INTRO: A ballast renewal and pointwork replacement programme will raise the quality of track on the Paris - Sud-Est line and extend its life
BYLINE: André Le Bihan
Chief Engineer, Permanent Way Department, French National Railways
AFTER 15 years of use by trains running at up to 270 km/h, France’s first high speed line was in need of some attention. Last year saw work commence on the progressive replacement of two track components: ballast and turnouts. The line is of wholly conventional design with ballasted track and CWR, although the specifications for rails, pointwork and tracklaying were somewhat more demanding than for other lines.
The ballast laid on the first section, completed in 1980 between Vergigny and Mâcon (Fig 1), was of rather mediocre quality. The turnouts were laid on wooden sleepers, as was usual on conventional lines at the time. These two elements have proved to be the line’s weak points, and SNCF decided to make a start on replacement during 1996.
The work is initially limited to the 220 km forming the central part of the line. It entails replacing all the ballast and 63 turnouts in a carefully planned sequence of operations that will take at least six years.
Organising the work requires considerable attention to logistics as there are several fundamental constraints. The line has to remain in operation to handle an average of 180 TGVs a day running at 270 km/h, while geographical constraints such as difficult access and grades as steep as 3·2% demand working methods and processes that are not used on other lines.
Comfort and safety on a high speed line depend on maintaining excellent track geometry. On the Paris - Sud-Est line this is achieved by a heavy-duty tamping programme married with rail grinding. To ensure that the best possible results are attained, a computerised planning system called Timon has been in place since 1986 to help maintenance planners in the decision-making process. This has helped keep the average cost of maintaining the line below that of a conventional line of the same age carrying the same volume of traffic.
However, the Timon system also revealed that track level was deteriorating much more quickly on certain parts of the Vergigny - Mâcon section than elsewhere (Fig 2). A programme of tests and sampling carried out over several years was put in hand, and this confirmed that the ballast quality used on this section was below par. In particular, the stone was physically deteriorating through wear so that the lower ballast layer was becoming clogged with small particles.
Next step was a statistical study based on close scrutiny of the samples that had been taken successively from the track, plus analysis based on the data available in the Timon system. This made it possible to establish, section by section, the optimal life of the existing ballast layer, and to draw up a programme of work to remedy the situation.
The initial treatment was not very expensive, and easily implemented; this was to raise the track level by 150mm by adding a layer of good quality ballast. But the effectiveness of this process only lasted a few years, as it did not tackle the problem of the original spoiled ballast on which the new ballast rested. On some sections the old ballast had become so degraded and polluted that it was essential to replace it.
A ’master plan’ was drawn up on the basis of priorities, avoiding for practical reasons a multiplicity of worksites scattered along the line. The plan sought to spread the cost so that there was not a sudden peak in expenditure. The work runs from 1996 to 2001, but the programme can be modified to suit any change in conditions.
On the northern section of the line built two years later, a superior quality of ballast was used, with the result that its performance is markedly superior. We do not envisage having to regenerate the ballast between Lieusaint and Vergigny until its life has reached 23 to 25 years.
Organising the work
Most of the ballast renewal work is undertaken in spring and autumn ’campaigns’. If the time needed for setting up and dismantling the worksites is deducted, this leaves around 20 effective weeks of work a year. Because of bad weather and the peak traffic periods caused by wintersports, no work is scheduled to take place in the winter. The summer is left free for the turnout replacement programme.
Several restrictions apply to commercial operations while work is under way. Trains must reduce speed to 120 km/h in the worksite zone, which is up to 4 km long, during the initial phase of removing ballast and installing the first replacement layer. During the final phase of work with levelling and stabilisation under way the speed of passing trains is 160 km/h.
Strict commercial considerations apply, and the impact of the works must not add more than 4min to normal timings. The work is arranged so that 10 trains an hour in each direction can pass the worksite.
Nightly possessions start at 21.05 on the track to be treated, and last to 06.55. Trains are routed over the second track - both tracks were signalled for bidirectional working when built. A complete 4h possession of both tracks is taken in the middle of the night.
With these conditions in force, the average work rate has been close to 700m a day for the ballast renewal, giving a maximum of 78 km a year. This includes the short sections where ballast is replaced during the summer as part of the turnout programme.
The worksite has the appearance of a similar one on a conventional line, but there are important differences: