CAPACITYINTER-URBAN

DEMAND

Melbourne
Boring more tunnels torelieve existing routes

Rhein-Ruhr
A premium service forGermany’s largest conurbation

TokyoPlans are afoot to increase
services in the busy Kanto area

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December 2015

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TRAMS | LIGHT RAIL | METROS | COMMUTER RAIL

December 2015 | Metro Report International42

TECHNOLOGY Superstructure Renewal

Photos: L Alejandro Chércoles

Renewing track superstructures

Metro Madrid has developed a renewal method to transform its
conventional ballasted track into a mass-spring system to reduce thetransmission of vibrations into the ground and adjacent buildings.
L ALEJANDRO CHÉRCOLES ASENSIO,JUAN ANTONIO MÁRQUEZ PICÓN,JORGE BLANQUER and MARKUS
HEIM*

Exposed turnout
areas are fitted withsupporting tracks, sothat the ballast canbe extracted.

Turnouts are the mostmaintenance-intensive sections of track in a metro network, with attendant highcosts. They are also the most common
source of vibrations that may be felt byresidents and buildings in the immediate
vicinity. One way to reduce the maintenance requirements of turnouts andlimit vibration is to replace conventional
ballasted track by a mass-spring system.
Following positive experience with previous installations, Metro Madrid is replacing six more turnouts this year.
The Madrid metro covers 293 routekm and has around 300 stations. The superstructure to be renewed in the tunnels
mainly consists of conventional ballasted
track with wooden sleepers and 54E1
rails. The depth of the ballast is generally
250 mm to 400 mm below the bottom of

the sleeper, and trains travelling at up to60 km/h have an axleload of 13 tonnes.
Track renewals are taking place atnight to avoid disrupting operations.
The greatest challenge is the short window available for track work, of just
2½ h. This means that around 75 nights
are required to replace a 60 m to 80 m
turnout section on conventional ballasted track by a ballastless turnout witha mass-spring system. Limited space,poor light and no ventilation make thework more difficult.
Mass-spring systems
Vibrations from trains can cause
noise to be heard in nearby buildings,
and the problem is exacerbated if services are frequent. Various measures canbe taken to reduce the transmission ofstructure-borne noise, and the correct
one will depend on specific technicalFig 1. The single
degree of freedom
principles for massspring systems.

*L Alejandro Chércoles Asensio is Head of Studies, Projects& Renewals, Track Section, at Metro Madrid. Juan Antonio
Márquez Picón is Head of Engineering & Maintenance of Civil
Works, Architecture, Track, Cleaning & Environment at Metro
Madrid. Jorge Blanquer is Head of the Track Engineering &Maintenance Department at Metro Madrid. Markus Heim isDevelopment Engineer, System Development Department, atGetzner Werkstoffe.

and economic factors. As a general rule,
the requirements are defined by national standards that limit noise and vibration. Installing under-ballast mats isthe most common method of reducing
vibrations from ballasted track, whereas
elastic rail pads or baseplate pads are often favoured for ballastless track.
The most effective method, although
the most costly, is to install a mass-spring
system. This entails the elastic decoupling of the entire slab from the subsoil.
As well as reduced vibration emissions,a mass-spring system has several advantages over ballasted track. These include
a longer service life with less onerous
maintenance requirements and lower
life-cycle costs, plus improved track
quality — which means a smoother ride.
The vibration-isolating effect of thesuperstructure is best explained using
the single degree of freedom principle(Fig 1)1. The theoretical system frequency is derived from the dynamic
effective mass of the slab and the train,
as well as the dynamic stiffness of theelastic bearing. The mass of the slab isusually predetermined at the planningstage, so the dynamic co-ordination isusually effected through the vibrationrelated properties of the elastomer.
To ensure the vibration isolation is aseffective as possible, the system frequency should be significantly lower than theexcitation frequencies. Irrespective of thetype of elastic bearing used, the vibration
isolating effect is only triggered when
the ratio of the excitation frequency tothe natural frequency is greater than √2.
If this ratio is smaller, the dynamic force
is distributed more intensely into thesubsoil. The system damping comprises
the damping of the elastic bearing andall the other damping mechanisms. Theisolating effect is usually expressed as alogarithm of the ratio between the input
and output forces.
Getzner Werkstoffe’s Sylomer polyurethane elastomer is being used forthe elastic base and side mats. The mat
is 25 mm thick; with a slab thicknessof 400 mm and an unsprung wheelset

mM

Mass of the system, from which theexcitation is emittedmZ

Additional massm

Total mass, composed of the mass of thesystem mM and the additional mass mZ

kSpring stiffnessG'DPSLQJFRHI¿FLHQW

FE(t) Exciting forceFF(t) Ground force, composed of the spring force
FK(t) and the damping force Fd(t)Metro Report International | December 2015

43

Superstructure Renewal TECHNOLOGY

The vibrationsuppressing matsbend easily andcan be cut using
conventional knives,
making them simple
to install.

Measuring results
The effectiveness of the vibration isolation measure used in a mass-spring
system can be estimated accurately using the single degree of freedom principle. Irrespective of the local conditions,the insertion loss is determined only by
the natural frequency and system damping. The measure is designed based on
the dynamic effective masses, which areusually predetermined, and the choiceof elastic bearing. More complex forecasts take into account other conditions,such as the stiffness of the tunnel invert,
the tunnel cross-section, the kind of soil
above the tunnel, the retroactive effectsof the elastic track on the vehicle andthe abrupt load generated in turnouts
when the train travels over the frog.
In Madrid, vibrations were measured at the street surface immediately
above the turnout frog at two different
turnouts before and after renovation.
Because of the gaps in the rails, abrupt
broadband track excitations above thefrequency range are to be expected70

4. Wettschureck R G, Heim M and Tecklenburg M. Langzeit-Eigenschaften der Unterschottermatten
im Münchner S-Bahntunnel nahe der Philharmonie Am Gasteig. Verkehr und Technik 57, Vol 1,
pp3-9, 2004.

8. Chércoles Asensio L A. Internal vibration measurement report, Metro Madrid, 2013.
9. Chércoles Asensio L A. Internal vibration measurement report, Metro Madrid, 2014.

403020Before renewal
After renewal

1081631Â5 63 125 250 500
Frequency Hz70

3. Wettschureck R G, Heim M and Tecklenburg M. Long-term efficiency of ballast mats installed inthe rapid transit railway tunnel near the Philharmonic Hall of Munich, Germany. Proceedings ofthe 10th International Congress on Sound and Vibration, Stockholm, pp403-410, 2003.

7. Daiminger W and Dold M. Langzeitqualität von Unterschottermatten nach 30 Jahren. Der
Eisenbahningenieur, Vol 6, 2014.

50

4

2. Wettschureck R G, Heim M and Tecklenburg M. Long-term properties of Sylomer ballast matsinstalled in the rapid transit railway tunnel near the Philharmonic Hall of Munich, Germany. RailEngineering International, Vol 4, pp6-11, 2002.

6. Langzeitreferenz vollflächiges Masse-Feder-System. Getzner Werkstoffe internal Case Study, 2013.

600References
1. Vibration insulation — Terms and methods. Verein Deutscher Ingenieure 2062 Blatt 2, 2011.

5. Dold M and Potocan S. Langzeitverhalten von Unterschottermatten aus Sylomer. Der
Eisenbahningenieur, Vol 11, 2012.

when trains pass over it. Before renovation, maximum vibration velocities ofmore than 0·2 mm/sec were recorded,
whereas measurements taken afterwards show a tenfold decrease.
Vibration velocity reduction of 8 to
16 dB was achieved for the frequency
range critical for secondary airborne
noise, 40 to 160 Hz, equating to a vibration-damping efficiency of 60% to 84%.
In the meantime, five 60 m to 80 m
turnouts in tunnel areas of the Metro
Madrid ballasted track have been converted into vibration-reducing massspring systems with no interruption toservices. Four further turnouts have been
converted but these required train services to be interrupted. Due to the extremely positive experiences, another four
turnouts in the Madrid metro network
are being converted this year, and Metro
Madrid expects to continue this renewalschedule in the coming years. Q

Vibration velocity level db re. 5x10-8 m/s

Renewal process
For safety reasons, speed restrictions
must be enforced over the section while
work is in progress, and the track should
be kept under close observation. Aftera topographic survey, work can begin
on removing the turnout and installinga supporting skeleton for both tracks
using customised metal isolated sleepers. The ballast can then be extracted,
leaving the track skeleton supportedwith concrete blocks and secured horizontally with wooden wedges or struts.
Before installing the elastic bearing, acompensation layer may need to be applied to the tunnel floor.
Installing the elastic bearing involves
filling the gaps that are opened up on thetunnel floor with polyurethane elastomer
mats. Full functionality of the massspring system can only be guaranteed if
the elastic isolation layer is fully formed.
It is therefore essential that a high levelof quality is maintained when forming
and working with the elastic isolation toavoid structure-borne noise bridges.
Once the mats have been laid on all
the lateral surfaces such as the support

structure and the shafts, the reinforcement can be installed and the initial 100
to 200 mm concrete support slab produced. The concrete is poured into thesubsoil through air shafts. Next, the turnout sleepers, rail fastenings and remaining
components are pre-assembled. Once theshuttering is complete for the longitudinal and lateral dewatering, shafts and signalling devices, the track panel is adjusted
and levelled using a track geometry car.
To conclude this stage, the concrete support slab is built up to its full height inseveral phases. At this point, temporarily
loosening the fastening system removes
stresses between the rails and anchors theconcrete slab support.
The shuttering and temporary supporting blocks can then be removed. The
gaps created by the supporting blocks
are lined with elastomer mats and sealedwith concrete. The concrete support
slab is now finished and is a uniform
400 mm in height. Welding the rails andfinal cleaning conclude the work.

Fig 2. The 1/3-octave
band spectra ofvibration velocity
levels weremeasured before
and after turnouts
were replaced at
two locations.

Vibration velocity level db re. 5x10-8 m/smass of 15% for trains with a 13 tonne
axleload, a natural system frequency of
20 Hz was calculated.
Sylomer and Sylodyn have been used
as elastic bearings of footings for over
30 years2-7. The mats are supplied either
in 1·5 m wide rolls or bespoke plates
made in advance, which simplifies theirinstallation in some cases. They bend
easily and can be cut to size using conventional knives.
In addition to the horizontal basemats, which are subjected to pressure, thevertical side mats contribute to the overall vertical stiffness because of their shear
stress. If this detail is neglected, higher
natural frequencies are generated, which
reduces the vibration isolation. The matscan be installed as lost shuttering and for
additional protection must be fully bonded using adhesive tape partly laid withplastic film to prevent structure-borne
noise bridges forming later.

6050403020
Before renewal
After renewal

100481631Â5 63 125 250 500
Frequency Hz