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Page 1: Under sleeper pads

Lakuši�, S., Ahac, M., Haladin, I.: Poskusno preverjanje tira z elastomernimi podloškami pod pragi

Experimental investigation of railway track with under sleeper pad

prof.dr.sc. Stjepan Lakuši�, dipl.ing.gra�. Maja Ahac, dipl.ing.gra�. Ivo Haladin, dipl.ing.gra�.

University of Zagreb, Faculty of Civil Engineering, Department of Trans-portation, Chair for Railways, Zagreb, Croatia

Abstract

The conventional railway superstructure consists of rails, elastic fastening systems and, in general, transverse concrete sleepers supported on ballast bed. Such tracks, in comparison with the structures on timber sleepers, have less contact elasticity between sleepers and ballast mate-rial. Ballast bed represents the weakest link in the entire track construction because of the crushed stone movements during dynamic loads. Their motion causes extensive abrasive wear on the contact areas of single stones and also breakings or even pulverization of some stones. The solution to this problem lies in the modification of the dynamic characteristics of vehicle-track system by installation of elastic elements that will ensure better load distribution in the track structure. Today, besides the usual resiliant rail pads and baseplate pads, there is an increased use of sub-ballast mats and especially under sleeper pads in the track structure. This paper pre-sents experimental investigation of vibration intensity carried out on the track with and without under sleeper pads. The results showed that the vibrations on track with under sleeper pads de-creased up to 30%, which is particularly important in areas sensitive to noise and vibration, at crossings, turnouts and railway tracks on bridges.

Povzetek

Obi�ajna klasi�na tirna konstrukcija je sestavljena iz tirnic, elasti�nega pribora za pri�vr-stitev ter praviloma pre�nih betonskih pragov vgrajenih v tirno gredo. V primerjavi s tiri na le-senih pragih imajo takšni tiri manjšo elasti�nost na stiku praga in tirne grede. Tirna greda pred-stavlja najšibkejši �len v celi konstrukciji tira, ker zaradi delovanja dinami�nih sil na tir prihaja do premikov zrn v tirni gredi kar lahko povzro�i trošenje in razpadanje tol�enca. Rešitev tega problema je v modifikaciji karakteristik dinami�nega sistema vozilo – tir z vgraditvijo elasti�nih elementov, ki zagotavljajo bolj ugodno razporeditev obtežbe v tiru. Razen obi�ajnih elasti�nih podlošk pod tirnicam se �edalje ve� vgrajujejo specialne elasti�ne podloge pod tirno gredo, ter posebej elastomerne podloške pod pragi. V tem prispevku je prikazana poskusna raziskava tira z in brez elastomernih podlošk pod pragi z ozirom na intenziteto vibracij. Rezultati raziskave so pokazali, da se vibracije tira z elastomernimi podloškami pod pragi zmanjšajo tudi do 30 % kar je posebej pomembno na obmo�jih ki so ob�utljiva na hrup in vibracije, pri kretnicah, križiš�ih ter pri tirih na mostovih.

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Lakuši�, S., Ahac, M., Haladin, I.: Experimental investigation of railway track with under sleeper pad

10. SLOVENSKI KONGRES O CESTAH IN PROMETU, Portorož, 20. – 22. oktobra 2010 ����

1 Introduction The conventional railway superstructure

consists of rails, elastic fastening systems and, in general, transverse concrete sleepers sup-ported on ballast bed or attached to some sort of stiff substructure. The basic requirement placed on each of these elements is as bal-anced and flexible load transfer from the ve-hicle through rails on lower layers of track structure with minimal construction and maintenance costs. When transferring the load there is a gradual decrease in the intensity of stresses as the load is transferred to the grow-ing contact surface between each lower layer of track structure. In addition to the size of contact area, stiffness of individual elements of track structure plays a significant role in the load transmission, i.e. the ability of these elements to damp the dynamic excitation (vibration) of the rails caused by passing ve-hicles.

2 Effects of concrete sleepers application Modern railway track with concrete

sleepers and ballast bed made of coarse stones, compared with the structures with timber sleepers, have two significant draw-backs:

� overall stiffness of the permanent way is higher,

� contact elasticity between the sleepers and ballast stones is lower.

Generally it can be argued that increas-ing the stiffness of the permanent way results in an increase of dynamic forces in the track structure. This statement must be taken with some caution, since the intensity of the dy-namic forces depends on the load frequency and total weight (damping possibility) of the whole structure.

Construction of railway tracks with con-crete sleepers of relatively high stiffness has resulted in changes in load distribution in ballast bed just below the sleepers. Ballast bed represents the weakest link in the entire track structure because of the crushed stone latent movements during dynamic loads which causes its compaction and abrasive wear. More pronounced and faster ballast stone wear is the consequence of the in-creased vehicle speed and axle loads, and also

lesser deflection of the rails due to increased overall stiffness of the permanent way.

Unlike the timber sleepers on which the largest bending moments and vertical deflec-tions occur in zones directly beneath the rails, in the case of concrete sleepers the largest deflection is occurring at their ends. These intense vertical displacements cause move-ments of coarse stones, creating a gap in the ballast bed just below sleeper ends. The con-sequence of this asymmetric settlement of ballast stones is decreased stability of the sleeper that leads to its tilting during vehicle passage. This tilting can cause significant damage to the sleeper itself and increased abrasive wear of coarse stones in its immedi-ate vicinity. Such a rapid decline of the track quality requires the implementation of ad-verse measures for ensuring safe and com-fortable ride, including operational speed limit, reduction of static and dynamic loads (reduction in axle loads, which can negatively affect railways capacity), and shortening the track geometry maintenance cycle thus in-creasing the maintenance costs. Due to their higher stiffness compared to timber, concrete sleepers have less ability to damp vibrations. Studies have shown that installation of con-crete sleepers can increase the track vibrations up to 5 dB [1].

Foreign experience shows that the solu-tion to these problems may lie in modification of the vehicle-track dynamic system charac-teristics in order to reduce stiffness of the track and in the installation of resilient ele-ments that would allow better load distribu-tion in the track. On high speed lines, beside the conventional resilient under baseplate pads and elastic rail fastening systems, there is an increased use of sub-ballast mats which reduce transmission of vibrations from the ballast bed to the track substructure and the subsoil. Disadvantage of such mats are their relatively high cost and difficulties during their installation (difficulties in proper com-pacting of ballast material) [2]. Lately, instal-lation of elastic, soft material between the sleepers and ballast material has developed as an alternative to sub-ballast mats and/or addi-tional possibility to increase the elasticity of the track.

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Lakuši�, S., Ahac, M., Haladin, I.: Experimental investigation of railway track with under sleeper pad

10. SLOVENSKI KONGRES O CESTAH IN PROMETU, Portorož, 20. – 22. oktobra 2010 ����

3 Resilient under sleeper pads Under sleeper pads are resilient pads at-

tached to the underside of sleepers to provide an intermediate elastic layer between the sleeper and the ballast (Figure 1). Swiss Railways (SBB) were first to use such pads in year 1986 [2]. In relation to the sub-ballast mats, this solution allows damping of vibra-tions in the upper parts of the track. The basic function of these pads is to reduce the effect of unequal stiffness of the track on wheel-rail contact forces and increase the surface area through which sleepers distribute stresses to ballast. Installation of under sleeper pads causes decrease in resonance frequency of permanent way and thus reduction of vibra-tion transmission from the sleepers to the ballast bed and track substructure.

Use of under sleeper pads enables: � reduction of forces transmitted from

sleepers to the lower parts of track structure (reduction of load that is transferred to the ballast bed is between 20 and 30%);

� reduction of stresses in rails and sleep-ers ensuring better and more uniform load distribution (reducing stress in the ballast bed up to 40%);

� increase of lateral track resistance (up to 9%);

� increase of the track elasticity and ver-tical displacements (deflections);

� improvements in track geometry; � reduction of ballast depth (up to 10 cm,

locally); � reduction of long pitch rail corrugations

in tight radius curves (up to 50%); � longer cycles of regular track mainte-

nance by tamping and reduction of op-erational and maintenance costs, espe-cially in the case of railway lines with heavy traffic load (2 to 2.5 times longer periods between tamping);

� reduction of vibrations with frequen-cies larger than 40 Hz (from 8 to 15 dB, up to 30% less vibrations);

� increase in travel comfort [3].

Figure 1. Ballasted track with under sleeper pads Under sleeper pads have been used for

special applications in track structures for about 20 years. However, during the last 5–10 years, a wider use of under sleeper pads has developed in most of the countries of central and western Europe, mainly because of the increased high speed lines construction. Sleepers with pads have been in use:

� in turnouts and crossings (at the places where the acces for tamping is difficult, in order to reduce the degradation of ballast material);

� under concrete bearers used in special locations such as expansion joints at the end of long bridges;

� on short bridges, and on the approach to, and running off bridges;

� in transition areas between ballasted and ballastless track;

� in order to reduce structure-borne noise and vibrations [4].

4 Vibration measurement The positive experiences of European

railway administrations on the implementa-tion of under sleeper pads have motivated Department of Transportation Engineering at the Faculty of Civil Engineering to implement proper investigation of the influence that sleeper installation mode has on the intensity of vibrations. This paper presents an overview of the proceedings and results of vibration measurements conducted in June 2009. at the testing site at „Beton Lu�ko“ company facili-ties. The aim was to investigate the vibrations damping capability of the individual compo-nents of track structure.

At the site, two test structures were placed on top of the 30 cm thick concrete slab, each structure consisting of two rails (60 E1) 1.20 m in length, attached to two concrete sleepers (PB-85-K) by elastic fastenings

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Lakuši�, S., Ahac, M., Haladin, I.: Experimental investigation of railway track with under sleeper pad

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(SKL-1). These structures differed by the type of sleepers bedding:

� structure TYPE 1 – sleepers are layed directly at the concrete slab,

� structure TYPE 2 – sleepers are layed in 35 cm thick ballast bed covering the concrete slab;

� and by the elasticity of contact between sleepers and structures lower layer:

� structure TYPE A – sleepers are layed directly at the the lower layer (concrete slab or ballast material),

� structure TYPE B – between the sleep-ers and the lower layer (concrete slab

or ballast material) there is a 3 cm thick layer of rubber material (under sleeper pad).

Material selected for the construction of under sleeper pad is a two-component mass for casting and/or injecting based on polyure-thane resins (Icosit KC 340 / 7), Shore stiff-ness 75±5 SH(A) [5]. This material has been applied in rail traffic for many years, primar-ily as a vibrations absorber and fastening material between the track superstructure and solid surfaces (concrete or steel bridge decks, concrete surfaces in tunnels) and also at crossings to ensure accurate track geometry.

Figure 2. Four different types of test structures

4.1 Concrete slab vibration

measurement procedure Due to the limited length of the rails

(1.20 m') there could be no real vehicle load-ing at the testing site. Therefore, vibration measurements of the tracks were conducted during the effect of impact load which simu-lates the wheel-rail contact forces due to ir-regularities of their running surface. Impact was achieved by lowering the weight of 3.66 kg (36.6 N) on the rail head, just above the fastenings. Weight was dropped down using guides from the heights of 5, 25 and 100 cm.

To obtain a more accurate assessment of the behavior of track structures during impact, simultaneous measurements of vertical accel-eration of rail (A-1), sleeper (A-2) and con-

crete slab (A-3) were carried out using three triaxial accelerometers placed as showed at the scheme in Figure 3. Measurement and analysis of results were preformed by using PULSE measurement system by Brüel & Kjær.

Figure 3. Scheme of accelerometers placement and impact force location

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Lakuši�, S., Ahac, M., Haladin, I.: Experimental investigation of railway track with under sleeper pad

10. SLOVENSKI KONGRES O CESTAH IN PROMETU, Portorož, 20. – 22. oktobra 2010 ����

Figure 4. Test structures TYPE A1 and B1 (left), TYPE A2 and B2 (right)

4.2 Results of concrete slab vibration measurements

Figure 5 shows time records of vertical vibrations (mm/s²) of concrete slab, measured by accelerometer A-3 during lowering of weight from 25 cm hight on the structures TYPE A1 and B1 rail head. It is evident that in the case of structure TYPE B1 (structure

with integrated elastomeric sleeper pad), ver-tical vibrations of concrete slab are less in amplitude and shorter in time. Similar results were obtained during all measurements, based on which it was concluded that the elastic sleeper pad really has a significant impact on reduction of overall vibration caused by im-pact load.

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Figure 5. Time records of vibration accelerations (mm/s²) measured by accelerometer A-3 placed on concrete slab of structure TYPE A1 (left) and TYPE B1 (right)

4.3 Analysis of the results of concrete slab vibration measurement

After integration of the recorded vibra-tion signals the effective value of vibration velocity (mm/s) in the frequency range from 1 Hz to 1 kHz and from 5 Hz to 250 Hz were

analyzed (Table 1 and 2). Figures 6 and 7 shows the third-octave spectrums of vibration velocities determined at the moments of maximum measured vibrations during lower-ing the weight from 25 cm at rail head for each different structure type.

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Lakuši�, S., Ahac, M., Haladin, I.: Experimental investigation of railway track with under sleeper pad

10. SLOVENSKI KONGRES O CESTAH IN PROMETU, Portorož, 20. – 22. oktobra 2010 ����

Autospectrum(PODLOGA) - InputPulse Time : Input : Input : CPB Analyzer

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Figure 6. Vibration velocity (mm/s) of concrete slab of structure TYPE A1 (left) and TYPE B1 (right)

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Figure 7. Vibration velocity (mm/s) of concrete slab of structure TYPE A2 (left) and TYPE B2 (right)

Table 1. Effective values of vibration velocity (mm/s) – TYPE 1

Frequency range TYPE A1 [mm/s]

TYPE B1 [mm/s] TYPE A1/ TYPE B1 Vibration reduction

[dB] 1Hz – 1 kHz 1.560 0.211 7.393 17.4 5Hz – 250 Hz 0.556 0.088 6.318 16.0

Table 2. Effective values of vibration velocity (mm/s) – TYPE 2

Frequency range TYPE A2 [mm/s]

TYPE B2 [mm/s] TYPE A2/ TYPE B2 Vibration reduction

[dB] 1Hz – 1 kHz 0.082 0.012 6.833 16.7 5Hz – 250 Hz 0.017 0.005 3.400 10.6

Comparison of the values obtained by

measuring vibration acceleration of concrete slab in logarithmic scale (Figure 8), for all four observed types of track structure, con-firmed the investigations presumption - intro-duction of flexible elements in the track struc-ture and reduction of stiffness by introducing elastomeric pads below sleepers and/or con-struction of ballast bed of sufficient thickness, decreases the transfer of vibrations from the sleepers to the lower parts of the track struc-ture.

Comparison of difference in vibration values (Figure 9) between the structures TYPE A1 and B1 showed that the installation of under sleeper pads can cause a small in-crease in the intensity of low frequency vibra-tions. This result conforms with the experi-ences and results of previous studies con-ducted by a number of European railway ad-ministrations. It is necessary to emphasize that the cause of this increased values of low frequency vibrations at tracks with under sleeper pads has not yet been cleared.

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Lakuši�, S., Ahac, M., Haladin, I.: Experimental investigation of railway track with under sleeper pad

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Figure 9. Difference in concrete slab vibrations values for test track structures with (B) and without (A) under sleeper pad

5 Conclusion Detailed knowledge about the behavior

of the railway track subjected to vertical dy-namic effects is necessary for precise deter-mination of stresses that occur in the tracks superstructure and substructure and of noise and vibrations that occur as their conse-quence. The size and transfer of these effects can be significantly affected by the introduc-tion of flexible elements in the track structure. Effect of elastic components in terms of their ability to damp dynamic effects depends on their position in the track structure and dy-namic characteristics (mainly hardness). The great advantage of applying under sleeper

pads lies in their compatibility with traditional fastening systems and concrete sleepers, and also conventional procedures and equipment for building and maintaining tracks.

From experiences gathered during meas-urements and analysis described in this paper, it can be concluded that the use of under sleeper pads in modern track construction enables significant improvements in terms of decreasing dynamic impacts on track and thus reduction of construction and maintenance costs. All the conclusions about the advan-tages and disadvantages of introducing under sleeper pads into the track structure have not yet been made at the European level, but so far positive experience in their application

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Lakuši�, S., Ahac, M., Haladin, I.: Experimental investigation of railway track with under sleeper pad

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shows that the installation of sleepers with elastic pad should become a common practice during reconstruction of existing and con-struction of new high speed railway lines.

References

[1]www.commentmgr.com/projects/1212/doc

s/CH-4.8%20Vibration.PDF [2] Plasek,O., Salajka, V., Hruzikova, M., Vymla-

til, P.: The Static Analysis of Track with Under Sleeper Pads, Railway Engineering Confer-

ence 2007, (ur: Forde, M.C.), London, UK, lipanj 2007.

[3] Lakuši�, S., Ahac, M.: Betonski pragovi s elasti�nim podloškama, Prometnice - nove tehnologije i materijali, (ed. Lakuši�, Stjepan), Zagreb: Gra�evinski fakultet Sveu�ilišta u Za-grebu, 2010.. Str. 579-616.

[4] UIC Project No. I/05/U/440, USP - Under Sleeper Pads, Summarising Report (4th Edi-tion), Be�, Austrija, ožujak 2009.

[5] www.epms-supplies.co.uk/p-Icosit-KC-340/7-(182).aspx