Method for slab track substructure design at a speed of 400 km/h

Abstract This paper presents a practical method for identifying the fundamental design parameters of a multilayered earth structure in ballastless tracks at an operational speed of 400 km/h. The properties of subgrade materials and layer thickness are determined based on vehicle–track coupled dynamic numerical analysis and integrated empirical models incorporating a threshold cyclic strain approach. The principle of the proposed method is to limit train-induced strain and deformation in the foundations in order to achieve satisfactory long-term track performance. Firstly, the critical velocity effect is introduced and briefly assessed for design purposes. Dynamic amplification factor (DAF) is obtained from the statistical analysis of simulated results of vertical vehicle–track coupled dynamic models. A trapezoidal stress pattern is used to characterize the mobilized stress distribution on the subgrade surface under two-axle moving loads of a vehicle on a slab track. The attenuation law of excess vertical soil stress in a substructure under traffic loads is consequently reproduced using Boussinesq’s equation. Next, two threshold strains are defined in accordance with the accumulation rate of residual subgrade settlement, predicted by empirical models as a function of the K 30 value (modulus of subgrade reaction) and plasticity index of the subgrade aggregate. Following this, the abovementioned results are synthesized into design criteria that can be readily utilized. The application of the proposed method is illustrated by an example of earthwork design. Field observations are finally presented for the validation of the developed approach. The design methodology will improve knowledge of slab track substructure design and is expected to extend the applicability of the current railway code to higher train speed conditions.