Ladder Sleepers Perform Well in Tests

The Structural Engineering Group of Japan¢ s Railway Technical Research Institute examined the reasons for the failure of longitudinal sleepers. It was concluded that these 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 JR Freight line.

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. 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 2.5 m. The minimum unit length is 5 m as obviously two pipes are necessary for structural and gauge stability. The maximum is 12.5m (five pipes) weighing about 4 tons, which is the most that can be conveniently handled at the work site.

Because of the 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. 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, much distribution is achieved by ladder sleepers. This shows the effect of a bogie exerting static wheel load; maximum pressure immediately under the wheels is approximately halved, and the pressure gradient is much smoother. Loading tests also showed that the settlement rates are 8 times lower under ladder sleepers than for conventional sleepers, but the improvement at joints is less pronounced.

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 700 mm by transverse sleepers introduced 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.