Ballast stiffness estimation based on measurements during dynamic track stabilization

Ballast compaction with the Dynamic Track Stabilizer (DTS) is known to improve the lateral track resistance by increasing the stiffness of the ballast. This paper presents two approaches for estimating the strain dependent ballast shear modulus G based on measurements during dynamic track stabilization. One approach uses the Hardin equation with its enhancement for coarse-grained soils to estimate the small strain shear modulus for a given grain size distribution curve. Since different shear strains are induced on an observed sleeper (with fixed location) by a bypassing DTS, the measurement of ballast shear strains with accelerometers on the top and the bottom edge of the ballast layer yield shear modulus degradation curves G/Gmax. It is shown for data from a regular maintenance operation, that these shear modulus degradation curves are in accordance with previous research. For the second approach, a SDOF model for the track-ballast interaction under harmonic loading (caused by the dynamic track stabilizer) is developed. Influence lines are used to estimate the mass, the mass moment of inertia and the geometry of an equivalent machine foundation (with spring and dashpot coefficients according to the Hall analog). An optimization algorithm is used to fit the model response to the measured data from field tests with the DTS. The resulting shear moduli show plausible values for loose and compacted ballast conditions, which proves the potential of the presented method as a basis for a future system for Intelligent Compaction (IC) with the DTS.