Signs of wear in the track bed
Klaus Lieberenz, Silvio Klügel, René Kipper
The track must be constructed and maintained in a way that enables it to carry the planned traffic loads and to provide the required travel comfort. When it is used it develops signs of wear in the track, subgrade or subsoil, and thus there is a deterioration in the quality of the track geometry. This is crucially dependent on the loads applied between wheel and rail, the transfer of the loads into the track bed system, as well as the elasticity and deformation behaviour of the entire system. So, damage may occur which can rapidly worsen and extend over long sections of the route. Its emergence should therefore be prevented or it should be dealt with as soon as a single error occurs.
Discontinuities in the area of the load transfer between wheel and rail, such as rail joints (fish-plated joints, welded joints or insulated joints), transfer areas in the common crossing and tongue areas of turnouts and crossings, rail surface defects (corrugations, waves, wheel burns or breakages), as well as cavities under sleepers lead to dynamic additional loads. These loads cause significantly higher stresses in the bearing system with increasing speed of trains. Initially, the track and then the ballast may be overstressed and there may be damage to the fixings and to the sleepers which may lead to smashed particles, particle rearrangement and settlement in the ballast bed. A typical sign for this is white spots on the surface. If the cause of the problem is not resolved, the successive overstressing increases and the damage spreads lengthwise in the direction of travel and in the depth.
Overload (δB < δp,1 + δp,1,dyn) and/or pollution of the ballast change the deformation and elasticity behaviour, the shear strength and load distribution in the trackbed. The ballast is overloaded and loses its carrying and drainage capability due to particle destruction, particle rearrangement and rounding off. Both track bed and subsequently also the subgrade/subsoil can no longer take the higher loadings without damage and detrimental deformations.
Overstressing by additional loading or lack of load distribution in the ballast (δp,2), and/or the increased deformability/reduced load-bearing capacity of the floor (δU) due to increased water content due to poor drainage (disturbed formation crossfall, inefficient longitudinal drainage) lead to initially higher elastic and then plastic deformation and damage to the load carrying capability (δU < δp,2 + δp,2,dyn or increased deformability/reduced load carrying capacity δU* < δp,2).
Ground in the subsoil that can easily deform such as fine-grained soil without sufficient shear strength, soft layers and displacement sensitive sands can be compressed or moved under dynamic additional loading and lead to settlement and long wave changes in track geometry.
Over short distances in longitudinal track direction, changing ballast bed modulus or track stiffness in transition areas lead to dynamic additional loads with overloading in the track. It can also lead to excessive wear of the subgrade and damage to the track, as well as consolidations, particle rearrangement and settlement in the track bed and the subgrade/subsoil.