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30.10.18

The Railway Test Facility

Source: RTM Oct/Nov 2018

John de Bono, research fellow at the Nottingham University’s Nottingham Centre for Geomechanics, sheds a light on their Railway Test Facility.

The University of Nottingham’s Railway Test Facility is a full-scale experimental facility for testing a section of full-scale railway track. It is essentially a large concrete pit containing subgrade, ballast and sleepers, and a loading frame which simulates passing trains. Designed and built by members of the university over 10 years ago, the facility has been used to gain much insight to the behaviour and degradation of ballasted rail track, as well as to test new ideas and concepts.

The concrete pit is 4.1m by 2.1m across and 1.9m deep. The lower half  of  the pit  is filled with subgrade (such as compacted silt), or for better repeatability built up with concrete slabs. The upper part of the pit is then filled with around six to  eight tonnes of ballast, depending on the desired depth. Three sleepers are positioned and embedded on top in accordance with industry spacing. A sturdy steel loading frame then straddles the pit, with three hydraulic actuators which are capable of applying large cyclic loads to the sleepers in sequence to mimic a passing train.

The hydraulic actuators are typically used to apply loads of 100 kN, which is considered representative of an axle load of over 20 tonnes. The loads are applied in wavelike (sinusoidal) form, with a phase difference such that when the middle sleeper is under maximum load (100 kN), the sleepers either side are both under approximately half load (50 kN). This was chosen to closely resemble how axle loads are distributed by the rails to the sleepers, and represents the passage of a moving train.

The laboratory set-up allows the subgrade, ballast and sleepers, as well as the loading, to all be controlled, and was designed to allow a complete full-scale study of the performance and degradation of the track. The use of several sleepers, loaded sequentially, makes it possible to capture the combined load effects from neighbouring sleepers.

This is one of the key aspects of the Railway Test Facility, which makes it possible to capture the combined load effects from multiple sleepers. In other words, this means that the middle sleeper and ballast are affected by the presence and loading of the neighbouring sleepers. This provides conditions much closer to reality than if a single sleeper were used, and therefore provides a more realistic indicator of real track/sleeper performance. Among the most common measurements taken when testing a sample of track is the sleeper displacement (settlement), which is measured at multiple points on any sleeper. The stress underneath the ballast and/or subgrade, as well as on the vertical walls, are also routinely measured to understand the stress distribution throughout the track-bed.

Since being built, the Railway Test Facility has been used to investigate the effects of the subgrade type, ballast mineralogy, sleeper material, geogrids, under sleeper pads, and tamping on the track performance. Performance is usually judged in terms of the sleeper settlement after a number of cyclic loads, with tests typically consisting of up to one million load cycles. The nature of the facility allows for any number of things to be measured; for example, aspects such as ballast degradation can be monitored, which was useful in quantifying the damage caused by tamping. This was achieved by using a refurbished Plasser tamping machine, and methodically inspecting the ballast for fragmentation and abrasion afterwards.

Outcomes from experiments using the facility have led to a better understanding of the optimal design and use of geogrids (e.g. effects of size, shape, and material properties), as well as the use of under-sleeper pads and the performance of alternative sleeper materials.

Current work continues with the same strategy, with the aim of improving current methods or developing new ways of minimising ballast degradation, and therefore improving performance and reducing maintenance costs associated with ballasted track. It is hoped one day that the Railway Test Facility will be used to test and develop the rail track of the future, using entirely new materials and concepts.

 

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