Under sleeper pads

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Transcript of Under sleeper pads

  • Lakui, S., Ahac, M., Haladin, I.: Poskusno preverjanje tira z elastomernimi podlokami pod pragi

    Experimental investigation of railway track with under sleeper pad

    prof.dr.sc. Stjepan Lakui, 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

    Obiajna klasina tirna konstrukcija je sestavljena iz tirnic, elastinega pribora za privr-stitev ter praviloma prenih betonskih pragov vgrajenih v tirno gredo. V primerjavi s tiri na le-senih pragih imajo takni tiri manjo elastinost na stiku praga in tirne grede. Tirna greda pred-stavlja najibkeji len v celi konstrukciji tira, ker zaradi delovanja dinaminih sil na tir prihaja do premikov zrn v tirni gredi kar lahko povzroi troenje in razpadanje tolenca. Reitev tega problema je v modifikaciji karakteristik dinaminega sistema vozilo tir z vgraditvijo elastinih elementov, ki zagotavljajo bolj ugodno razporeditev obtebe v tiru. Razen obiajnih elastinih podlok pod tirnicam se edalje ve vgrajujejo specialne elastine podloge pod tirno gredo, ter posebej elastomerne podloke pod pragi. V tem prispevku je prikazana poskusna raziskava tira z in brez elastomernih podlok pod pragi z ozirom na intenziteto vibracij. Rezultati raziskave so pokazali, da se vibracije tira z elastomernimi podlokami pod pragi zmanjajo tudi do 30 % kar je posebej pomembno na obmojih ki so obutljiva na hrup in vibracije, pri kretnicah, kriiih ter pri tirih na mostovih.

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

    10. SLOVENSKI KONGRES O CESTAH IN PROMETU, Portoro, 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.

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

    10. SLOVENSKI KONGRES O CESTAH IN PROMETU, Portoro, 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 510 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 difficul