When the local maintenance workshop finds that track tamping cannot improve the track geometry quality durably, the ballast bed needs cleaning.
Fine particles make the ballast bed age faster. The quality of the ballast bed is crucial for the durability of the track. When the amount of fine particles increases, cavities form more often. Later on, white spots develop. They indicate progressing wear on the ballast bed, particularly if water cannot drain, and the white spots become increasingly wet.
The resistance to breakage and abrasion depends on both, the ballast used and the loads to which it is subjected. This includes the axle load, sleeper type, stiffness of the subsoil, line speed etc. The progression shown is symbolical, the deterioration of the ballast at around 500 million load tons is subject to large, local fluctuations which can vary widely.
The diagram below shows the typical track quality progression. With a standard deviation of 0.3 mm, the initial quality is remarkably high. After a short consolidation phase, during which the track geometry deteriorates more quickly, the track geometry deteriorates uniformly following the typical progression rate. When the thresholds are reached, track tamping restores the initial condition of the track grid. As the ballast ages, the track geometry deteriorates more quickly and the durability of the tamping results decreases.
Consistent analysis of the track geometry quality, particularly by using fractal analysis and ground penetrating radar information, makes it possible to objectively determine the time for ballast bed cleaning or track renewal.
Cleaning the ballast bed at the right time saves money in the long-term.
