Testing trackbed stiffness

Consultant URS has a long and distinguished record in transportation asset management. Robert Armitage, URS’ UK director of transportation asset management, reflected on this at the recent launch of its new Rail Trackform Stiffness Tester (RTST).

The team’s origins date back to the 1990s when Scott Wilson (since acquired by URS) and the University of Nottingham established a joint company to capitalise on both organisations’ expertise in pavement engineering. Originally focusing on highway and airport pavements, the company saw an opportunity to apply some of the investigation, assessment and design techniques from these fields to the rail industry. A trackbed technology team, which is now part of URS, was established to implement this idea.

Through its current framework with Network Rail, URS has delivered trackbed investigation and design services across the UK rail network for a number of years. This work informs Network Rail’s track renewal specifications on plain line, switches and crossings and enhancement projects.

The original pavement engineering organisation outgrew its offices at the University Science Park and moved some years ago to URS’ office in Chilwell, where the RTST launch event took place.

Measurement challenge

The need to improve trackbed stiffness measurement techniques is recognised in the Railway Technical Strategy, which sets out a long-term vision for the country’s rail network. International research also shows that poor, non-uniform stiffness has a direct influence on track deterioration and, in extreme cases, can lead to derailments.

URS has responded to these challenges with the RTST which delivers safety, operational and productivity improvements compared to previous industry methods. The new machine also gives an improved measurement of layer stiffness, allowing for better assessments of the causes of trackbed failure and compliance with specifications.

At the recent launch event, Robert and his colleague Dr Matthew Brough, URS’ operations director for transportation asset management, looked at the history of trackbed asset management in which URS has played an important part. Matthew is well qualified to discuss this as he leads the URS pavement, trackbed and materials consultancy teams. He is also a member of the Track Stiffness Working Group chaired by Network Rail, which aims to better understand track stiffness as well as its measurement and impact upon the network.

It all started with the FWD

URS’ market leading trackbed asset management service began in 1992 when the company introduced the falling weight deflectometer (FWD), already in use in the highway sector, into railway asset applications. The FWD is used to establish the layer stiffness of a highway pavement structure, a property that is of critical importance in assessing the pavement’s condition and ability to sustain the loadings that it needs to carry. The stiffness of a railway trackbed is of equal significance and the innovation was to adapt the FWD used for highway investigations so that it could also be used in rail applications.

In 1996, a team of trackbed specialists was established to undertake rail trackbed investigation, design and asset management consultancy. The team produced another key innovation around this time. Up to this point, the standard method of checking the nature and condition of trackbed materials below sleepers had been to excavate trial pits in the four foot and visually examine what was present. This approach had limitations in terms of the information obtained, and also suffered from obvious problems with regard to health and safety and speed of execution.

The URS team conceived the idea of the Automatic Ballast Sampler (ABS), a methodology that takes core samples by driving sample tubes down through the ballast into the track formation and subgrade. The samples so obtained are then examined, logged and sub-sampled for further testing under laboratory conditions.

Trial holes and ABS samples obviously only gave trackbed information at the specific discrete locations where the sampling was done. To interpolate between those locations something else was required. Ground Probing Radar (GPR) was the solution, as it enables the identification of layers or strata within the trackbed. Since the nature of each layer can be identified precisely at each ABS sample location, the GPR data allows the engineer or technician to follow the layers through the trackbed from ABS location to ABS location.

The FWD was used to obtain ballast, sub-ballast, formation and subgrade stiffness to support the ABS and GPR data in helping to understand the behaviour of the track in those areas. FWD was also used to establish critical velocity, an important speed limiting parameter on railways at which dynamic interaction causes enhanced track deterioration. However, the time taken to get the FWD apparatus into rail mode and onto the track meant that it was cumbersome and its effectiveness as a track asset information tool was limited.

URS developed a full-scale trackbed to validate the RTST and use in future research.

Another step forward

URS introduced its ‘Total Route Evaluation’ methodology for trackbed asset management in 2002. This methodology collated all trackbed data with bespoke track-quality metrics, enabling trackbed maintenance and renewal decision-making to be based upon performance rather than condition.

The Mast Operated ABS (MOABS) followed in 2012. This improved site worker safety and increased the efficiency of the trackbed sampling process. The new device effectively automates the driving of the ABS sampling tube, removing the need for a human operator to hold the pneumatic hammer. It incorporated winching and jacking systems for lifting heavy equipment and for withdrawing the sampler from the trackbed. All the apparatus was carried on a rail trolley for ease of site access and MOABS gained full Network Rail product acceptance.

These developments left the FWD behind and opened the way for further innovation. URS had the required ideas and saw an opportunity in the form of the RSSB/ Railway Industry Association (RIA) Innovation Competition. In 2013, the company submitted proposals for its RTST and was successful. Its proposal was joint winner and an award of £200,000 enabled URS to design, build and test the RTST.

The competition judges recognised the operational, technical and health and safety benefits of the RTST and its huge export potential, as well as the industry need for developments in this field.

Andrew Broadbent, RSSB head of research and development, commented: “URS’ Rail Trackform Stiffness Tester is a major step forward for surveying different trackbeds for railways in Britain and potentially, beyond. We’re delighted to have provided the financial injection and support to URS as the 2013 winner of the RSSB/RIA Innovation competition.”

RTST launch event

Andrew described the research and development activities of the rail industry and the importance of the work of innovative companies like URS. He described how RSSB supports R&D through a range of programmes funded by government and industry.

There were demonstrations of both the MOABS and RTST in URS’ trackbed test facility. This comprises a full-scale trackbed constructed to Network Rail specifications alongside development of the RTST in order to validate the new technique against existing processes. It was very convincing to see the MOABS in action, showing how much quicker and safer it is than the old ABS system.

The RTST is a clear improvement on the previous industry technique. The whole apparatus is mounted on a transport frame that can move along on rubber-tyred caterpillar tracks and then switch to rail wheels, both systems being hydraulically powered and braked. When positioned correctly, easily achieved with the umbilical cord control system, the loading beam is lowered onto the test sleeper, the array of geophones is positioned onto the ballast and the falling weight is dropped to apply the load to the trackbed.

The technique is able to replicate the loading requirements of high-speed or heavy-haul lines through the use of an increased range of pulse-loading conditions on the RTST. The weight is fully enclosed within the machine, which greatly minimises safety risk. The geophones measure the deflection response of the ballast, sub-ballast, formation and subgrade enabling the assessment of layer stiffness. With further analysis of the time history response of the subgrade, critical velocity can also be established.

Each test is very quick to perform. Added to the fact that the ‘on-tracking’ and ‘off- tracking’ of the machine each take around 15 minutes, compared to about two hours for the old rail FWD, the speed of operation means that far more track measurement work can be undertaken in any given track possession than was possible with the FWD. This should mean that it is now feasible to take trackbed stiffness measurements more widely than ever before, greatly assisting the understanding of track asset condition and the monitoring of its changes over time and under traffic.

A tour of URS’ UKAS accredited laboratory facilities followed. These are extensive and comprehensive, enabling the company to offer a wide range of testing of materials from highways, airport pavements and rail tracks. Tests include chemical contamination analysis or physical properties, fatigue tests and more. Bespoke test machines developed by URS, such as the Springbox and Aggregate Flow Test, have been used for a variety of applications including accelerated ballast deterioration testing and the assessment of stoneblower aggregate performance. URS also has laboratories for testing asphalt and bitumen – materials that can potentially be applied to rail as asset owners look to asphalt tracks as future trackform solutions.

Unlocking Innovation

Jim Lupton, RIA deputy technical director, is spending a significant proportion of his time managing the Unlocking Innovation scheme, which runs regular workshops for industry clients and suppliers to share their challenges and solutions. With widespread recognition that innovation is the key to improving the UK’s rail system, Jim is keen that initiatives like the RSSB/ RIA Innovation Competition continue to support research and development for companies such as URS.

Commenting on URS’ success, he said: “It is inspiring that in just 18 months the prize helped develop a concept into a fully-functional rail mounted test machine with significant client interest. We are very pleased to have helped make this happen and it shows the value of the work that RIA, RSSB and others are doing to help innovators access support.”

The RTST is now market ready and will be deployed to various test sites over the coming months. URS’ Matthew Brough explains: “We already have a number of UK projects secured for the RTST and have also received enquiries from overseas rail operators. We are using our new test facility for other projects that will further research trackbed stiffness and its influence on track performance. If the industry moves towards a performance based specification for track renewals, we see the RTST as the best machine to give the required stiffness data for compliance with Railway Group standards. The RTST can also give rapid method of assessment and post remediation validation, supporting infrastructure owners as they increase the remediation of formation below track. We also see potential markets in slab and asphalt track evaluation, as the machine has the capability to measure a range of trackforms. I would like to thank the Track Stiffness Working Group, RIA and RSSB for their ongoing support in making this happen and Tek-Co for a quality design and build service.”