TIMEOUT FOR TECH
TIES AND BALLAST
TIME-TESTED FOUNDATION Both are essential for a rugged, lasting railway. By Gary T. Fry, Ph.D., P.E., Vice President, Fry Technical Services, Inc.
W
elcome to “Timeout for Tech with Gary T. Fry, Ph.D., P.E.” Each month, we examine a technology topic about which professionals in the railway industry have asked to learn more. This month our subject is ties and ballast. The basic concept for a railway is hard steel wheels rolling on hard steel rails— the most efficient form of ground transportation for heavy loads ever conceived and implemented. A basic question for a railway is how best to support the rails under the heavy concentrated forces of the moving wheels. A very effective answer comes in the form of a simple construction technique—ties confined within a consolidated layer of crushed rock, or ballast. Ties have been made 52 Railway Age // November 2023
successfully from a variety of materials: wood, concrete, steel, and even plastic. Ballast is generally made by crushing hard rock, such as granite or trap rock, into sharp-edged pieces ranging in size between 1 and 2 inches. Once placed and consolidated, such a ballast layer drains water very well (a critically important function) and is stiff ly resistant to the tri-axial forces generated by heavy moving trains. Using this type of construction, heavyhaul railways have been built and operated successfully through immensely varied terrain and weather conditions around the globe. For example, Figure 1 shows rails, ties, and ballast on a heavy-haul railway above the Arctic Circle along a mountain grade in Norway. Figure 2 shows rails, ties and ballast on a heavy-haul railway in the desert plains of the western United States.
Despite the vastly different locations, terrains, climates and political jurisdictions, the railway tracks look more or less the same and utilize the same basic components: rails, ties, and ballast. Ties perform two main functions as part of the track structure: holding the rails to gauge (in essence, tying the rails together); and transferring tri-axial forces between the rails and the ballast—vertical forces, lateral forces and longitudinal forces. It is important to recognize in general that there is no ability for the ballast particles to “pull” on a tie. Therefore, forces transferred between tie and ballast rely on friction and also direct compressive contact pressure between tie surfaces and ballast particles. We will begin by considering the transfer of vertical wheel forces and the manner in which vertical pressures vary among the railwayage.com