Geomechanical assessment of an inert steel slag aggregate as an alternative ballast material for heavy haul rail tracks

Abstract

The geomechanical behaviour of railway ballast is a key aspect in the performance of ballasted tracks, mainly under increased axle loads and/or train speeds. Recently, and driven by the circular economy paradigm, some researchers have evaluated the use of alternative ballast materials as replacement for traditional natural crushed rocks. This work presents a comparison between the geomechanical behaviour of a granite aggregate and the one of an electrical arc furnace steel slag aggregate, with the trade designation of ‘Inert Steel Aggregate for Construction’ (ISAC). Both materials were designed for heavy haul railway track applications and showed similar particle size distribution (PSD) curves established by the standard gradation AREMA N. 24. Laboratory tests were performed on scaled down ballast specimens to evaluate both the macro-structural behaviour under cyclic loading triaxial tests (long-term permanent deformation and resilient modulus under various stress paths) and the micro-structural behaviour (particle breakage in cyclic loading and single-particle crushing strength). With a view to analise the structural response of the railway track and to compare the influence of the two aggregates, numerical simulations by FEM are presented, using static loading and non-linear elastic material models for both the ballast and the sub-ballast layers. The models were calibrated on basis of laboratory test results, to obtain the elastic vertical displacement on the rail top level and the stress distribution on the track foundation layers. The results suggest that the use of steel slag aggregate as ballast material is promising. When compared with the granite aggregate, the ISAC demonstrated to have a higher crushing strength of its particles; a greater tendency to stabilize the permanent deformation (PD); a lower particle breakage after PD tests; and a higher resilient modulus. The results of the numerical modeling showed that the rail presented less maximum elastic vertical displacements (approximately 3% on average) and that the foundation soils underwent less maximum vertical stresses (approximately 4% for a 32.5 t/axle loading and 9% for a 40 t/axle loading), in the model with ISAC in the ballast layer.