T RACK DE S I GN FO R MAI N T EN A N C E

Optimization at track transition zones byspecially engineered PU pads
A chain is only as strong as its weakest link applies equallyto a railway network, as the time at which track maintenance has to be carried out is determined by its mostsensitive section, usually a transition zone. These arefound in areas where the continuous support provided
to the superstructure and bedding is interrupted and thestiffness of the track substructure alters, like at interfaces
between slab and ballasted track, open track traversing abridge, tunnel or culvert.
Discontinuities may also occur in systems of the same
type. Due to the varying degrees of stiffness and theassociated deflection differences, any abrupt change intrack parameters from one type of superstructure to another result in increased dynamic stress. A rail vehicle has
to cross a step, which can lead to sudden increases in thewheel/rail forces. These impacts significantly accelerate
the wear of the superstructure and lead to ballast settlement and damage to individual components.
Across the transition zone from ballasted track to slab
track, settlement of the ballast as a result of its movement
and wear is unavoidable. It is therefore necessary to tamp
the track at regular intervals to prevent voids and hollow
areas underneath the sleepers.
The slab track, because of its solid design, exhibits less oreven no settlement, resulting in the running surface ofthe ballasted track becoming lower than that of the slabtrack. When combined with a local change in stiffness,the resulting height difference places a significantly increased dynamic stress on the super structure that cangenerate excessive loads around the rail seats.
With time the frequently occurring wear indications intensify the problem like white spots due to excessive
ballast abrasion, settlement caused by high specific loads
and short pitch corrugation on the rail surface. Hollowness
and voids underneath the sleepers can result in overloading, the breaking of rail clips, bolts and sleepers, or evenfracturing of respective components. Any short pitch
corrugation in the transition zone can be seen as a phenomenon arising from the widely differing natural frequencies of the different superstructures.
Increased maintenance

Such damage scenarios lead to increased maintenance oftransition zones and higher costs. Data from the Netherlands show that transition zones require between two andfour times as much maintenance as normal stretches oftrack.
The proportion of transition zones is vanishingly small,
implying that the somewhat higher investment costs,
because of using the best possible elastic superstructurecomponents and a technically optimized solution, arerecovered very quickly.

Transition zones with varying track stiffness need extra maintenance and absorb
disproportionate maintenance effort.

The main criteria that need to be satisfied are:l

Reducing dynamic effects at local changes in track
stiffness.

l

Adjustment of existing track stiffness differences.

l

Lowering settlement especially in the transition zone
to fixed constraint points.

l

Optimal length of the transition zone with respect tocosts and benefits.

The classical ballasted track is usually too stiff and has tobe joined to softer zones, e.g. a slab track with highly
elastic rail seats.
For optimal results, it can be recommended that thestiffness change is such that the computed deflection
difference between the individual sections is no morethan 0.2 to 0.5 mm.

Martin Quirchmair
System Development
Getzner Werkstoffe

For the transition length, an engineering rule of thumbis to make the transition as long as necessary (benefit)
while keeping it as short as possible (cost). Dependingon the source, the 0.5 sec, 0.7 sec or 1 sec rule is frequently
used i.e. the length is determined by the time it takes tocross it, without it being shorter than the absolute minimum length of the distance between the bogies.
Common approaches

Normally, the aim is to distribute the local discontinuity
in the track parameters across a wider area. The change
in stiffness should be carried out continuously, or in smallsteps, in order to minimise the dynamic stress on thesuperstructure, leading to splitting the transition intoindividual sections.

Harald Loy
Head of SystemDevelopment

Rail Business [Focus-India] January 2016

27

TRA C K D ESIGN FOR MAINTEN ANC E
more quickly begins at an excitationof about 30 Hz.

0Proven in the field

40
Forces30

2Deflections
320

Force kN

Deflection mm

1

10

optimized Transition Zone
unoptimized Transition Zone

40Transition sleeper number

On high-speed lines, transition zones
can extend over six or more sections.
The typical constituent parts of thesesolutions usually comprise guardrails,
ballast bonding, or transition slabs.
These solutions satisfy the demands
very well, but are expensive and, froma commercial perspective, cannot be
used along standard tracks. Simpler,
lower cost solutions are used in many
applications.
Elastic superstructure components
based on PU (Polyurethane) offer
one alternative or addition to thesepartial solutions. The special properties of this material allow the stiffnessof the superstructure to be definedvery precisely (elastic properties),while its complementary relationshipwith the ballast provides protectionin the long term (plastic properties).
Optimization with elasticity

The use of high-quality elastomers
enables undefined levels of stiffness
to be replaced by defined ones. Depending on the superstructure, adap-28 Rail Business [Focus-India] January 2016

tation of the stiffness can be performed using rail pads, baseplate pads,
under-sleeper pads, sub ballast mats,mass-spring systems or elastic insert
pads for sleeper boots.
This is where the strengths of highquality PU products, such as Getzners Sylomer® and Sylodyn®, come tothe fore. In contrast to rubber-basedmaterials, no softeners are used thatmight diffuse out during the lifetimeof the material and stiffness remains
unchanging and defined for its entire
service life. The properties of PU materials exhibit a high degree of variability. Sylomer® and Sylodyn® materials can be tailored to the product andsupplied with outstanding dynamic
properties or, if desired, with a highplastic character.
The interlocking of ballast stones withthe pad reduces settlement and leads
to less ballast movement, improving
the stability of the overall railway superstructure.
The adaptability offered by PU enables a broad range of products withfinely graded degrees of stiffness andmaterial properties, in turn allowing
the varying track parameters acrossthe individual sections of the transition zone to be perfectly matched. Inaddition, the top ballast layer is stabilised by becoming embedded in thepad. There is less vibration, reducing
the ballast movement. The criticalfrequency range within which the ballast stones in the ballast bed wear

On standard tracks, various track
types are often connected directly
without intervening sections. The
problems arising can be significantlyalleviated, even retrospectively, bydividing the transition into a numberof sections. The fitting of additionalelastic elements can be achieved without rebuilding the entire superstructure, as in the case of construction ofa bridge in Mexico.
Here, a stretch of ballasted track had
been connected to a slab track in theclassical way. As no particular attention had been paid to this critical transition zone, characteristic white spots
associated with ballast abrasion became apparent very quickly. Further,
the effect of high dynamic forces had
loosened the fastening bolts and theslab track surface was damaged.
To compensate for the unevennessof the damaged slab track, adjusting
plates from Sylomer® were fitted between the rail seat and the concrete.
Soft, elastic Sylodyn® rail pads placed
directly under the rail foot ensured
excellent load distribution and good
dynamic properties. In the transition
zone, 25 sleepers were padded withelastoplastic Sylomer®, which markedly reduced settlement and provided
a smooth transition with considerably
less wear.
Modern simulation methods help engineers develop an all-embracing design that takes account of the differing
elastic layers. A transition zone thathas been optimized in terms of stiffness and settlement helps sustain
track quality for longer and increases
the availability of the network. Laboratory experiments and in-situ measurements round off the theoreticalstudies. This facilitates the targeted
use of rail pads, baseplate pads or under-sleeper pads. Existing conflictpoints can even be retrospectively
neutralised without having to rebuild
the entire superstructure.
The amount of knowledge and experience gathered internationally in recent years will enable to help network
operators quickly and effectively
whenever individual transition zone
problems occur.