A significant portion of the maintenance costs for track is caused by insufficient substructure and, above all, by poor drainage conditions. Plasser & Theurer formation rehabilitation machines create a stable foundation for the permanent way.

Insufficient load-bearing strength of the subsoil poses a substantial technical problem because the stability of the permanent way as well as the track geometry is endangered to a high degree. Enormous expenditure for track maintenance will be the result, the service life of the entire permanent way material will be reduced substantially. Consequently, poor subsoil causes enormous extra costs. Rehabilitation of sections of track with subsoil problems is therefore absolutely essential for many reasons.

The installation of formation protective layers (FPL) is acknowledged as the most effective solution when it is necessary to raise the load-bearing strength of the substructure of track installations. Best results are achieved by a formation protective layer consisting of a sand-gravel mixture which is placed under the ballast bed and compacted.

Excavation and formation construction using RPMW 2002-2

Today, high quality substructure improvement can be performed in most cases by rail-mounted methods using high-capacity formation rehabilitation machines in continuous working action. However, this process requires the movement of great quantities of material for excavation of spoil and the supply of new material. Material recycling of existing used material on the worksite and the placement of geosynthetics contributes towards reducing these quantities quite substantially.

Perfect construction of the new formation - AHM 800 R

The idea of incorporating a recycling process for the ballast was first introduced in 1994 when the excavating machine AHM 800 R went into service on the ÖBB. The top layer of ballast and formation material are removed using two separate excavating chains working one behind the other. Only the old formation material is taken away, while the old ballast is screened and then crushed in the machine, mixed with sand-gravel, placed in the track and consolidated.

The installation of a formation protective layer is performed without dismantling the track. At the same time geotextiles and other geosynthetics can be installed in one pass.

Recycling of the old ballast brings savings of up to 50% in new material and fewer costs for transport to and from the site, dumping, specialist disposal, recultivation and energy. The achievable working output is around 40 to 80 m/h depending upon the thickness of the new formation protective layer. The track closure periods can be reduced by up to 50% and this will also reduce the operational hindrance costs.

The properly installed protective layer with geotextiles brings a high initial quality of the track with a high quality reserve. The maintenance expenses are minimised and the service life of the track extended. This quality reserve has been proven by the ÖBB on the basis of the evaluations of the MDZ quality figures of the track recording car runs: the previously usual tamping intervals of approx. 2 years have been extended considerably which has brought a drop in the maintenance costs.

Ballast recycling using cone crusher

The RPM 2002/RPMW 2002-2 built by Plasser & Theurer which perform the fully mechanised insertion of a formation protective layer with subsequent placement of ballast using recycled ballast material. The RPMW 2002-2 separates the ballast from cohesive material using a star screen (system Wiebe) as well as a high pressure water spraying unit before it is processed by the cone crusher and vibrating screen for re-use.

Excavation range of PM 200-2 R

At the present time, the latest technological development in formation rehabilitation using a sand/gravel mixture is the PM 200-2 R made by Plasser & Theurer which is also a machine for formation rehabilitation machine with integrated ballast recycling. Here the old ballast - as on the RPM 2002 - is sharpened and then returned to the track together with new ballast. As a new method, a ballast washing plant has been incorporated which frees the ballast of cohesive material. Due to a separate clarification plant on the machine for the wash water, the water consumption can be kept very low with highly efficient cleaning of the ballast.

This line of development in the technology of formation improvement shows the trend towards separate excavation of spoil and old ballast, consistent ballast reprocessing (recycling) and best possible profiling and consolidation of the formation. The possibility to insert geosynthetics achieves a reduction of the quantities of FPL gravel and together with the high cleaning quality and the integrated ballast recycling a greatly reduced requirement of new ballast.

Optimum consolidation and quality control enable traffic to be resumed at speeds up to 70 km/h. A further important point is the cost-efficiency: it was possible to raise the work output with substantially improved quality of work from less than 30 m/h (PM 200) to up to 110 m/h (PM 200-2 R).

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