Evaluation of Critical Dynamic Stress and Accumulative Plastic Strain of an Unbound Granular Material Based on Cyclic Triaxial Tests

Critical dynamic stress (σ(cri)) and accumulative plastic strain (ε(p)) are primary indicators regarding the dynamic stability of unbound granular materials (UGMs). This study aims to seek an effective method to evaluate the dynamic stability of UGMs used in railway subgrades. First, the dynamic characteristics of an UGM used in railway subgrade bed construction were investigated by performing a series of large-scale cyclic triaxial tests, with the results showing that ε(p) versus cycle number (N) curves can be categorized into stable, failure, and critical patterns. Grey relational analyses were then established, where the analyzed results demonstrated that the ε(p)–N curve pattern and final accumulative plastic strain (ε(s)) of the stable curves are strongly correlated with the moisture content (w), confining pressure (σ(3)), and dynamic deviator stress (σ(d)). The analyzed grey relational grades distributed in a narrow range of 0.72 to 0.81, indicating that w, σ(3), and σ(d) have similar degrees of importance on determining the ε(p)–N curve patterns and the values of ε(s) of the UGM. Finally, a data processing method using a back-propagation (BP) neural network is introduced to analyze the test data, and an empirical approach is developed to evaluate the σ(cri) (considering the effects of σ(3) and w) and ε(s) (considering the effects of σ(3), w, and σ(d)) of the UGM. The analyzed results illustrated that the developed method can effectively reflect the linear/non-linear relationships of σ(cri) and ε(s) with respect to σ(3) and/or σ(d). The σ(cri) approximately increases linearly with increasing σ(3), and a simple empirical formula is proposed for the σ(cri). In addition, ε(s) and its variation rate increase non-linearly with increasing σ(d) but decrease non-linearly as σ(3) increases.