult of varying excitation mechanismsthat are caused by high dynamic forces.2
Higher costs
Maximising network availability is
the top commercial priority for infrastructure managers. Transition zones
only account for a fraction of network
length, but incur a disproportionate share of maintenance outlay. Eachyear, North American railroads spend
around $200m to maintain them, whilein Europe the figure is in the region of€85m. Data from the Netherlands, forexample, show that transition zones require between two and four times theexpenditure of plain line sections3, whileother railways suggest a factor of eight.
This financial impact means that there
is a clear incentive for asset owners tooptimise the methods they use to treat
individual transition zones, as even themost advanced track design and support options would recoup their costs
very quickly. Depending on axleload andspeed, the main criteria that infrastructure managers need to address are:• reducing dynamic effects at local
changes in track stiffness;• adjustment of existing stiffness differences in the track;• lowering settlement, especially in thetransition zone, to fixed constraintpoints;• defining the optimal length of a transition zone for efficient maintenance.
In most cases conventional ballasted
track is too stiff and has to be joined tosofter zones, such as a slab track segment with highly elastic rail seats. This

raises the question of what differences indeflection or stiffness should be permitted in the transition zone, and over what
distance the transition should extend.
Various guidelines have been established over the years. It is often recommended that the stiffness change is suchthat the computed deflection difference
between the individual sections is no
more than 0·2 mm to 0·5 mm. As far as
the length of the transition zone itself isconcerned, an engineering rule of thumbcan be applied: the overarching aim is tomake the transition as long as necessary(benefit) while keeping it as short as
possible (cost). Depending on the case,a 0·5 sec, 0·7 sec or 1 sec duration is frequently specified, based on the length oftime a train takes to cross it. However,short structures and high speeds would
require very long and expensive transitions. A compromise therefore has to befound. Furthermore, the transition zone
should never be shorter than the distance between the vehicle bogies.4
Established approaches
Transition zones have long beenrecognised as particularly sensitive elements, and numerous approaches havebeen adopted to alleviate the problem.
But existing methods have significant
downsides, sometimes making track
maintenance more difficult or expensive.
Typically, track engineers seek todistribute the local discontinuity in the
track parameters across a wider area.
Fig 3. Signs of wear in the transition zone:
white spots (top right), short pitch corrugation(centre right) and track settlement in theballast (below).

The change in stiffness should be carried out continuously, or in small steps,
in order to minimise the dynamic stresson the superstructure. This essentiallysplits the transition into several sections.
On high speed lines, transition zones
can extend over six sections or more. But
this approach represents the high endof the spectrum, and typically requires
a variety of measures usually including
guardrails, ballast bonding, or transition
slabs (Fig 4). While this method has been
honed in the light of experience overmany years, it is probably too complex forthe majority of transition zones on conventional lines. Simpler options are often
used to lower the cost, but these normallyaddress only part of the problem.
The defined use of elastic superstructure components based on polyurethane
could provide an additional or alternative mitigation method. The propertiesof this material allow the stiffness of theFig 2. Defects occur
at a transition zone
from ballasted toslab track.

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TRACK MAINTENANCE Methodology

Fig 4. Guard rails along the transition(above). Bonding as a way of stabilising theballast (right).
Fig 5. Various elastic
products can be used
to manage the track
structure through atransition zone.

Fig 6. Damage toslab track withloosened fastening
bolts (below) canbe resolved byinstallation of plastic
compensating plates
and elastic rail pads
(centre, right).

superstructure to be defined very precisely
in terms of its elastic properties, while its
complementary relationship with the ballast provides protection in the long termusing the material’s plastic properties.
Defined elasticity
The use of high-quality elastomers
enables undefined levels of stiffness tobe replaced by defined ones. The deflections in the individual sections of atransition zone can then be modified ina targeted manner. Depending on thetrack structure, changes to the stiffness
can be made using rail pads, baseplatepads, under-sleeper pads, sub-ballast
mats, mass-spring systems or elastic insert pads for sleeper boots (Fig 5).
In contrast to rubber-based materials, products such as Sylomer andSylodyn use no softeners that might
diffuse during the lifetime of the material. To all intents and purposes, thestiffness remains constant and definedfor their entire service life.
Both materials can be tailored tooffer either highly dynamic properties
or highly plastic ones. When used forrail pads and baseplate pads, the ratio
between dynamic and static stiffness iscritical. Plasticity is out of the question. On the other hand, plastic deformation is a desirable attribute in othersituations, such as where under-sleeper pads are used for ballast protection.
Here it increases the contact area andsignificantly lessens the contact pressure between the ballast and the sleeper. The interlocking of ballast stones
with the pad also reduces settlement
and leads to less ballast movement,improving the stability of the overall
track structure.
The adaptability offered by polyurethane allows for a broad range of products to be developed with finely graded
degrees of stiffness and material properties. This means the materials can be
precisely aligned to match the varying
track parameters through each sectionof a transition zone. A further advantage
of polyurethane is the positive impact onsettlement. The top ballast layer is stabilised by becoming embedded in the pad,and vibration is reduced which in turn
reduces the ballast movement. The critical frequency range within which stonesin the ballast layer wear more quickly
begins at an excitation of around 30 Hz.

Any reduction in vibration amplitudes
in this frequency range increases theservice life of ballasted track.
Learn from experience
The fitting of additional elastic elements, such as under-sleeper pads, railpads or baseplate pads, can be achieved
without rebuilding the entire superstructure. Fig 6 shows a typical example of a bridge on the Ferrocarriles
Suburbanos network in Mexico City. A
section of ballasted track had been connected to a slab in the normal way. As
no particular attention had been paid
to this transition zone, the characteristic white spots associated with ballast
abrasion became apparent very quickly.
High dynamic forces then caused loosening of the fastening bolts and the surface of the slab track was damaged.
To try to resolve the problem, calculations were carried out to identify themost suitable elastic support products,
which were then carefully matchedwith one another to create a smoothtransition zone. To compensate for theunevenness of the damaged slab track,
bespoke plastic adjusting plates madefrom Sylomer were fitted between therail seat and the concrete. The pads

Railway Gazette International | August 201537

Methodology TRACK MAINTENANCE

placed directly under the rail foot are
made from softer, elastic Sylodyn. Thisensures good load distribution and dynamic properties. In the transition zone,
25 sleepers were padded with elastoplastic Sylomer, which markedly reduced settlement. In combination, thesemeasures considerably reduced wear at
the transition zone.
It is always desirable to incorporate
preventive measures when the track is
first laid. For example, on a private coalrailway in Germany, a transition zone
was managed through a transition slab.
To increase the contact area between theballast and the transition slab, a newlydeveloped plastic sub-ballast mat wasused for the first time. The contact areaachieved during in-house laboratory experiments was around 34%, a figure that
reduces the load between ballast andtransition slab by a factor of six to eight.
This stabilises and protects the ballast in
the transition zone.

Fig 7. Deflection
measurements
at the transition
under heavy haulconditions (top row).
Determining thecontact area in thelab (bottom row).

In-situ measurements aimed at verifying the transition calculations were carried out in autumn 2014 and are being
repeated this summer. The experience
gained from these measurements is being
fed back into our development of a computer model to ensure our elastic support
products can be tailored precisely to any
given transition zone (below). QReferences
1. Freudenstein S, Geisler K, Mölter T, Mißler M and Stolz C. FesteFahrbahnen in Betonbauweise [Concrete slab tracks], Betonkalender,
Sonderdruck, Ernst&Sohn, 2015.
2. Grassie S L. Rail corrugation: characteristics, causes, and treatments,
Review Paper, Germany 2009.
3. Zuada Coelho B E. Dynamics of railway transition zones in soft soils,Dissertation, Delft University of Technology, 2011.
4. Schneider P. Einsatz von Schwellenbesohlungen in Übergangsbereichen,Präsentation 2. Schwarzenberger Bahn-Gespräche, 2011.

ANALYSIS

Advanced modelling aids design
There is a direct correlation between the loadon the superstructure in the transition zone andthe quality of the design. The targeted use ofnon-linear material data enables various elastic
beddings to be compared and the best option
selected to minimise the stress placed on thetrack structure in the transition zone. Q
0Â00Â5Deflection mmlaboratory measurements, have been stored for
all the elastic elements used in the model. This
allows us to account for the non-linear materialproperties. Bespoke elements, such as guardrails
or transition slabs, can also be included.
In addition to rail deflection, the static anddynamic models provide other useful data,including rail foot tension, sleeper torsion orsupporting forces.
Fig 8 shows how the deflections in the differing
sections of track merge uniformly after optimisedmodelling. The reduction of load in the supporting
forces at the transition point is evident, andalthough additional force peaks occur at the endsof the individual sections, they are much smaller
than those occurring prior to FEM modelling.

Optimised transition zoneUnoptimised transition zone

1Â01Â52Â02Â53Â03Â5Fig 8. FEM model (below), showing the resulting
maximum rail deflection (right) and maximum force
on rail supports (below right).

4Â00510152025303540303540Sleeper number

45403530

Force kN

Most computer models calculate rail deflection
using analytical methods such as theZimmermann method — an infinitely long,continuously-bedded beam. These models havebeen verified many times and, in a homogeneousenvironment with constant track parameters,deliver extremely high quality results. However,when computing the critical points in thetransition zone where there is a sudden increasein stiffness, the model reaches its limits.
Finite Element Modelling provides a meansof computing the transition zone as a whole.
Separating the model into a finite number ofelements enables us to analyse each section as
well as the points of discontinuity.
The structure of the FEM model consists of anumber of elastic elements. Depending on theapplication, differing materials can be extruded
in layers underneath the sleeper. This accounts
for the bedding of both the ballast and thesubstructure, as well as that of the elastic
elements such as sub-ballast mats and undersleeper pads. In practice, the sleeper is tamped
underneath the rail seats. Therefore the centre
part of the sleeper has been cut out in order tocreate a realistic scenario (Fig 8).
Elements such as baseplate and rail pads
can be added separately to the model tosimulate the structure of a particular rail
seat. Empirical material data, derived from

252015

Optimised transition zone
Unoptimised transition zone

10500510152025Sleeper number