Thumping good trackbed testing

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Understanding the condition of substructure is vital for maintaining good track geometry. With trackbed investigation now recognised as good practice, some engineers might dismiss this statement from the 2012 Rail Technical Strategy (RTS) as obvious. However, the issue is how to obtain this understanding with increasingly limited possession times for trackbed surveys. So it’s hard to disagree with the RTS’s call for improved trackbed assessment techniques.

One company developing such techniques is URS, which has a single supplier framework contract with Network Rail for trackbed investigation and design. Its trackbed investigation work has developed techniques that are now industry standard practice. This has also resulted in two innovation awards. In November 2010 URS’s “Total Route Evaluation” techniques won British Expertise’s Innovation award. In March its Rail Trackbed Stiffness Tester was a joint winner of the innovation award at the Railway Industry Association (RIA)’s Technology and Innovation Conference as reported in last month’s the rail engineer.

Total route evaluation

In 2001, a report to the Rail Regulator showed that Railtrack was spending £200 million per annum on track renewals yet not achieving the required track quality improvements. At the time, trackbed investigation was traditionally undertaken by slit trenches up to 200 metres apart. This was slow and labour intensive and did not identify issues between sample points and beneath the ballast, which often led to incorrect track renewals specification. Such investigations are essentially required to assess trackbed condition and track layer stiffness, the elastic rail deflection under wheel loading. The problem is that the subgrade, buried under the track, is the primary determinant of overall track stiffness. Hence there is a clear relationship between trackbed stiffness and track quality.

In addition to the requirement to optimise track renewal specifications, an understanding of the subgrade is needed to determine the track’s critical velocity. This is the speed at which excessive ground vibrations occur as the waves from trains’ shock loading on individual sleepers resonate with the ground’s natural frequency. Typically, this starts to be an issue on soft subgrade at speeds of 140 km/h.

So it is not surprising that trackbed investigation is the subject of various research projects. For its part, Scott Wilson (since acquired by URS) started trackbed research 15 years ago through a joint venture with the University of Nottingham that became URS’s Pavement, Trackbed and Materials Consultancy. This led to the development of the Total Route Evaluation (TRE) methodology which uses a number of techniques including Automatic Ballast Sampling (ABS), Ground Penetrating Radar (GPR), Springbox testing and Falling Weight Deflectometer (FWD).

ABS abstracts a ‘core’, typically of 0.9 metres, which is analysed for its chemical and engineering properties. This technique has several advantages over historic trial pitting techniques, one of which is that many samples can be taken in relatively short track possessions.

GPR assesses the thickness and condition of upper trackbed layers and provides information between ABS sampling points. It is a good technique for initial trackbed assessment at a route level but, if used to specify renewals, has to be calibrated using ABS data.

The Springbox, developed by URS, is a 250mm cube filled with test material. This has spring-loaded faces, the forces on which are measured when a pulsed vertical load is applied to the test sample. It is particularly useful for stiffness assessment of ballast for new railways. As will be seen, for rail, FWD is a novel technique still under development.

TRE in action

URS uses TRE for several overseas rail infrastructure asset owners as well as in its national framework contract with Network Rail for trackbed investigation and design. This requires site investigations at over 500 track renewal sites per year with testing of recovered materials undertaken in its dedicated laboratory in Nottingham. The results are then used to design track renewals in accordance with local trackbed conditions.

TRE has also been applied on a route basis, and for track enhancements. Previously, on the West Coast project, TRE was employed to ensure the effectiveness of the ballast cleaning programme. Using desktop studies and high speed GPR calibrated from material testing, a “toolkit” was developed to provide indicative particle size distributions of material layers to assess residual ballast life to optimise ballast cleaning to save millions of tons of material. More recently, TRE was used on the Great Northern Great Eastern upgrade to determine residual ballast life and trackbed limitations. This also tested the suitability of various alternative trackbed treatments.

To enhance operator safety, URS recently developed its Mast Operated Automatic Ballast Sampler (MOABS), which has now received product acceptance following a year of intensive development.

This is used to drive one metre long tubes into the trackbed at depths of up to two metres, and replaces the previous ABS technique. The tubes have plastic liners which hold the sample. A SERB (Specialist Excavation of Railway Ballast) machine collects representative ballast samples for fouling assessment and for Springbox testing.

On the other side of the world, increased iron ore production required a 250% increase in rail traffic with axle loads potentially increased to 40 tonnes on a 426 km long rail line in Western Australia. This required GPR surveys with detailed intrusive investigations at discrete locations to assess trackbed condition and determine the optimum maintenance and renewal strategy including the use of ballast cleaners. TRE has also been applied in Malaysia and Jamaica to ensure trackbed was fit for planned tonnage and linespeed increases.

Measuring the bounce

The idea of testing a pavement by measuring the effect of a weight dropped on it originated in France in 1963. However French (and British) road engineers were sceptical and the following year the idea was taken up by Denmark’s National Road Laboratory which, by 1975, had developed a practical working model of the Falling Weight Deflectometer (FWD). By the 1980s FWDs were in widespread use, particularly in Sweden and the Netherlands. During the 1990s the different interpretations of FWD data resulted in European guidance for incorporation in national

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