www.railwaygazette.com | April 2012
INDIAVinay Mittal
IR Chairman’s vision
for growth
PAGE 30

INDUSTRY FOCUS
CAFDeveloping a portfolio ofmodular designs
PAGE 52

Mitigating vibration using
under-sleeper pads
Reprint from Railway Gazette International 4/2012
Dipl.-Ing. Dr. Harald Loy

TRACK DESIGN | RESEARCH

Mitigating vibration using
under-sleeper pads
Installation of concrete sleepers fittedwith Sylomer pads.

ADDED BENEFITS Primarily used to improve the stability and life of ballasted
track, under-sleeper pads have been found to provide a cost-effective wayof reducing the transmission of ground-borne noise and vibration.
Dipl-Ing Dr Harald Loy
System Development
Getzner Werkstoffe

URight: Fig 1.
Transmissionof vibration inthe area aroundrailway lines.

40

nder-sleeper pads are primarily used to improve thestructure of ballasted track,helping to maintain an evensurface, improve the grip between ballast and concrete sleepers and avoid thedevelopment of hollow areas underneath the sleepers. However, ongoing
research suggests that these pads canalso ofer an economical way to mitigate problems of structure-borne noiseand vibration.
Noise and vibration are aninevitable but unwelcome side efectof our mobility. A moving train causes
mechanical vibration from wheel/rail contact, which is transmitted
through the subsoil. his can be very
annoying for residents in properties
alongside the line. he problem can be
aggravated if the vibration is increased
by resonance within the buildings,
or if structure-borne noise arises as aresult of higher-frequency vibration.
Secondary airborne noise emitted by
the structures can oten be heard as a
muled rumble inside the buildings.

Railway Gazette International | April 2012

his sound is sometimes enhanced by
the primary airborne noise on open
track (Fig 1).
Tackle the source

Where it is considered necessary
to reduce the impact of noise andvibration, particularly in urban areas,
it is widely accepted that it is mostefective to concentrate on reducing
vibration at the point of emission.

he quality of the railway
superstructure has a considerable
inluence on the generation ofvibration. he more even the structure,
the lower the excitation when a train
passes. In a conventional ballastedtrackform, the track itself is ‘loating’,
and repeated static and dynamic loads
result in changes to the geometry overtime. hese deviations lead in turn toadditional acceleration of the wheelsets,
and the resulting forces further alter
the quality of the track bed. Over time,
hollow areas appear below the sleepers(Fig 2) and the rail surface wears, whichfurther increase the rate of change. As
the system swings more and more,
emissions increase, until the structure
is returned to its original geometry bylining and tamping.
he time taken for this deterioration
process is largely dependent on
the initial quality of the track soestablishing as good and stable astructure as possible should be theprimary objective when laying track.
Elastic elements such as under-sleeper
pads can be a valuable contributor to ahigh-quality structure, ensuring evenness and resilience in the trackform.
In recent years, various European
railways have been adopting USPs,
mainly as a way to improve trackbed
geometry and protect the ballast.
Adding an elastic layer under concreteRESEARCH | TRACK DESIGN
Fig 2. Hollow
area formationis reduced byunder-sleeper
pads, thanks tomore consistent
deflection of
the concrete
sleepers.

sleepers avoids a hard interface withthe ballast, allowing the stones tobed into the padding material. hisincreases the surface area and avoids
excessive contact forces, leading toincreased stability, less settlement andreduced wear of the track components(RG 1.11 p42). Studies by the Technical
University of München showed thatpads with a bigger contact area ofer afurther increase in lateral resistance1.
Tests by Austrian Federal Railways
found that the use of USPs almost
completely eliminated the formationof hollow areas under the sleepers2.
Deviations in track geometry weresigniicantly reduced where pads wereused, and ÖBB therefore adopted themas standard for all new main line track
and turnouts.
Vibration insulation

As well as reducing noise andvibration by providing a smoothertrackbed, the highly elastic materials
can also reduce emissions signiicantly
thanks to their physical insulation
properties.
he eiciency of an elastic
component is mainly dependent on themass, stifness, and damping factors.
Ideally, the natural frequency of theresulting vibratory system should be
far lower than the insulating excitation
frequency. As elastic components, thepolyurethane materials Sylomer andSylodyn have been found efective in
reducing emissions. Depending on
local requirements, under-sleeper pads
made of such materials can be provided
with a more or less pronounced
damping component by adjusting
the dynamic stifness, in order toreduce excessive peaks or resonancein the vicinity of the natural frequency.
As a rule, the higher the dynamiceiciency achieved through the choiceof material, the greater the vibration
protection.
he vibration reduction of elastic
elements is quantiied in terms of
the insertion loss3. his describes therelative efect of a mitigation measureagainst a reference situation. For
example, it could show how the 1/3
octave band spectrum of structureborne noise would change if USPs wereinstalled, assuming all other inluences
remain constant, for example the samevehicle, speed and rail roughness.
As the elastic element inluences thesystem as a whole, the frequencydependent insertion loss could varywith other properties of the track,subgrade and/or vehicles.
Fig 3 shows a series of insertion loss
curves measured on diferent lines
with various types of polyurethane
under-sleeper pads. he natural
frequencies of the superstructureswith USPs generally lie between 30 Hz
and 40 Hz. In the relevant range for
emitted structure-borne noise and20

relevant

Insertion loss dB

reduction

secondary airborne noise, above 50 Hz
the insulation eiciency varies between
4 dB and 14 dB at 63 Hz across all typesof pad.
Values of 4 dB to 7 dB are attainablewith regular elastic pads. But toaddress structure-borne noise it isworth considering highly-elastic
pads, which can achieve insulation
eiciencies of 11 dB to 14 dB with an
acoustically-optimised superstructure.
Multiple measurements found thatthe performance of these types of pad
signiicantly exceeded expectations.
For ease of understanding, a 10 dB
improvement corresponds to an
insulation rate, or a reduction invibration, of 69%.
With frequencies below 50 Hz, thereis hardly any ampliication, whilst
below 25 Hz a further reduction of up
to 8 dB is apparent. his is probably
due mainly to the better bedding of thesleepers in the ballast.
Vibration insulation theory leads
us to expect a lowered eiciency
at some point due to the inluenceof soter rail pads, and this can befound between 100 Hz and 160 Hz.
Nevertheless, the reduction almost
always remains positive, without therisk of ampliication which had beenfeared.
heseindings
prove
thefundamental suitability of USPs for
reducing vibration and secondary
airborne noise, but selecting the right

Highly elastic USPs...
...
Elastic USPs
...
...
...
...
Average overall curve100

Left: Fig 3. Measured
insertion loss with various
types of under-sleeper pads.

Below: Fig 4. Use of undersleeper pads in the ballast
structure of a single-bore
tunnel.

amplification

-104

8

1631·5

63

125

250

Frequency Hz

Railway Gazette International | April 2012

41

dB

dB

TRACK DESIGN | RESEARCH

HzFig 5. Modelapproaches forforecasting theinsertion loss ofunder-sleeperpads. In thesemi-empiricalapproach (right),
Range 1 shows
the offset
for improved
trackbed
quality; Range
2 considers
the alleviativeinfluence of softrail pads; andRange 3 shows
an optional
declining curve
taking accountof the decreasing
influence of thesubgrade.

Fig 6. A sandwich
under-sleeper pad
using different
materials toprovide multiple
functional layers.

pads for the local application is vital,
both on open tracks and in tunnels(Fig 4).
Modelling performance

he measurements in Fig 3 showthat the efect of USPs is frequencydependent. One relatively simple option
to forecast the vibration mitigation
performance mathematically is to use
an impedance model3,4,5. Originallydesigned for ballast mats, this can inprinciple also be used for calculations
involving under-sleeper pads. heinsertion loss indicates the ratio ofthe vibration velocity amplitudes in
the ground for track without elastic
components to those when suchcomponents are installed.
In addition to the spring impedanceof the elastic material, the terminating
impedance of the subgrade is taken into
account. With a soter subgrade in thefrequency range greater than 125 Hz
this can typically lead to a declining
curve with a decreased efect in the 1/3
octave band spectrum, as shown on thelet in Fig 5. he possible reduction in
the insulation eiciency in the 100 Hz
to 160 Hz range with sot rail pads isnot seen. Nor does the model show
the positive changes measured in thefrequency range below 25 Hz thanks to
the improved trackbed.

Hz

Another approach for better
forecasting of track behaviour might beto adopt a ‘semi-empirical’ model using
multiple ranges (Fig 5, right). However,this has yet to be explored fully, and an
empirical model may require more sitemeasurements for calibration.
Primary airborne noise

Measurements suggest that the useof USPs has no signiicant impact
on the emission of primary airborne
noise. Although a short-term increaseamounting to a maximum of 5 dB
within the frequency range 50 Hz to
100 Hz must be expected following
installation, this is ofset by a reductionof up to 5 dB in the higher frequency
spectrum above 100 Hz6.
hese comparisons use a new track
without USP as a reference, and it
should be borne in mind that thegeometry of that track will deteriorate
more quickly, which could lead toa signiicant increase in airborne
noise. For example, rails with shortpitch corrugation may result in an
increase of more than 15 dB in noiselevels when a train passes. Given theimproved long-term quality of trackofered by the use of USPs, the shortterm impact on primary airborne noise
appears negligible, although further
investigation is needed.
Improved pads

Under-sleeper pads that arepredominantly used to improve
trackbed geometry ideally consist of a
resistant material with a visco-plastic
property. his allows the ballast stones
to become properly embedded, evenif the pads are very stif. On the other
hand, pads used primarily for vibration
insulation require a soter, dynamicallyhighly-elastic material with low
damping properties.
42Railway Gazette International | April 2012

Ideally, both approaches arenecessary for efective vibration
protection. A combination of diferent
materials could be used to reducethe creation of disruptive vibration
by ensuring a stable geometry, andreducing the transmission of vibration
at the same time using the physicalinsulation principles. Fig 6 shows sucha sandwich pad arrangement. he sotand acoustically highly-efective elastic
layer made from Sylodyn is embedded
between a mechanical adhesion meshnext to the concrete sleeper and a viscoplastic layer on the ballast side. hisseparates the various functions in orderto cater for the difering requirements.
Our research suggests thatappropriate under-sleeper pads canbe used to reduce vibration within thefrequency range relevant to structureborne noise by more than 10 dB.
Nevertheless, we have not establishedany signiicant impact on primaryairborne noise. Technically-optimised
pads ofer an economic improvement
for vibration protection in traditional
track structures, but the choice ofmaterial is of vital importance. heinitial results from using under-sleeper
pads to mitigate vibration have beenvery promising, and research into thisapplication is continuing. lReferences
1. Iliev D. Evaluations of sleepers equipped with USP– elasticity, contact stresses and lateral resistance.
Getzner Bahnfachtagung Schwarzenberg, October2011.
2. Auer F. The influence of elastic components on
track behaviour. ÖVG Tagung, Salzburg, Vol 104, p53.
3. DIN V 45673-4 (2008-07): Mechanical vibration –resilient elements used in railway tracks – Part 4:
analytical evaluation of insertion loss of mountedtrack systems.
4. Wettschureck R and Kurze U. EinfügungsdämmMaß von Unterschottermatten. Acustica Vol 58,
1985, p177.
5. Müller G and Möser M. Taschenbuch der Technischen Akustik. 3rd Edition, 2003, p545.
6. Behr W. Innovation Project LZarG – effect of different USP in real tracks. Getzner Bahnfachtagung
Schwarzenberg, October 2011.