Lifting, lining and consolidating

Ballasted track is still the most widely used design of permanent way worldwide. Ballast was originally chosen as the load distributing element due to its high availability, however, the demands on the system have increased significantly since then. Today ballast is far more than an indiscriminate pile of excavated material. Together with other components, it forms a highly active system. Clear chemical, geometric and physical reference values define the national requirements for the ballast. Over the useful life of the system these properties change and so does the overall behaviour. 

The system flexes under the applied load. The resulting bending line makes it possible to distribute the loads on several sleepers and dissipate them. The load transfer, however, has its effect on the ballast. Due to different wear mechanisms in the ballast and in the subsoil a part of the settlement appears plastically. The result is track geometry faults, horizontal and/or vertical deviations from the desired track geometry (target geometry). At the latest after reaching the defined limits (EN 13848) the track geometry must be corrected, in order to allow the safe operation of the system. Track geometry faults significantly affect the riding of vehicles on the system. Comparable with bumps in the road, transport vehicles can compensate for this irregularity depending on their speed. If that does not work anymore, a derailment is the logical consequence. Railway Infrastructure Managers begin to take action to maintain the track ballast long before reaching this threshold. The reduction of the dynamic impact on the overall system, the restoration of the basic elastic properties, and the guarantee of the desired riding comfort are at the forefront of this. It is very important for the overall system, in particular, to prevent excessive formation of cavities and to create homogeneous sleeper support. 

In order that the correction values determined in this way can be implemented, the existing ballast has to be supplemented in some places with new ballast. Depending on the correction procedure selected, the lifting and lining values of the machine give the necessary information about the desired vertical and horizontal change of position.  The machine lifts the existing track skeleton and moves it in the desired direction. Under the track skeleton a free space is created which is filled in by the parallel compaction and consolidation process. There are various technologies on the market for this compaction process, two of which are described below. In this context the fundamental differences are first described, more detailed information is available on the manufacturer’s site.

In 1953, Franz Plasser and Josef Theurer developed the first hydraulic tamping machine. The use of hydraulics allowed the non-synchronous operation of tamping tines that were opposite one another. The uniform pressure on all tamping tines on the ballast guarantees the creation of a homogeneous support^[4]. Studies^[2]showed the importance of the choice of parameters (frequency and amplitude) on the durability of the tamping and the targeted consolidation (lifting of the track skeleton) to the ballast bed. Plasser & Theurer chose 35 Hz and an amplitude of 4 to 5 mm, a parameter set based on the testing performed by Fischer.

In combination with an adequate squeeze time these parameters are considered as a basis for the durability of the tamping. While the tamping tines are squeezed by means of the hydraulic cylinders (tamping force 10 to 12 kN), they are vibrated with a constant amplitude by a rotating eccentric shaft. In combination with a rotating mass this design allows stress peaks to be bridged and the tamping operation to be carried out in a short time. ^[5]

The basic parameters of the tamping procedure, have remained the same until this day over the many years of international experience. On new machines the speed regulation is used to reduce the flow behaviour at different frequencies in order to reduce the penetration resistance.

In both of these processes, it is important to ensure that the tamping depth of the tines is chosen sufficiently large depending on the selected track form (design height of the superstructure, rail, rail pads, fastenings and sleeper). If it is too low, it may be that the squeeze action damages the bottom edge of the sleeper. On the other hand, too large a tamping depth can lead to a low consolidation under the sleeper. The rules and regulations of DB Netz AG consider that 20 mm is the ideal distance between the upper edge of the tine plate and the lower side of the sleeper as the ideal tamping depth.