Direct shear behavior of ballasts under differential cyclic normal loading conditions

Ballast is a common construction material, which often faces complex stress disturbances from nature and human activities. In this study, we conducted a series of direct shear tests on ballasts under constant and differential cyclic normal stresses. The effects of normal loading/unloading timespan ratios (ξ), applied peak normal stresses (σ_p), and shear rates (v) were explored. We found that the normal loading and unloading stiffness is related to ξ, σ_p, and v. Also, as ξ, σ_p, and v increase, the peak shear stress increases. The changing pattern of the time lag between the peak normal stress and peak shear stress is only related to ξ. When ξ ≤ 1/1, the peak shear stress lags behind the peak normal stress, showing dynamic weakening of shear strength. When ξ > 1/1, the peak shear stress and peak normal stress occur simultaneously, showing dynamic strengthening of shear strength. Moreover, it always shows dynamic strengthening for different σ_p, while increasing v makes the shear strength transitions from dynamic weakening to dynamic strengthening. In the normal stress loading stage, the shear stress increases linearly, whereas in the normal stress unloading stage, the shear stress curves exhibit non-linear evolutions, resulting in differences in the slope coordination between the shear stress and normal stress. Our findings provide valuable insights into mechanical behavior of ballasts under irregular stress disturbance, aiding in mitigating associated risks in railway engineering.