Ballast stiffness estimation based on measurements during dynamic track stabilization

Abstract

The Dynamic Track Stabilizer (DTS) is used to compact the ballast after track tamping. Ballast compaction with the DTS is known to increase the lateral track resistance and thus the (shear) stiffness of the ballast. The shear stiffness of railway ballast can be expressed as a strain dependent shear modulus G. The conducted research yielded two approaches for estimating the shear modulus of ballast, which are based on measurements taken during dynamic track stabilization. Firstly, the Hardin equation, with its enhancement for coarse-grained soils, is used to estimate the small strain shear modulus for a given grain size distribution curve. This provides a foundation for estimating shear modulus degradation curves (G/Gmax). These curves are calculated by measuring ballast shear strains with accelerometers at the top and bottom of the ballast layer, as well as the stress distribution within the ballast layer. For a specific track section, different shear strains and stresses are observed for a bypassing DTS, resulting in the aforementioned shear modulus degradation curves. The results of a regular maintenance operations demonstrate that these shear modulus degradation curves align with those described by other researchers for similar material. Secondly, a SDOF model was developed to analyze the track-ballast interaction under harmonic loading, which is caused by the dynamic track stabilizer. For this model, the mass and mass moment of inertia of the track and the geometry of an equivalent machine foundation are estimated using influence lines, with spring and dashpot coefficients according to the Hall analog. Furthermore, the model response is fitted to the field test data using an optimization algorithm. The resulting shear moduli show plausible values for both loose and compacted ballast conditions. Consequently, this method can serve as the basis for an Intelligent Compaction (IC) system with the DTS in the future.