Abstract:
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.
