Experimental Identification of Dynamic State Boundary Surface of Red Mudstone Fill Material

Red mudstone fill material (RMF) has been used as a substitute fill material in railway subgrades, but its allowable dynamic response under long-term cyclic loading has not been clearly identified. This paper presents an experimental identification of the dynamic state boundary surface (DSBS) of RMF under different water contents and cyclic stress levels. A series of single-stage cyclic triaxial tests with up to 50,000 loading cycles was performed on compacted RMF specimens. The DSBS was illustrated by the isolines of loading cycles and cyclic stress in terms of permanent axial strain, strain amplitude, axial strain rate, equivalent Young’s modulus, and dissipated energy. Clear two-stage behavior was observed in the evolution of permanent deformation, equivalent Young’s modulus and dissipated energy, indicating that the effect of loading cycles should be incorporated into the state boundary framework. The DSBS based on deformation-related variables showed pronounced water content dependence, but the gradual variation in these variables led to ambiguity in identifying the shakedown ranges, especially the creep limit. In contrast, the dissipated-energy-based DSBS exhibited a stable lower boundary at high loading cycles. Two characteristic dissipated energy levels, 0.01 kPa and 0.04 kPa, were therefore defined as the boundaries for the shakedown limit and creep limit of RMF, respectively, regardless of water content and cyclic stress state. The energy classification was supported by saturated RMF tests and field excitation data from the Dazhou–Chengdu railway. These results suggest that dissipated energy can serve as a unified indicator for evaluating the stability of RMF subgrade, and that the DSBS provides a practical framework for characterizing the allowable dynamic response of clayey fill materials.