Geogrid slope reinforcement, track refurbishing for German rail lines

Compiled and edited by Ron Bygness

Hallstadt rail crossing

An existing railroad crossing in the Hallstadt section of railroad track 5100, from Bamberg to Hof (Saale), had to remain open for rail traffic, but had to be replaced by an underpass for the street. An underpass for Michelin Street, under the railroad tracks, had to be lowered in this project, which also widened the street over a length of 400m.

An alternate solution was necessary to overcome the difference in height, approximately 5m, to the east of the tracks. For this reason, the plan was to build a traffic circle (roundabout), constructed with an embankment at km3.588 on Michelin Street (Figure 1). Because of unfavorable subsoil conditions, the traffic roundabout embankment was built on a deep foundation of mortared, vibrocompacted columns in combination with horizontal reinforcement atop the vertical support members.

It was necessary to tie down the sides of the embankment— which had slopes of 45° (inside) to 60° (outside)—with a geogrid reinforcement. About 10,000m² of geogrid was used as the horizontal reinforcement over the compacted columns. Further, the geosynthetic slope reinforcement was completed with an additional 10,000m² of geogrid. A 3-dimensional erosion protection matting (TRM), manufactured from UV-stabilized polymer random-fibers, was placed on the outer layer.

The combination of detailed designs, appropriate product selection, and professional installation resulted in a project completed to the satisfaction of the customer, DB Projektbau GmbH, Projectzentrum Nürnberg (German Railroad Ltd., from the Nuremberg project center).

Track refurbishing: Seelze–Wunstorf

DB Netz AG (the German rail network), completed a program of track refurbishing in an area west of Hannover between the communities of Seelze and Wunstorf in northcentral Germany. The aging track in this area did not fulfill the higher requirements demanded by modern railroad traffic. The track still rested on old wooden ties, and the trackbed itself also had to be renewed.

Soil testing revealed that broad areas along the right-of-way could not be expected to absorb the required load-bearing capacity (cohesive subsoil and very heterogeneous). Thus, reinforcement of the subgrade protective layer (SPL) was essential to the completion of the project.

Due to limited space conditions along the track, reinforcing measures in excess of 35cm would have made reconstruction necessary in some long sections. Such reconstruction would have made the project so expensive that improvements would have been postponed for the foreseeable future. The use of one layer of geogrid made it possible to limit the SPL to a maximum of 35cm while improving load-bearing capacity, thus making the overall project feasible.

The work was done on a track-bound basis (i.e., the removal and placement of bulk materials, ties, and rails were performed entirely from the existing old track, with the appropriate trackprocessing equipment).

First, the old track was lifted by a cleaning machine to remove the track’s crushed stone bed and existing SPL. This exposed a subgrade on which the geogrid was installed, then the old track was put down on the geogrid (Figure 2). In the next step, a sand spreader/compactor lifted the old track again and put down a new 35cm-thick SPL. This was followed by applying a crushed stone layer, according to specifications.

The last step was to remove the old track and replace it with new rails on new ties. This was completed by a rapid replacement train. Construction monitoring confirmed the improved load-bearing results provided by the geogrid. Whereas the installation of an SPL without reinforcement could have improved CBR values by about a factor of 3, the use of geogrid produced an improvement by factors of 6-7, according to the manufacturer.

Ron Bygness is the editor of Geosynthetics magazine.