INTRO: Trials in Namibia have proved that using specialised sleepers to elevate the rail foot above the ballast bed can keep the rail head free of wind-blown sand
BYLINE: Prof Dipl-Ing Dr techn Klaus Rießberger
Graz University of Technology, Austria
Chief Engineer, TransNamib
TRADITIONAL ballasted track was not designed for use in the desert climates that exist on all continents, where arid conditions and near-continuous winds can lead to sand drifts blocking railway lines.
Unlike snow which tends to melt under pressure, sand is able to withstand high wheel loads, and the risk of derailment therefore increases dramatically above a certain level of coverage. Sand accumulating against the side of the rail tends to be blown over the rail head, and while this improves adhesion conditions, it also increases friction and therefore wheel and rail wear.
These conditions are often exacerbated by the infrequent train service on railways serving sparsely-populated desert areas, as the natural track-clearing action of a moving train does not apply. When trains do operate they must often proceed at a reduced speed with the crew keeping a close watch out for sand drifts, usually cleared by hand, although track machines are sometimes used.
Remedies have included erecting barriers to prevent the sand from reaching the track, but this is not a simple task and usually only effective for a limited time. A better solution is to elevate the rail to a level higher than that usually found on traditional ballasted track, creating a gap between the rail foot and the ballast. Unlike a continuous opening, in accordance with the laws of fluid mechanics a series of such gaps accelerates the air flow with a higher potential for carrying grains of sand, creating a self-clearing effect. The gaps also mean that drifting sand is not forced to flow onto the rail head, but is carried under the rails and across the track, reducing wheel and rail wear.
Smoothing the flow
The ’sand’ sleeper designed for desert conditions has two rail chairs between 100mm and 120mm in height, cone-shaped with an elliptical cross-section to optimise performance whatever the wind direction. Careful consideration was given to avoiding any sharp corners or edges which might disrupt the smooth air flow and cause drifting sand to accumulate.
Wind flow was tested on a one-third scale model in a water tank at Graz, with water speeds simulating various wind velocities approaching the track axis from different directions. The sand content was also varied1.
So far, the sand sleeper has been produced with a prestressed concrete beam, with rail chairs having slack steel reinforcement. For initial trials the Fist type of direct-mounted, elastic fastening was adopted, as it is used widely throughout southern Africa and has useful properties including a comparatively high moment around the y axis, activated by traction and braking forces.
In 1999 an 80m section of track on the TransNamib network was laid with sand sleepers manufactured by Grinacker of South Africa. To compare the performance of the design, a section was laid with a type of sleeper used on dockside lines that also has an elevated rail position but more angular chairs. A third section was laid on larger, conventional sleepers, but with a lower ballast top between the sleepers.
After three years in service during which several sandstorms took place, the design of the sand sleeper was revealed to be effective. As can be seen in the top photograph, its use keeps the track completely free of sand when drifts accumulate on the adjacent section laid on the larger sleepers, while the section of conventional track furthest from the camera is almost completely covered by sand.
The sand sleeper is also more effective that the similar dockside sleeper, whose sharp corners cause turbulence and the accumulation of sand deposits. Confirming the results of water tank trials, the accelerated air flow under the rail removes sand from even the top layers of ballast. Should a drift cover the rail, this wind action should clear sand once holes have been made in the drifts lying under the rail.
Even in extreme environments such as desert, solutions to the problems facing track engineers can be found by the considered application of the laws of physics. Further development of the sand sleeper concept should be warranted by the scale of the drift problem facing the world’s desert railways.
CAPTION: The effectiveness of the sand sleeper design can be seen in the foreground on the test section in Namibia. The less-effective conventional sleepers with lower ballast height can be seen in the background
CAPTION: The elliptical cross-section of the rail chairs of the sand sleeper (left) allows the sand to blow through the track below the rails. The design proved more successful than the angular ’dockside’ sleeper (below)
1. Ramon-Rosales R. Modellversuchen mit neuartigen Schwellen zur Reduzierung der Verwehungsgefahr von Eisenbahngleisen in Gegenden mit Flugsand. Diploma thesis, Graz University of Technology, 1995.