Most railways are built on the ground, and run for much of their length within or on geotechnical structures such as cuttings and embankments. The principles of soil mechanics, which underlie the practice of geotechnical engineering, may equally be applied to a ballasted trackbed.
A number of aspects of railway geotechnical engineering are being investigated at the University of Southampton, with the financial support of the Engineering and Physical Sciences Research Council (EPSRC) and Network Rail through a Strategic Research Partnership on Future Infrastructure Systems. Some of these are outlined in this article. In addition to EPSRC and Network Rail, funders and collaborators include the rail Safety and Standards Board (RSSB), Mott MacDonald, London Underground, GeoObservations and Arup.
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 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 1.5 m below the surface of an embankment. The apparatus is shown in Figure 1.
The laboratory tests are being complemented by field studies, in which changes in soil water content and ground movements in response to vegetation and weather events are being monitored within a road cutting near Newbury (Smethurst et al, 2012) and on two railway embankments.
At one of the railway sites (Hawkwell, near Southend; Briggs et al, 2013), the effect of controlled tree removal has been investigated (Figure 2). Although tree removal reduced summer shrinkage dramatically, deep seated soil suctions established over years, if not decades, were also lost over a period of a few years, which may have an adverse impact on the long-term stability of the slope. The data have been used to validate a numerical modelling technique, which has demonstrated the potential benefits of removing or coppicing the trees near the top of the earthwork while leaving in place the trees near the bottom.
This may reduce seasonal shrink/swell movements while retaining sufficient suctions to ensure the stability of the earthwork.
In a different series of tests using similar apparatus to that shown in Figure 1, vertical stresses are being cycled at a frequency mimicking train passage. This is part of an investigation into the possible effects of heavier trains.
Further funding has recently been obtained from EPSRC for a collaborative project called iSMART (Infrastructure Slopes: Sustainable Management and Resilience Assessment).
Working with five other UK institutions, this will enable the continued monitoring of instrumented slopes, the development of more advanced models of slope failure mechanisms, and the assessment of the effects of climate change.
Discrete pile stabilization of railway embankments and cuttings
Individual discrete piles are an attractive and economical way of stabilising an infrastructure slope, falling somewhere in terms of cost between a continuous retaining wall and a softer solution such as lime stabilisation. However, a lack of clarity concerning the mechanisms of slope/pile interaction has led to uncertainty in the appropriate methods of calculation for use in design.
Monitoring of ground movements, pile movements, pile bending moments and pore water pressures associated with discrete pile stabilisation schemes for cutting and embankment slopes at Grange Hill (Figure 3), Mill Hill and Hildenborough (Kent; Smethurst and Powrie, 2007) has led to a wealth of data and valuable insights into the various mechanisms of soil/pile interaction that may occur (Figure 4). This has led in turn to the development of design methods that can be used with confidence to ensure that the maximum benefits and economies are achieved in given conditions.
Field investigations of track behaviour
Geophones attached to the sleepers and digital image analysis are now routinely used in our research to investigate the deflections of sections of railway track as trains pass. Early work investigated the phenomenon of ballast migration (Figure 5), which may occur on canted track traversed by fast (~200 km/hour) trains. It involves the gradual migration of the ballast down the cant so that the high end of the sleeper is exposed and the ballast gathers in a heap against the low rail.
The technique has also been used to investigate the performance of transition zones from embankments onto stiffer substructures such as underbridges and culverts (e.g. Coelho et al., 2011). It is now being deployed to investigate the behaviour of switches and crossings and the effectiveness of track remediation schemes (Figure 6).
The University of Southampton sleeper testing rig (Figure 7; Le Pen and Powrie, 2011) is being used to investigate the performance of different combinations of sleeper type and ballast specification, with and without under sleeper soffit pads.
Rig tests typically involve up to three million cycles of a 20 tonne axle load, representing a cumulative load of 60 MTG 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 a sidewind. Settlement data from a typical test running to three million cycles are shown in Figure 8.
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 has been built and is being used to measure dynamic stiffnesses, both directly and indirectly, at frequencies between 50 and 1000 Hz under a range of preloads.
Development of structure in railway ballast
It is well-known that ballasted track gradually settles as a result of trafficking, owing to deformation of the sub-base and/or of the ballast itself (Figure 8).
Traditionally, the track is returned to the required level by lifting and tamping; unfortunately, this process destroys the structure that has been developed by trafficking resulting in a relatively much softer response during the initial reloading.
This is illustrated by the results of triaxial tests on a scaled ballast simulating the effects of cyclic loading, tamping and reloading (Figure 9).
To complement the laboratory simulations using scaled ballast, methods have been developed to recover samples of trafficked ballast from below sleepers during track renewal operations, and to assess the arrangement and orientation of particles using computed tomography (CT) scanning.
Figure 10 shows sampling in progress, and Figure 11 a typical CT scan. Initial analysis suggests that the development of structure in ballast is associated primarily with densification and an increase in the number of particle to particle contacts, rather than gross particle reorientations.
Further insights are being gained through numerical discrete element analyses at the particle scale, using particles representative of real ballast generated by means of the ‘Potential Particle’ approach (Harkness, 2009).
A typical numerical model of a triaxial test specimen made up of ballast particles is shown in Figure 12.
William Powrie FREng is Professor of Geotechnical Engineering and Dean of the Faculty of Engineering and the Environment at the University of Southampton.
His main technical areas of interest are in geotechnical aspects of transport infrastructure, and sustainable waste and resource management.