01.05.12
Track for the 21st Century
Source: Rail Technology Magazine Apr/May 2012
RTM hears more about the latest research from Track 21, led by Professor William Powrie.
Track 21 is a research programme funded by the Engineering and Physical Sciences Research Council (EPSRC), which aims to develop a systematic understanding of the fundamental science underpinning railway track behaviour, and then use this to bring about a step change in track system performance. The intention is to develop quantitative models of the interactions between the key elements of foundation and sub-base, ballast and sleepers which can be used to assess how the system as a whole behaves. These models will then be used to inform key decisions regarding whole life costs, critical zone improvement and design for minimum maintenance or high speed.
The Programme is led by Professor William Powrie of the University of Southampton, and in addition to the team at Southampton involves academics at the Universities of Birmingham and Nottingham and input from a variety of industry partners including Network Rail, HS2, LUL, RSSB and RIA.
Amongst other activities, the past 18 months has seen the development, construction and commissioning of specialist apparatus for advanced laboratory tests, which forms the focus of the remainder of this article.
Deformations of earthworks subjected to cyclic seasonal changes in pore pressure due to vegetation effects and enhanced traffic loading effects
Cyclic shrink/swell deformations of earthworks embankments can cause major problems for rail infrastructure owners in terms of maintaining the required track levels. Fatigue effects might also lead to the gradual failure of such embankments over a several decades.
These aspects are being investigated in cyclic triaxial tests in which 70mm diameter specimens of Lias Clay embankment fill from a site near Bristol are being subjected to variations in pore water pressure of 100kPa, while maintained in a total stress state representative of a depth of 1.5m below the surface of an embankment. The apparatus is shown in Figure 1 (right).
Similar apparatus is being made ready for tests in which total stresses are cycled at a much higher frequency, mimicking train passage. In both cases, the laboratory tests are complemented by field and full scale studies: collaborators include Network Rail, RSSB, Mott MacDonald, Geo- Observations and Arup.
Effects of principal stress rotation on different types of track subgrade
A torsional hollow cylinder apparatus has been set up and a testing procedure developed, following Powrie et al (2007), to investigate the effect of the principal stress rotation associated with train passage on a variety of railway foundation soil types. Particular emphasis is being placed on the effect of clay content and the time interval between loading events.
Investigating the effectiveness of ballast and sleeper modifications
The Southampton sleeper testing rig has been upgraded to enable the long-term performance of different combinations of sleeper type, ballast specification and under-sleeper pads to be investigated. The rig and some typical results are illustrated in Figure 2. Again, this work is being complemented by field studies to investigate the effect of interventions on critical zone (e.g. switches, transition zones and underbridges) to be assessed in a quantitative and scientific way: collaborators include Network Rail and URS.
The rig tests typically involve 106 load cycles, representing a cumulative load of 20 Megatonnes and perhaps between a year and a decade’s real use with no maintenance interventions. Lateral as well as vertical loads can be applied, simulating the effects of curving at a cant deficiency and/or sidewind loading. Handling the intensity of data produced is a significant challenge. In addition to the effect of sleeper/ballast modifications, we are also investigating the influence of contamination with finer particles such as coal and sand. Complementary tests are in progress in the Nottingham sleeper testing rig, which simulates the effect of vertical loading under a moving train over a group of three sleepers.
In addition to the load/deformation response, the effect of any track system modifications on noise and vibration must also be considered. The dynamic stiffness of the ballast bed at higher frequencies is important in this respect, as it influences the transmission of vibration into the ground or supporting structure as well as acoustic radiation from the track. A dedicated test rig to investigate high frequency ballast stiffness and noise mitigation effects is nearing completion. This will allow dynamic stiffness measurements to be made, both directly and indirectly, at frequencies between 50 and 1000 Hz under a range of preloades.
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