Effects of principal stress axis rotation on cyclic deformation characteristics of rail track subgrade materials

The cumulative plastic deformation of rail track subgrade materials under moving wheel loads is a complex problem. The plastic deformation and resilient modulus of subgrade materials subjected to moving wheel loads are a function of Principal Stress Axis Rotation (PSAR). Difficulties associated with the control of PSAR in conservative experimental methods lead to inaccurate estimations of the actual plastic deformation characteristics of rail track subgrade. Therefore, this study modifies a torsional multi-ring shear apparatus to evaluate the deformation characteristics of subgrade materials under moving wheel loads. The performance of this modified apparatus is evaluated by comparing the experimental results with those of small-scale model tests on an asphalt roadbed rail track with sandy subgrade. The evaluation suggests that the modified multi-ring shear apparatus has an excellent capability to estimate the deformation characteristics of rail track subgrade under moving wheel loads. A series of modified multi-ring shear tests is then conducted to investigate the effects of the loading conditions, the subgrade density, and the loading frequency on the deformation of subgrade materials under moving wheel loads. The results of these tests show that cyclic single-point load tests underestimate the actual deformation characteristics of subgrade materials irrespective of the subgrade density and the loading frequency. Furthermore, the ratio between the plastic cumulative deformations obtained from the moving wheel loading and from the cyclic single-point loading, referred to as the “ratio of axial strain” (Rs), is introduced to evaluate the plastic deformation characteristics of rail track subgrade materials under moving wheel loads only, using the results of cyclic single-point loading tests. Finally, an empirical formula is proposed to predict the cyclic plastic deformation of rail track subgrade materials as a function of the number of loading cycles.