TECHNICALJOURNAL OF
PROCEEDINGST H E PER M A NENT WAY INSTI TUTION &T H E UNIO N O F EU ROPEAN RAI LWAY E N G I N E E R AS S O CI ATI O N S

16TH OCTOBER 2018 - MANCHESTER CONFERENCE CENTRE UK
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JOURNAL OF PROCEEDINGSTECHNICALElastic elements
in track influencing
total track costs
and reducing
vibrations
INTRODUCTION
Under Sleeper Pads (USP) are associated withhaving a positive impact on track behaviour ofballasted track and reducing vibration.

IMPACT OF UNDERSLEEPER PADS ON TRACKSUSTAINABILITY
Starting with the impact on track behaviour
one question should be addressed: Why doconcrete sleepers need further development?
The implementation of concrete sleepers has
given track much better stability in increasing
the lateral track resistance and thus lowering
the maintenance requirements. The one world
record run in 1955 with 331 km/h destroyed
the wooden sleeper track. There were twomain reasons: little lateral track resistancedue to the light superstructure and the manual
maintenance not allowing continuity of track
quality, neither in track laying nor within track
maintenance.
These problems were solved impressively long
before Under Sleeper Pads were in use byimplementing mechanised track renewal andmaintenance and heavy superstructure with 60
kg rails and concrete sleepers. After the world
record run in 2007 at 574.8 km/h, the track was
opened for operation without any maintenanceaction necessary. Furthermore, concrete
sleepers are cheaper than wooden ones andshow a longer service life. Thus, not only loads
and speeds could be increased but also thelife cycle cost of superstructure was reduced
by using concrete sleepers. However, concrete
sleepers still show one main disadvantage
compared to wooden ones: the contact area
between the sleeper and ballast is very small.
Tests using black paper below the sleepers
allowed the contact area between ballast andsleeper after tamping to be identified. The
used ballast in Austria has a maximum size of
63 mm. The use of a dynamic track stabiliser(DGS) showed significant differences in the
results as presented in figure 1.

On the one hand the results show theeffectiveness of track stabilising; on the other
hand, high ballast degradation when using
concrete sleepers is evident. The hard-to-hardsituation in this contact area causes only a few,
highly overloaded contact points forming thestarting points for a limited number of “forcepaths” through the ballast structure rather thanallowing the entire ballast structure to act as asleeper-support. Edges and corners are quickly
cracked off, leading to these unpredictable
initial settlements. As the number of “forcepaths” varies under neighbouring sleepers, thesleepers settle differently, causing initial errors;
equal to a reduced initial quality and increasing
the rate of deterioration. To summarise: thebigger the contact area, the smaller the initial
settlement is and thus the differences between
these settlements; the higher the contact area
between sleeper and ballast the higher the
initial track quality.
These results have been very promising froma technical point of view. However, installationof Under Sleeper Pads can cause additional
investment costs. To analyse the efficiency ofthis additional investment from an economic
point of view, the technical effects (track quality
behaviour) must be analysed over the entire
service life of track (life cycle costing) as just alife cycle approach allows the evaluation of theeconomic efficiency of various track types. In
order to do this the behaviour of track must be
analysed, as understanding of track behaviouris the precondition of forecasting it and thus
evaluating life cycle behaviour and life cycle
cost of track. Within an extensive research
program at Graz University of Technology incooperation with the Austrian Federal Railways
Infrastructure, track behaviour over time wasanalysed.
All the following results described are
based on the extensive data of the AustrianFederal Railways. Testing Under Sleeper
Pads had already started in the late 1990s.
Implementation of USP in ballasted track inAustria began roughly 20 years ago.

AUTHORS:Peter Veit
Graz University of Technology, Austria
Stefan Vonbun
Getzner Werkstoffe GmbH, Austria

The tests and the analyses of the changeof track behaviour led to the general
implementation of under sleeper pads
for concrete sleepers at Austrian Federal
Railways. The respective regulations arein power since 2007. Figure 2 depicts theimplementation of Under Sleeper Pads on the
network of the Austrian Federal Railways overtime. There are still some wooden sleepers
installed e.g. on sharp curves, sidings andbranch lines. Concrete sleepers without USP
are built in mainly in case of single sleeper
exchange and short sections of track renewal.
The reason is to avoid too many places where
superstructure with and without USP meet,as these two types of superstructure show
significantly different behaviour.
Track behaviour is evaluated when analysingtime sequences of track recording data. In thecase of USPs, up to 2017, 60,000 sections
with USP have been compared with respective
sections without USP. It can be shown thattrack deterioration is cut in half with regards
to levelling-lining-tamping. This is caused byan increase of the contact area between thesleeper and the ballast bed. Measuring in trackand lab shows up to three times higher contact
areas using USP. This reduces the stresses
in the ballast bed. Furthermore, it also has avery positive impact on track service life as
in case of good subsoil worn out ballast islimiting track service life. In Austria, there arealso hundreds of switches equipped with USP.
Based on the analyses of track behaviour a life
cycle cost evaluation was conducted taking
the above described effects on track behaviour
into account.

TRACK QUALITY BEHAVIOUR
When describing track quality behaviour,
various conditions such as transport volume,
type of superstructure, quality and status of allcomponents, alignment (radii), ballast quality,
quality of sub-layer as well as sub-soil, thefunctioning of the dewatering system, positionof stations, bridges and turnouts must be taken

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into account. Therefore, a data-warehouse was
set up covering track recording car data (initial
status, present status, and quality figures), typeand age of superstructure and sub-structure
and transport load. The research was basedon this data for the main railway networkof Austrian Federal Railways covering time
sequences over the past 17 years (figure 3).
This structure allows the comparison ofdifferent types of superstructure for a big
number of sections facing the same boundary
conditions. The comparisons can be done
every 5m for a data set of 4,000 km in total
and thus provides a big number of comparable
sections. This allows the identification of theeffects of initial quality as well as calculating
the specific deterioration rate for a given setof boundary conditions and different types ofsuperstructure.
The results are very promising as the ballast
is identified to be the element limiting the
economic service life of track.

This underlines the expectations of Under
Sleeper Pads as stresses in the ballast bed
are reduced due to the increasing contact area
between sleeper and ballast (figure 3a).

LIFE CYCLE COSTING
Life Cycle Costing (LCC) is a process whereby
costs of various project alternatives are
considered for building and maintaining an
asset. These alternatives must all provide
the same level of service and benefits to beeffectively compared using LCC. Total costs
under the LCC methodology are initial designrespectively the costs of re-investment andconstruction, ongoing maintenance costs andscrap value or disposal cost for the previous
asset. All these costs are gathered for eachalternative. Then the user discounts the costs
back to present day Euros and summarises
them. Engineering departments typically haveproblems with convincing management toaccept a higher initial cost in order to saveon maintenance or to allow longer service

lives. The LCC analysis plainly demonstrates
the total cost which may aid in management
discussions.
However, there are other decision factors thatneed to be addressed: risk, future availability,
best maintenance practices and environmental
concerns, just to mention a few that may beunique to a certain project. Some of thesefactors may be addressed by increasing future
costs. As LCC addresses only those decision
factors that can be stated monetarily, costs ofoperational hindrances become important indescribing costs of reduced availability.
Application of LCC techniques provides
management with an improved awarenessof the factors that drive cost. Thoroughanalysis of the construction and the ongoing
maintenance are outlined in detail so as tomake omissions more obvious to the LCC
creators. It is important that the cost drivers are
identified as completely as possible so that theultimate decision makers can make the mostinformed decision.
Two attributes of permanent way make life
cycle costing especially useful in the fieldof railway infrastructure: an extremely longservice life and a strong relation between initialquality, maintenance demands and service
life. Investment determines the initial track
quality, while maintenance affects future track
quality and service life. Both investment andmaintenance strategies must be considered
together, as focusing on investment strategies
or isolated maintenance regimes will lead to
sub-optimal decisions.

Figure 1: Contact area sleeper – ballast

It is difficult to predict the total life cycle cost of
long-lasting assets because of the likelihoodthat there will be unexpected changes
related to component costs and maintenanceproductivity. Moreover, LCC analyses usuallyfocus on the factors and cost categories thatare most affected by the alternatives that are
being investigated. Therefore, the LCC values
that result from a study are not necessarily
complete or accurate or suitable for decisionmaking.
It is much better to compare different options
by looking at the differences in life cycle costs.
These differences can be directly tied to thefeatures that are expected to differ among
the various options. The best option will be
the one that is expected to have the greatest
reduction in LCC from the base case. Notethat the LCC is expressed as an annuity (i.e.
as an equivalent annual cost over the life ofa project). Sensitivity analyses are generally
conducted as part of life cycle cost evaluations
to attach critical values for sensitive input data.
The discounting rates depend on the service
life of the project calculated. Discounting rates
of a maximum of 5% net are generally in use
for service lives of 30 and more years.

Figure 2: Implementation of under sleeper pads at Austrian Federal
Railways72

One of the most important figures within LCC
is the service life. Unfortunately the service
life is not a given fixed value, but is influenced
significantly by the initial quality and themaintenance executed.

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Furthermore, it must be differentiated between
the technical and the economic service life,
as the technical service life can be increasedby expensive maintenance actions as single
sleeper exchange or by limiting the operationin speed and/or axle load. However, this mustnot be the target as it leads to high costs andpoor performance. The economic service life
is the time span resulting in the annual cost.
The economic service life is reached as soon
as the additional maintenance necessary to
prolong service life is more costly than the
reduction of depreciation due to the prolonged
service life.

Figure 3: Analyses of track behaviour over time

It can be mathematically proven that the ratioof the maintenance intervals is indirectly
proportional to the that of deterioration rates.
In other words, this means that a halved
deterioration rate leads to a doubled interval for
tamping actions and points out the importanceof the rate of deterioration. Furthermore,
research has shown, that increasing
maintenance cycles due to high quality leads toa remarkable increase of service life. However,if the maintenance level is reduced leading
to poor quality, service life will be shortened.
These effects depict the overwhelming
importance of track quality, namely the initialquality and the deterioration rate, for service
life, maintenance demand and thus the life
cycle cost of track.

RESULTS COMPARING
STANDARD TRACK AND TRACKWITH UNDER SLEEPER PADS

Figure 3a: Reduction of ballast contact pressure with USP

As the deterioration rate differs widely due tothe various boundary conditions, comparing
track with and without USP requires comparing
these two types of ballasted track facing thesame boundary conditions. Elastic footingsfor concrete sleepers have been underinvestigation for quite a long time throughout
Europe. It is obvious from the test results thattrack deterioration is reduced dramatically due
to the use of Under Sleeper Pads (figure 3).
The general results showed that:•••the initial quality is increased by 18 percent
the tamping cycle can be prolonged
without loss of quality in a range from2.00 to 3.00
the service life should increase by morethan one third

The initial quality of USP tracks is better
than that of conventional tracks as the initial
settlements of track are reduced and with
reducing these settlements to their absolutedifferences, the initial errors are reduced
automatically.

Figure 4: Internal rate of return for the additional investment into undersleeper pads

This data forms the input data for calculating
the economic efficiency of USP. The
calculation is based on Austrian cost figures.
The additional cost of USP is 30% of sleeper
costs including fastenings. In order not toassociate the results to a specific cost, levelnumerous sensitivity analyses have been73

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Figure 5: Development of track quality
carried out in varying specific cost data.
The economic evaluation shows that thisreduction leads to enormous savings interms of total LCC justifying the relatively low
additional investment cost. However, savings
are higher, the higher traffic loads are (figure4). Therefore, the Austrian Federal Railways(ÖBB) started to implement USP equipped
concrete sleepers as a standard solution on
highly loaded tracks (more than 30,000 daily
gross tons per track), sharp curves (less than
600 m), high speed (more than 160 km/h), andturnouts, first.
As further results for other boundary conditions(e.g. curved track) are also published in the
UIC leaflet, in the following additional results
regarding initial quality, quality after tamping
and expected service life will be discussed,
based already on more than 60,000 crosssections compared. Figure 5 shows typicaltrack quality behaviour for a section of 54,000
gross tons per day and track, on the left side
conventional ballasted track with rails 60E1 on
conventional concrete sleepers, on the righthand side the same superstructure but with
USP. The vertical red line shows the time ofrenewal, the green line a tamping action.
However, this is just a specific result.
Therefore, in figure 6 the quality after tamping
and the deterioration rate is given for all
checked sections. The quality deterioration
rate is reduced from 0.14 to 0.07 as was
expected by the theoretical calculations. The
quality after tamping is reduced from 0.5 mmstandard deviation to 0.3 mm. As the analyses
are based on Austrian data, all tamping actions
were stabilised.
These results underline the technical andconsequently the economic efficiency of Under
Sleeper Pads. Figure 7 summarises thecharacteristics of wooden sleepers, concrete
sleepers and concrete sleepers with Under
Sleeper Pads. The durability of the concrete
sleepers stays the same, the lateral trackresistance is increased as the ballast stones
can press into the pads.

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Durability of track increases as the criticalelement of the ballasted track, the ballast,
faces lower loads.
In Austria, concrete sleepers are the cheapest
solution for the available investment, concrete
sleepers with Under Sleeper Pads are thesame price as wooden sleepers. However,cost of track renewal increases by 5%, service
life is prolonged by more than one third andmaintenance can be reduced down to 50%(average values for track on good sub-soil withproper dewatering systems). These facts resultin an average reduction of total life cycle costby one third.

VIBRATION ISOLATION WITH
UNDER SLEEPER PADS
While USPs are excellent for reducing
maintenance costs, which has been alreadyoutlined in detail, they are also a cost efficientand effective measure for reducing vibrations
and secondary air-borne noise next toballasted track superstructure (refer to figure8). Highly elastic pads offer a simple method
for reducing vibrations on railway lines thatis cost effective in comparison with Under
Ballast Mats (UBM). In addition, they exhibit
all the positive properties of elastoplastic USP,
such as an increase of contact area and thus
reduction of LCC.
One advantage of USP over UBM is, that
retrofitting can be executed a lot easier with
USP, as sleepers can be exchanged one byone. Installation of UBM in an already existing
track would be a lot more complex and costly.
The vibration attenuation performance of
UBM is of course higher, due to the fullsurface decoupling, the bigger mass andthe lower stiffness of the elastic layer. The
decision between USP and UBM is a trade-off
between total cost (product plus installation)
and attenuation performance and has to betaken on the basis of project specific boundary
conditions.

Elastic sleeper pads with vibration isolating
characteristics are a very effective measure for
reducing secondary air-borne noise. Since thevibration and sound emissions of the railway
track are also significantly dependant on thequality of the superstructure, the more even the
superstructure is, the lower the emissions are.
In addition to this effect, the physical principleof vibration isolation plays a big part in the
reduction of emissions. The performance ofa vibration isolation solution is dependent on
factors like overall mass, stiffness and system
damping. By means of inserting an elastic
element such as USP, an oscillating system iscreated.
In the best case the frequency of the system isway below the spectrum of exciting frequencies
that should be isolated. This is based on theprinciple of the one mass one spring single
degree of freedom oscillator. The dynamic
stiffness of the USP used must be tailoredto the specific installation situation and theconstraints of the project. Only if the technicalsolution is carefully engineered will the USPs
be able to exploit their full vibration isolation
performance.

VALIDATION OF VIBRATIONISOLATION WITH USP
The vibration attenuation performance ofelastic elements in the installed state is
generally quantified by the insertion loss.
The insertion loss describes the relative
performance of an attenuation solution
compared to a reference situation, answering
the question of how the one-third octave bandsof the structure borne noise change after
installing USP. To enable this comparison, aceteris paribus principle should be followed:
the train, the speed, the roughness of the railand other parameters shall ideally be the exact
same or at least very similar to make a valid
insertion loss calculation possible.
Depending on the maximum permissible rail
deflection, padded sleepers achieve insertion
losses in the order of 10 dB(v) to 15 dB(v) (at63 Hz). 10 dB(v) of reduction are equal to an

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which are ideal for effectively minimisingvibrations. Deliveries were made both to the
CEMEX sleeper works and directly to London
Underground. This renovated section of theDistrict Line runs through very narrow tunnels,
meaning that there is little or no gap between
the sleepers and the drainage channels
adjacent to the tunnel wall. A particularchallenge in this case was preventing an
increase in the level of sound transmission
through the tunnel walls. The elastic Sylodyn®
bearings were for the first time placed not only
on the bottom of the sleepers, but also on theends (refer to figure 10), thus reducing thetransmission of noise and vibrations throughthe tunnel wall.

Figure 6: Impact on quality and track deterioration by installing USP

Figure 7: Characteristics of ballasted track with different types of sleepers
isolation performance (reduction) of 69%!
Secondary air borne noise arises due to thesound emission of a structure that is stimulated
to vibrate, for example by a passing train.
This applies in particular to metal structures,such as steel bridges and viaducts.
Measurements in track comply with thetheoretical calculations based on the
impedance model. Most recently a railway
track in Poland was assessed by experts from
Wroclaw University of Technology and the
R&D department of Getzner Werkstoffe. The
dominant one-third octave band of passing
trains can generally be found at 63 Hz. The
use of Sylomer® USPs resulted in an insertion
loss of 11.6db(v), which is equal to an averagevibration reduction of 74 per cent. Vibrations
are reduced at frequencies above 31.5 Hz
compared to the track without any elastic
elements. In the range of the natural frequency
only a very small resonance amplification could
be observed. These investigations, especiallyat the relevant frequency range of 63 Hz,
demonstrate the fundamental suitability of USP
in reducing vibrations and secondary airborne
noise.

PROJECT EXAMPLE: LONDONUNDERGROUND
The track of the London Underground District
Line was completely renewed between
Paddington and High Street Kensington
stations. Around 173 million passengers use
this line every year. Starting in July 2011, thisvery busy section of track, which is also partof the Circle Line, was upgraded with new
ballast, rails and brand new sleepers with pads
supplied by Getzner Werkstoffe. The sleeper
pads protect the track superstructure andreduce the level of vibrations caused by theunderground trains.
They also have a beneficial effect on the
numerous dwellings alongside the track, as thedecrease in vibrations noticeably improves theresidents’ quality of life.
Providing elastic bearings for this stretch of
the District Line was the first major sleeper
pad project for London Underground: thevibration protection requirements called forthe use of full-surface Sylodyn® sleeper pads,

Around 7,000 pads for concrete sleepers
and roughly 1,000 pads for timber sleepers
were used on a stretch of track extending
over approximately 2.5 kilometres. CEMEX
fitted the elastic bearings to the concrete
sleepers directly in its sleeper factory. London
Underground itself fitted the pads to the timber
sleepers.
Structure-borne noise measurements
taken following the installation verify theeffectiveness of the vibration protection
solution. “A significant reduction in
groundborne vibration has been achieved ina number of neighbouring properties. Somelong-term residents living next to the track haveeven written and thanked us for providing themwith greater peace and quiet”. Additionally,
the sleeper pads supplied by Getzner required
no changes to the installation programme,
methodology or equipment for the trackrenewal. “Overall, the use of Getzner sleeper
pads on this project has been a great success”,
stated Mike Barlow, Principal Project Engineer
from London Underground.
Other successful vibration isolation projects
with USP in the UK include the Birmingham
Arena Tunnel and numerous projects withturnouts and other track sections with special
requirements.

SUMMARY
Under Sleeper Pads are a state-of-the-art
technical solution reducing both lifecycle
costs as well as reducing vibrations in railway
tracks. Being already a standard product incountries like Austria, Germany, France and
Italy, in recent years the positive effects of USP
demonstrated also led to introduction of theseelastic elements in the UK.
As of 2018 more than 200.000 sleepers in the
UK (mainline and urban railway) have beenequipped with polyurethane USP. The biggest
number of these USP have been installed for
LCC reduction reasons; a smaller number forvibration isolation purposes. This development
can also be observed worldwide, as the
adoption of USP into best practices for railway
superstructure construction continues tospread across all continents.

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To sum up all theoretical calculations andpractical experiences, it can be stated that only
concrete sleepers with Under Sleeper Pads fulfil
the demand for a more sustainable and costeffective superstructure.

FURTHER LITERATURE
Veit, P.; Marschnig St.: Under Sleeper Pads –Economic Evaluation, TU Graz, project reportfor UIC working group, 2012
Veit, P.; Marschnig, St.; Berghold, A.:
WINS – Wirtschaftlicher Nutzen vonSchwellenbesohlungen, project report forAustrian Federal Railways, 2010, Graz
Marschnig, St.; Berghold, A.: Besohlte
Schwellen im netzweiten Einsatz, ZEVrail, 2011,
BerlinFigure 8: Vibrations and structure-borne noise in the surroundings ofrailway lines

Auer, F.: Zur Verschleißreduktion von Gleisen in
engen Bögen, doctoral thesis, 2010, Graz
Auer, F.: Der Einfluss von elastischen
Komponenten auf das Verschleißverhalten
von Gleisen in engen Bögen, paper at 39th
Conference Modern Rolling Stock, 2010, Graz
Loy, H; Combating structure-borne noise andvibrations by putting pads on sleepers – theireffect and limitations, ETR Austria, December
2012
DIN 45673-3: Mechanical vibration – resilient
elements used in railway tracks – Part 3:
Experimental evaluation of insertion loss ofmounted track systems
Loy., H; Kwiatkowska, E.; Biskup, M; Measuring
the vibration-isolating effect of an elastic railway
superstructure in Poland, Global Railway
Review, Volume 24, Issue 1, 2018
Potocan, S; Vibration protection for
Birmingham’s Arena Tunnel, European RailTechnology Review, 2011

Figure 9: Measured insertion loss curve of superstructure with USP in
Krakow, Poland

Figure 10: concrete sleeper with USP including a special elastic bearing on both front ends of the sleeper for
additional vibration isolation (left: schematic 3D image, right: photograph).
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