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Supporting roll Geogrids provide a solution to railway track ballast problems on soft and variable subgrades.

Successful application of geogrids for reinforcement of railway track ballast has led to their use in a major track-laying project on the Chiltern line in central southern England. A key advantage for this application is the reduction in track maintenance on soft and variable subgrades.

Improvements to the Chiltern Railways route between London Marylebone and Birmingham Snow Hill are well under way. The project includes doubling the 30km section of single track between Princes Risborough and Bicester North.

This multimillion pound investment, along with new trains, will enable Chiltern to double the frequency of its services and reduce journey time on this alternative route between Birmingham and London by 30 minutes.

Peter Try, director of Railtrack Midlands, said: 'We have made a commitment to improve the rail- way infrastructure and together with our contractors Mace and Laing GrantRail, we will strengthen this important link between the Midlands and London.'

Increasing services means relaying of the old up line, reinstating track which was axed during the Beeching rail infrastructure review of the 1960s.

Work started at Princes Risborough and is moving onward to Bicester North, with completion expected by the summer. Continuously welded rail will smooth the ride and concrete sleepers will be used throughout - a total of 46,000 along the 30km stretch. There will be 140,000t of new ballast.

Geogrid reinforcement, which is approved by Railtrack for the structural reinforcement of track ballast, is being used in areas where the subgrade strength is soft or variable. Consultant Scott Wilson Railways carried out the track design for the joint venture contractor Laing GrantRail.

Based on the results of a trial pit investigation a single layer of Tensar SS30 geogrid is being used in conjunction with geotextile separator and filter in areas of soft and variable subgrade. In some areas this has allowed the use of a thinner than usual sand layer beneath the geotextile as the presence of the geogrid will reduce subgrade erosion. A minimum of 250mm of ballast is placed over the geogrid.

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