T R A CK ELASTOMER IC P ADS

30 years old, as good as new for 30 more
Measurements of the structure-borne noise show no
significant changes till now in the effectiveness of thisparticular method for reducing levels of structure-borne
noise.
Laboratory verifications

A 1200 x 1200 mm² sample of the product, taken at
approx. 18 cm of ballast depth, was subject to visual andlab checks to compare with as-installed parameters andincluded a DIN 45673-5fatigue test, plus static and dynamic stiffness. The under-ballast mats, lying in water on
the floor of the tunnel, were dried before testing. Surface
indentations caused by the ballast were clearly visible andthe load distribution layer in contact with the ballast wasin very good condition, exhibiting some insignificant
plastic ballast grain impressions, but was not damaged,
and no perforations. The increase in the contact area
minimises the specific load between the ballast and thefloor of the tunnel and the two resilient layers underneath
are also completely intact.
Excavated ballast showing intact mats

Wolfgang Daiminger
Müller-BBM,
MunichMirko DoldProductManagement,
Getzner Werkstoffe

Track engineers often express doubts about the longevityof properly engineered ballast mats, used to overcome
earth quality or, in this case, noise isolation problems. An
effective investigation on mats installed 30 years ago ina Munich tunnel for noise isolation of an underconstruction music hall have demonstrated that age need
not wither the physical characteristics or the functionalityof such mats. Technologies have further evolved in thesethree decades and current fitments can be lifelong.
Sylomer® B851 mats easily withstood the extremely highloading of more than 1,300 mt imposed on it and the highstructure-borne noise insulation requirements placed
continue to be fulfilled. With the mats immersed in waterand low ballast depth (min. 18 cm around the top of thesleeper), the resulting higher specific load and the mechanical stress apparently had no adverse effects on theeffectiveness of the under ballast mat. From this investigation and other long-term studies and that the measured
static/dynamic characteristics reveal only slight differences
over the when-installed values, it can be assumed that theunder ballast mats will remain fully functional for at least
another 30 years. If the under ballast mats were notpresent, higher operating costs would be incurred due tothe more frequent maintenance of the track and it is quitepossible that the entire superstructure would have had tobe replaced.
Getzner Werkstoffe Sylomer® (static bedding modulus
of 0.008 N/mm³) under ballast mats were retrofitted in1983 over a 345 m length in both tunnel sections using
a specially developed process. The superstructure consistsof standard ballasted track with S 54 rails, timber sleepers
at 60 cm intervals, about 30 cm ballast bed height under
the sleeper, axle load of 160 t and 80 kph maximum speed
using the E420 trainsets (since replaced by E 423). Study
results from a 2003 check are also available for comparison.

20 Rail Business [Focus-India] May 2016

Stiffness

200 x 200 mm² sample for static load deflection curve(load range up to 0.25 N/mm² at a 0.16 kN/s test velocity
revealed that the bedding modulus determined was within
the 1983 specification.
The dynamic stiffness, determined using the so-called
direct method according to ISO 10846-2 under the conditions specified lies just below that obtained in 2001.
The laboratory tests show that the effectiveness of thestructure-borne noise measurements carried out in 1983
remains undiminished.
For verifiable and reliable results, all the peripheral parameters that might influence the generation of structureborne noise like effects of consolidation, water, quality ofthe sleepers and rail fastening, must be kept as constantas possible. Special attention was paid to the roughnessof the rail surface that becomes a decisive factor whencomparing the measurements eg. short-pitched corrugation can increase the level of structure-borne noise by up
to 20 dB at a frequency of around 200 Hz. The rails inthe vicinity of the measuring points were simply reground,
not replaced.
The marks caused by the rotating grinding discs of therail grinding train were also clearly visible along the outerside of the rail beyond the running surface. A major change
in the peripheral conditions, however, was caused by thecomplete changeover in the type of train units, ET 420
only units were in use, but by 2003, all the light-rail traffic
was ET423 units, so the evaluation after 30 years of operation has to be confined to units of this type.
Comparison of static/dynamic stiffness characteristics
StaticDynamic

2012
0.00900.03922000
0.00870.03961983
0.00830.0388

Ballast mats: protecting their charges

The measurements

Rail grinding in Jan 2013 was followed by measurements in May,
with four points representing a selection of the 1983/2003 measuring points. It was possible to fit the sensors to the fastening
points as originally used in 1983 (aluminium plates bonded tothe tunnel wall).
The individual train movements were evaluated in the form of
Max-Hold third octave spectra (rms, time constant "Slow" 1
second), exactly the same as the one earlier. For each measuring
point, energetic mean values of the Max Hold third octave spectra
were calculated from the individually evaluated train movements,
with velocity level third octave spectra between 4 and 315 Hz.
The effectiveness (insertion loss) of the implemented measure,
represented by difference in velocity levels, is computed fromthe difference of the results before and after installation. Thearithmetic means of the velocity level differences for three measuring points, studied in 2001 and 2013, were also compared.
Positive values indicate a reduction, negative values an increasein the level of structure-borne noise (measurements in 1983 and2001 involved ET420 trainsets, whereas ET423s were in use by2013). However, a few movements with ET423s were also measured in 2001. The vibration results on the tunnel wall from the2001 measurements on ET423 units displayed lower levels in
the frequency range below the 63 Hz third octave band. In thehigher frequency range above the 63 Hz third octave band,
somewhat higher levels for movements involving ET420 units
were recorded. The differences in level between the results ofmovements with ET420 and ET423 units were determined andthe arithmetic mean calculated for the measuring points and one
reference point from 2001.
Clearly, ballast mats offer modern solutions to track issues
and wider application, with engineered solutions for specificproblems is a key option.