Ground/Track/Train Systems Interactions

Named Investigators: Prof. William Powrie (University of Southampton)
Researchers: Dr Chris Jones, Dr Jeffrey Priest, Dr Li-Ang Yang, University of Southampton; Dr Michael Burrow (University of Birmingham)

Background:
Field measurements made during project A1 have provided data on the effective stiffness of track systems, and the variations in stiffness that can occur as a result of differences in the degree of sleeper support. Increased track deflections may lead to increased vehicle dynamic loads and long term damage. Vehicle dynamics studies have not been able to include this information as the computer simulation packages used do not allow accurate representation of variations in track system support conditions, and only very simple track dynamics models are used. Similarly, as both high speed mainline and metro rail networks extend by means of underground construction into densely populated and vibration-sensitive urban areas, there is a need to apply the new insights into the geotechnical response of the track and the ground to dynamic loading from trains to improve the prediction of the generation and propogation of groundborne vibrations.

Objectives:
The main aim of the project is to study and improve the understanding of the interactions between the ground, the track system and train vehicle behaviour, especially recursive processes giving positive feedback. The specific objectives are:

To continue the development of instrumentation and carry out further field measurements to understand quantitatively and mechanistically the behaviour of track systems of different types subjected to different loads
To gather data on the geophysical properties of the ground and vibrations at the same sites
To carry out further laboratory stress tests to investigate the susceptibility of different subgrade materials to cyclic principal stress rotation (PSR)
To assess the ISVR 2.5-dimensional method for modelling ground vibrations from railways by analysing the results from the test sites
To develop an analytical model for track system response that includes a frequency dependant ground stiffness
To identify and quantify the parameters needed to model track system behaviour in a vehicle dynamics study
To develop models that enable the interdependance of vehicle dynamic response, the generation and propogation of groundborne vibrations and the geotechnical characteristics of the track system and the ground to be determined.

Summary of methods:
Field monitoring of dynamic loads and displacements and S- and P- wave velocities, frequency response functions of the ground and track and the vibration response of the track to known train loadings; laboratory cyclic hollow cylinder tests; and 3- and 2.5- dimensional dynamic finite element, boundary element and/or finite difference modelling.

Work Done:
Project A4 combines a number of different, independent tasks in addition to tasks which are interlinked. At present most of the work to date has involved these independent tasks.

Work has continued on the development of instrumentation for site monitoring. This has included the updated remote video monitoring system to include high speed digital picture acquisition for use on high speed train lines. Remote datalogging has been implemented to increase the number of site locations available for monitoring . The high speed remote video monitoring has been successfully trialled on a section of the Channel Tunnel Rail Link (CTRL) and the West Coast MainLine (WCML) and shown to be effective at quantifying ground deformation at the train speeds encountered. The remote datalogger has been trialled within the lab environment and is currently awaiting NR approval to be validated under working site conditions.

Further field measurements have been conducted on both the WCML and CTRL to understand qualitatively and mechanistically the behaviour of differing tracks to differing loading conditions. On the WCML worked has been conducted to assess the effect of high speed tilting trains (Pendolino’s) on track behaviour. Initial studies have shown that Pendolino’s increase the magnitude of load and frequency possibly leading to increased degradation of the ballast. On the CTRL work has been conducted to investigate the mechanism for lifting of the ballast during a train package. Collaborative work was done with CTRL, UoB and UoS using vibrational and aerodynamic measurement techniques. Initial results suggest that the ‘flying ballast’ phenomena can not be attributed to individual effects but may be in combination given certain site conditions.

Site measurements and analysis have also continued on the Great Central Railway (GCR) near Loughborough through Dr Michael Burrows. A range of geophysical measurements on the Victorian embankment have been conducted to characterise the geotechnical structure railway embankments.

Advanced 3D modelling and 2D dynamic modelling have been conducted to understand ground behaviour underneath a railway. Work has been conducted to investigate stress changes during loading, especially in the context of ‘principal rotation of stress’. Dynamic modelling work has considered dynamic stress changes in the track as a function of speed, braking and acceleration and track geometry defects. Work has also be conducted updating the numerical code for modelling ground vibrations from railways. The 2.5 dimensional boundary elements code previously developed has been incorporated into a more systematic code that can me more easily maintained. This means the boundary elements are now coupled with a much more capable set of finite elements. 2 person months have been utilised on the project to date.

Work Remaining:
Specific tasks remaining include;

To develop an analytical model for track system response that includes a frequency dependent ground stiffness
To identify and quantify the parameters needed to model track system behaviour in a vehicle dynamics study
To assess the ISVR 2.5-dimensional method for modelling ground vibrations from railways by analysing the results from the test sites

Outputs

Presentation from RRUK Workshop 2008
Poster from RRUK Workshop 2008
Priest, J. A., Powrie, W., Le Pen, L., (accepted). Performance of canted ballasted track during curving of high speed trains, 1st International Conference on Transportation Geotechnics
Thomas, A.M., Burrow, M.P.N., Rogers, C.D.F., Chapman D.N., Metje, N., Gunn, D. and Nelder, L. (Accepted for 2008). Electromagnetic Characterisation of a Victorian Railway Embankment Fill Material. 3rd International Conference on Site Characterization. 1st to 4th April, 2008. Taipei, Taiwan.
J.E. Chambers, D.A. Gunn, P.B. Wilkinson, R.D. Ogilvy, G.S. Ghataora a d M.P.N. Burrow R. Tilden Smith (Accepted for 2008). Non-Invasive Time-lapse Imaging of Moisture Content Changes in Earth Embankments Using Electrical Resistivity Tomography (ERT). 1st ISSMGE International Conference on Transportation Geotechnics. 25-27 August 2008, Nottingham, UK.
D.A. Gunn, J.E. Chambers, H.J. Reeves, G.S. Ghataora, M.P.N. Burrow, P. Weston, D. Ward, J.M. Lovell and R. Tilden Smith (Accepted for 2008). New geophysical and geotechnical approaches to characterise under utilised earthworks1st ISSMGE International Conference on Transportation Geotechnics. 25-27 August 2008, Nottingham, UK.