Detailed quantification of internal erosion in railway subgrades under cyclic loading

Extreme weather events and intense rainfall increasingly make railway subgrade structures susceptible to internal erosion. While extensive research addresses internal erosion under steady-state conditions, the combined impact of cyclic loading and upward seepage flow, critically relevant to railway subgrades, remains largely underexplored. This study experimentally investigates internal erosion in Group A/B coarse fills under these complex hydromechanical conditions. Dye tracing and particle image identification techniques were employed to track the depth of origin and composition of eroded particles. Three erosion modes – stable, transitional, and failure - were identified and described using a power-law relationship. Higher hydraulic pressures accelerated particle loss throughout all soil layers. For instance, at 5 Hz, total eroded mass sharply increased from 15 g (at 5 kPa) to over 318 g (at 11 kPa). Increased loading frequency intensified hydraulic gradient fluctuations and shifted the primary erosion zone downward, resulting in more severe erosion. Dye tracing revealed that under stable conditions, over 80% of eroded fine particles (<0.25 mm) originated from upper layers. Conversely, in transitional and failure modes, particle loss extended to deeper layers, including coarser particles (up to 2 mm). For example, under 11 kPa pressure and 15 Hz loading, approximately 37.6% of the total loss originated from the two bottom layers. Hydraulic measurements further confirmed that both increasing hydraulic pressure (peak gradient rising from 1.45 to 8.45) and loading frequency (interquartile range of gradient expanding by 2.6 times) intensified soil instability. These detailed quantitative can provide data to support filter layer design and the development of constitutive models for predicting internal erosion in railway subgrades. • Internal erosion quantified under cyclic loading and seepage conditions. • Dye tracing and image analysis pinpoint origins of fine particle loss. • Higher hydraulic pressures accelerate erosion across specimen depth. • Increased loading frequency shifts primary erosion zone downward.