INTRO: Longitudinal precast concrete beams laid in stone ballast provide continuous support to the rails and reduced ground pressure for the same overall weight as conventional sleepers
BYLINE: Dr Eng Hajime Wakui
Chief EngineerStructural Engineering GroupJapan Railway Technical Research Institute
FROM THE 1940s to the 1960s, experiments were carried out in France, Japan and the Soviet Union with longitudinal sleepers laid in parallel pairs under the rails. None was successful, and the idea was abandoned in favour of individual transverse concrete sleepers replacing wood. Twin-block concrete sleepers joined by a steel tiebar are also in common use.
In 1993, the Structural Engineering Group of Japan’s Railway Technical Research Institute (RTRI) examined the reasons for the failure of longitudinal sleepers. It was concluded that these pioneering designs had not paid enough attention to maintaining the track gauge, and they required a greater weight of concrete than transverse sleepers. Nevertheless, it was considered that the basic principle of placing parallel pairs of sleepers directly beneath the rails was sound.
Since then, ladder sleepers have been developed and tested in service. They were installed experimentally in February 1996 on a JR Freight line of 1067mm gauge after extensive tests at RTRI. The length of the experimental section is 100 m, and by June 1997 it had carried 20 million gross tonnes. Tests on a heavy haul version will start shortly at the Association of American Railroads’ Transportation Technology Centre near Pueblo in Colorado.
Several designs for different types of track structure have been developed using ladder sleepers. While most will be laid in stone ballast, designs include track resting on a mortar, asphalt or concrete base suitable for viaducts or in tunnels. Patents for ladder track have been filed in a number of countries.
Track can be tamped
Many attempts have been made to produce concrete trackbeds which require little or no maintenance. While these have been successfully used on structures, notably on Shinkansen routes in Japan and on metros in tunnel, permanent way engineers have been reluctant to abandon stone ballast because of the ease with which the line and level can be adjusted by tamping to compensate for the settlement of earthworks.
The fundamental difference between ladder sleepers and concrete trackbeds is that the former are designed to be laid in ballast and tamped by machine to maintain the required rail top level, and adjust the alignment if necessary. Obviously, the tamping heads have to be turned through 90°, but the sleepers are flexible enough to be lifted by a tamping machine and the principle is almost the same as for cross-sleepered track.
Pressure exerted on the ballast is much less variable, and as a consequence the rate of settlement is far lower than that experienced with conventional sleepers. The sleeper and rail act together as a composite beam which to some extent bridges across weak spots in the formation. All this points to a big reduction in track maintenance costs.
Ladder sleepers are manufactured as two longitudinal concrete beams joined by transverse steel pipes which act as gauge ties. The finished product looks like a ladder laid on the ground - hence the name.
The standard distance between two transverse pipes is 2·5m. The minimum unit length is 5m as obviously two pipes are necessary for structural and gauge stability. The maximum is 12·5m (five pipes) weighing about 4 tonnes, which is the most that can be conveniently handled at the work site.
At 625mm intervals, pairs of shoulders for Pandrol clips are cast into the top of the sleeper. The rail can therefore be supported continuously, except for a short distance at the sleeper ends. In addition to the rail pad, a continuous ballast mat can be laid under the sleeper.
The purpose in having different length units is to enable curved track to be laid. By using an adjustable insulated spacer between the Pandrol shoulder and the rail, or by using Pandrol shoulders on a moveable baseplate, a curve of 200 m radius can be laid using the 5m long units. The 12·5m units can be used on curves over 2 000 m radius.
Because of its excellent crack resistance, pretensioned partially-prestressed concrete (PPC) was adopted for the longitudinal beams. Newly developed indented prestressing wires of three or five strands, which have very good adhesion to the concrete, are arranged close to the top and bottom surfaces where cracks are most likely to occur (photo p589). Stress transfer between the indented wires and the concrete relies solely on this bond.
The dimensions of the longitudinal beam were mainly dictated by the need to incorporate the cast iron shoulders for the Pandrol clips within the cross-section, which is 400mm wide at the upper surface and 155mm deep under the rail seat (Fig 1).
The width is increased on the inside by 30mm where the pipes used as ties enter the beam. These pipes are 76mm in diameter and 9mm thick. They are deformed at the ends to prevent them from rotating or being pulled out of the concrete.
With the rail and sleeper acting as a continuous flexible beam, sleeper ends obviously become a weak point where differential movement has to be constrained.
RTRI is currently developing a number of types of joint. The aim is to connect the longitudinal beams within a galvanised steel channel using sufficient through bolts to resist the shear forces. However, at intervals it will be necessary to allow longitudinal expansion and contraction of the sleepers to take place at the joints.
Because of their configuration, ladder sleepers are highly resistant to track buckling in hot weather. However, they do not offer the same resistance to longitudinal creep as transverse sleepers. As experience shows this to be necessary, transverse steel plates which react against the ballast can be installed between the beams.
One benefit of the ladder sleeper is the ability to bridge across weak spots in the subgrade where partial subsidence has occurred. The effect of unsupported gaps 1·8 and 3·6m long symmetrically under both beams and asymmetrically under one side beam was analysed. These calculations were used to determine the crack resistance and the ultimate capacity of the longitudinal beam as well as the diameter and thickness of the pipe.
These are extreme cases, and serve to emphasise that ladder sleepers can enhance safety under conditions where the subgrade has partially collapsed, for example after a washout or earthquake.
Drop-weight impact tests were performed to simulate the effect of wheel flats. These confirmed that ladder sleepers had been correctly designed to withstand such impacts.
As regards ballast pressure, the much more even distribution achieved by ladder sleepers is well brought out by Fig 2. This shows the effect of a bogie exerting 80 kN static wheel load; maximum pressure immediately under the wheels is approximately halved, and the pressure gradient is much smoother.
It is therefore no surprise that repetitive loading tests showed settlement rates to be about eight times less under the middle of ladder sleepers compared with conventional track (Fig 3). However, settlement is greater at the ladder sleeper ends, emphasising the need for more development in this area.
Heavy haul possibilities
RTRI has produced a new design for ladder sleepers scaled up for use on heavy haul lines with up to 40 tonne axleloads. A 1067mm gauge version has already been installed within the steel works of Sumitomo Metal Industries in Japan where the axleload is 30 tonnes. The 40 tonne standard gauge version has been installed in the High Tonnage Loop at the AAR’s Pueblo test centre in Colorado.
On the new design, the beam width has been increased to 450mm. Rail fastenings are spaced at 750mm intervals, and as there are still four between each gauge tie, these are 3m apart. Sleepers are therefore manufactured in lengths of 6, 9 and 12m.
Other track forms
The fact that the rail and sleeper are in continuous contact and act together as a composite beam means that a smaller rail section could support for the same axleload, especially where a heavy rail section such as 70 kg/m is used.
The fact that rail is supported at intervals of about 700mm by transverse sleepers introduces vertical wheel motion at sleeper passing frequency, which can be a factor in generating corrugation and noise. Ladder sleepers should therefore reduce both noise and ground-borne vibration, especially when combined with resilient wheels.
On viaducts and in tunnels where it is desirable to lay a concrete trackbed, ladder sleepers still offer advantages. They can either be laid on a resilient mat on the track bed, or as a floating track insulated from the trackbed by discrete resilient mountings (left). o
CAPTION: A 12·5m long ladder sleeper panel placed on a levelled ballast bed; lifting rings are attached opposite the tubes forming the gauge ties
CAPTION: Tapered insulators with serrations are used to accommodate rail curvature
CAPTION: Arrangement of reinforcing and prestressing wires within a ladder sleeper
CAPTION: Fig 1. Dimensions of the ladder sleeper installed in 1067mm gauge track in Japan
CAPTION: Fig 2. Peak ballast pressures exerted by a bogie with an 80 kN static wheel load are halved by ladder sleepers and pressure gradients are smoother
CAPTION: Top: One of the designs being tested for joining ladder sleepers
Above: Steel plates can be placed between the longitudinal beams to prevent creep
CAPTION: Ladder sleepers designed for 40 tonne heavy haul axles are being tested at Pueblo
CAPTION: An experimental section of ’floating track’ laid at RTRI has resilient track supports under the ladder sleeper
CAPTION: Fig 3. Settlement rates are eight times lower under ladder sleepers than for conventional sleepers, but the improvement at joints is less pronounced
Les ’traverses en échelle’ donnent de bons résultats aux essais
Des dalles longitudinales préfabriquées en béton tenues à l’écartement voulu par des tubes en acier fournissent un support continu au rail et agissent avec le rail pour donner une dalle composite. Lorsque posées dans du ballast en pierre, elles donnent une pression au sol réduite et plus uniforme pour le même poids total que les traverses conventionnelles. Plusieurs types de voie en échelle sont proposés et une conception a été mise au point pour les lignes de transport lourd avec des charges par essieu de jusqu’à 40 tonnes’Leiterschwellen’ erbringen gute Testergebnisse
Betonfertiglängsträger, die mit Stahlrohren auf Abstand gehalten werden, bieten der Schiene fortwährenden Halt und haben mit ihr zusammen die Wirkung eines Verbundträgers. In Steinschotter gelegt, ergibt sich ein reduzierter und gleichmäßigerer Bodendruck bei gleichem Gesamtgewicht wie konventionelle Schwellen. Es wurden verschiedene Arten von Leitergleisen vorgeschlagen; ein Modell wurde für Schwerguttransportstrecken mit Radsatzlasten von bis zu 40Tonnen entwickeltBuen comportamiento de las ’vías escalera’ en pruebas
Existen unas vigas longitudinales de hormigón prearmado mantenidas en sus par