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Interlock action

British Rail Research trials carried out on the rolling road rig at its laboratory in Derby demonstrated that inserting geogrids in the ballast can help extend maintenance intervals by minimising settlement where tracks lie over soft ground.

Obviously in these conditions track settles more rapidly, and much of this is due to vertical deflection of the underlying soft subgrade. The cyclical vertical loading on the ballast can result in lateral creep of ballast particles leading to permanent settlement of the track.

The use of a geogrid with high profile ribs which provide a good interlock with the ballast can limit the lateral creep of the ballast particles and hence reduce the settlement and the rate of deterioration of the vertical track geometry.

Tensar biaxial grids are used to reinforce granular sub-bases, capping layers and railway ballast placed over weak and variable soils.

When granular material is compacted over Tensar grids it partially penetrates and projects through the apertures creating an interlocking action between the particles and the grid. The mechanical interlock creates a flexurally stiff platform which distributes load evenly, reduces rutting and minimises differential settlement.

In the British Rail Research trials Tensar SS biaxial geogrids were used within the ballast layer beneath a track over a soft subgrade and compared with the performance of ballast without geogrid over both soft and firm subgrades.

The tests indicated that by installing the geogrid in the ballast over the soft subgrade the rate of settlement is substantially reduced and approaches that of a track on a firm foundation. The geogrid also reduced the elastic deflections by almost half when compared with a soft subgrade.

British Rail's London to Edinburgh East Coast main line set a precedent for the effectiveness of geogrids for ballast reinforcement.

The line passes through the low-lying peaty fens of Cambridgeshire. Peat has often proved a difficult subgrade for railway construction, especially for modern high-speed railways where precise track alignment is so important.

A length of track near Peterborough had required considerable maintenance due not only to the gradual erosion of ground levels over the years, but more importantly to lateral and vertical migration of the ballast under dynamic loading.

To remedy this, a review was carried out in 1990 by engineers at the Eastern Region's headquarters in York. In considering the role of geosynthetics, the reinforcing effect of a grid was identified as potentially the most advantageous function. The 50mm single-sized stone can mechanically interlock into the geogrid apertures, thereby controlling lateral movement of the ballast.

The overall purpose was to increase the line speed from 165km/h to 200km/h. Tensar SS geogrids were installed during the ballast renewal operation as a trial in 1989. The trial included seasonal wet and dry periods as the underlying organic peats govern the response of the foundation.

As this proved successful a total of 5km was installed during 1990. On these occasions it was shown that the grid could be installed efficiently and that such an operation could easily be assimilated into routine track maintenance.

In 1996, Railtrack's zone civil engineer Peter Kiernan reported: 'Over the last five to six years the general condition of the track in this area has shown a much greater stability throughout the annual cycle of climate changes than it did prior to the work being carried out.'

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