The effects of under-sleeper pads on sleeper-ballast interaction

Under-sleeper pads (USPs), typically made from polyurethane, are used by railways in certain parts of the world to reduce ballast settlement and consequently lengthen the ballast tamping cycle. The rationale behind this relatively new addition to the conventional ballasted track structure is that the pad increases the contact area between the angular ballast particles and the underside of the concrete sleeper, with the effect that ballast breakdown and total track settlement are reduced. This paper describes two experiments on the effects of USPs on four aspects of sleeper-ballast interaction, namely contact area, contact pressure, ballast settlement and ballast breakdown. Static and dynamic tests up to 1 million loading cycles were performed under controlled laboratory conditions on concrete sleepers with and without USPs. Sophisticated pressure sensors revealed an increase in contact area from 12% to 35% for static loading tests, and from 8% to 20% for dynamic tests, with a resulting 70% reduction in contact pressure. In addition, a 44% reduction in ballast settlement and a 23% reduction in ballast breakdown were achieved by the introduction of USPs. In conclusion it is argued that the introduction of USPs specifically on heavy-haul lines would offer significant advantages with respect to ballast settlement and breakdown. These advantages are most likely to lengthen general ballast tamping and screening cycles, resulting in significant life cycle cost savings. Keywords: under-sleeper pads, ballast, concrete sleepers, ballast settlement, ballast breakdown