TheDFCCIL JOURNAL
ISSUE VI, MARCH 2020

Heavy Haul Superstructure
Improvements at Hot Spotswith Under Sleeper Pads

Sanjay Risbood
Getzner India Pvt. Ltd., 24 Tejas Society.
Kothrud, PUNE 411038. INDIA

Stefan Vonbun
Getzner Werkstoffe, Herrenau 5,
Bürs 6706, Austria

Dr. Ferdinand Pospischil
University of Innsbruck,
Unit of Intelligent Transport Systems,
Technikerstraße 13, Innsbruck 6020, Austria

Dr.Harald Loy
University of Innsbruck,
Unit of Intelligent Transport Systems,
Technikerstraße 13, Innsbruck 6020, Austria

The following contribution presents information regarding Under Sleeper Pad(USP) technology in ballasted track
and its benefits with regards to maintenance reduction in heavy haul lines. It refers to the latest findings of acoordinated system of elastic elements in railway superstructure and especially at so-called ‘hot spots’ like turnouts
and transition zones. The state of the art finite element modelling is compared with in situ measurements andfeedback from track operators.

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1. Introduction
In the railway superstructure USP are primarily used for ballast protection and to improve track quality. They
increase the contact area between concrete sleepers and the top ballast layer, reduce the formation of hollowness
beneath sleepers and lower superstructure settlements. Next to the standard track Under Sleeper Pads can also be
used to smoothen the deflections of transition zones and increase the life cycle value of all track components.

Figure 1: Padded concrete sleepers with polyurethane USP installed in track.

1. Methods
High quality superstructure systems are based on evenness and resilience and therefore a lower force excitation
due to passing trains. These forces alter the track bed quality. Hollow areas below the sleepers and signs of wearon the wheel and rail surface, both of which arise over time, increase these processes as well as being the result ofthem. The track vibrates more and more, thereby also increasing the emissions. By tamping and adjusting, thesuperstructure has to be returned to its original position. The period of time this deterioration takes is largely
dependent on the initial quality of the track superstructure. The creation of the conditions necessary for a good,
durable line that is as inherently stable as possible should therefore be the primary goal when installing new track.
In this context, evenness and resilience are important starting points for a high-quality superstructure system.
Especially at higher velocities, axle loads and train frequencies, problems in the quality of the track superstructure
have a great impact on the degradation, the maintenance and comfort of the passengers. Through the definedarrangement of elastic elements, such as Under Sleeper Pads, the railway track edges nearer to achieving thisgoal.

103Figure 2: High contact area of elastoplastic polyurethane
USP drastically reduces ballast contact pressure.

Arranging polyurethane (PU) Under Sleeper Pads beneath concrete sleepers prevents a hard impression directlyon the ballast. The upper-most layer of ballast can bed into the padding material, increasing the contact area(from 2-8% without padding or EVA padding, to 30-35% with PU padding, refer to Figure 2) and thereby also
avoiding excessive contact pressures. The larger ballast contact area and more even bedding lead to increased
stability of the ballast bed, less track settlement and reduced wear to significant track components. Laboratory
and field testing are presented showing the positive impact of the under sleeper pad on lateral resistance andsettlement of the track.

Figure 3: Deflection measurements at the transition zone (top). Determining the contact area in the lab (bottom).

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3. Results
With the results from open track data the next step is tohave a closer look upon hot spots like transition zones
between different superstructure types or turnouts.
These abrupt changeovers along different
superstructure constructions manifest themselves as a
discontinuity in track parameters, such as deflection
and bedding and result in accelerated wear of thesuperstructure.
The defined introduction of elastic elements into thetrack can drastically reduce rates of wear. Polyurethane
products have been proven in the laboratory, but evenmore importantly: in the field. Modern simulation
methods help engineers to develop an all-embracing
design that takes into account the different elasticlayers. A hot spot that has been optimized in terms ofstiffness and settlement helps sustain track quality for
longer and increases the availability of the rail
network.
To extend the longevity of a hot spot the approach ofoptimization is as follows: dividing the hot spot into a
number of sections, with the stiffness gradient
minimised over a longer length. Various elastomers
can be used depending on the specific function inquestion. Laboratory experiments and in-situ
measurements round off these theoretical studies. As
the aforementioned sample projects illustrate, thisfacilitates the targeted use of Under Sleeper Pads.
Existing conflict points can even be neutralised
retrospectively without having to rebuild the entire
superstructure, because retrofitting of these products isalso possible.

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3.1Transition zones
Transition zones can be found anywhere in a railway network where a change in superstructure build-up occurs(refer to figure 4). Very common transitions zones are:(1) Transition from ballasted track to slab track(2) Bridge approaches(3) Tunnel to open track (very high difference in the stiffness of the track sections)(4) Transition from open track to turnout and other special track work
A change in superstructure results in a sudden change in bedding modulus (stiffness), USP technology is used tosmoothen this stiffness change. The length of transition zones is determined by the speed of the trains running on
the line, often the “one second rule” applies: every stiffness step of the improved transition zone should have alength consistent with the distance the train travels in one second.
For a train travelling at e.g. 100kph this distance would be 28m. If the stiffness is adjusted in two steps with two
different USP types, the total length of the transition zone would be 56m. The absolute minimum for reasonabletransition zone length is the bogie to bogie centre distance.

Figure 4: Typical transition zone in a bridge approach situation.

3.2 Turnouts
Compared to straight track, for which calculations are easy to conduct due to the relatively homogenous geometry
with constant rail profiles and sleeper mounting surfaces, calculating elastic elements in turnouts is far morecomplex. The main reasons for higher effort include the varying profiles of the rails, the additional construction
elements and the generally strong variations in the sleeper conditions. These parameters result in varying degrees
of vertical load deflection. High-quality elastic elements, perfectly tailored to one another using moderncomputational methods, guarantee improved track bed quality and ensure enhanced availability of the railnetwork.

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Figure 5: Using different PU USP types to optimize a turnout.

Figure 6: Smoother rail deflection due to defined elasticity introduced by PU USP.

Reference at EDFC:
One turnout in the Eastern DFC at Daud Khan is equipped with PU Under Sleeper Pads since 2019.The
performance monitoring of this padded turnout is currently in progress to confirm the advantages of using PUR
USP for reduced turnout maintenance. For demonstrating the benefits, the padded turnout is compared with anon-padded turnout on the same line.

107The positive effects of Under Sleeper Pads have long been proven in railway tracks across the globe and PU USP
are now standard components in the track superstructures of Deutsche Bahn (Germany), ÖBB (Austria), RFI (Italy),
Network Rail (UK) and SNCF (France) amongst other.
Excerpt of global references in Heavy Haul:
Heavy haul operators have already successfully adopted PU USPs in their networks, significantly decreasing
their track maintenance efforts.
•FMG (Australia)•VALE, MRS (Brazil)•DaQin Coal Line (China)•Malmbanan (Sweden/Norway)

Heavy haul operators have already successfully adopted PU USPsin their networks, significantly decreasing theirtrack maintenance efforts.
Conclusion
With the introduction of PU based Under Sleeper Pads (USP),several railway operatorshave alreadytaken a huge
step towards maintenance reductions in the superstructure and therefore an efficient and sustainable railway
track. With the help of laboratory and in-situ track measurements the finite element model was calibrated and isready to improve superstructure hot spots such as transition zones, bridge approaches and turnouts. Positive
feedback from railway operators around the globe proves the link between theory and reality. Long-timemeasurements show the importance of a good track quality and emphasize the positive effects of Under Sleeper
Pads and their improved contact area.

Benefits of USP:•

Under Sleeper Pads increase tamping intervals by at least a factor of two. (60.000 cross-sections
examined in the network of ÖBB, Austrian Federal Railway)•Maintenance costs and operational hindrances are greatly reduced.

•Service life of track assets are increased by 20-30%•Economic benefits are proven by Technical University of Graz (Austria), the world’s leading experts inlifecycle management and LCC analyses.

MARCH 2020108

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