Dynamic properties and shakedown behavior of red clay under intermittent cyclic loading

Red clay, widely used as a subgrade material in southern China, requires a reliable evaluation of its dynamic behavior to ensure infrastructure safety. Long-term cyclic triaxial tests were conducted on red clay from typical, complex subway subgrades to investigate its dynamic properties and shakedown behavior under intermittent cyclic loading. Results show that intermittent cyclic loading, especially with multiple amplitudes, causes greater axial plastic strain and lower post-cyclic strength than continuous loading. These effects diminish with increasing confining pressure. Notably, axial strain partially recovers during loading intervals, with recovery ratios depending on the number and sequence of pauses. Based on the rules of cumulative plastic strain rates and cumulative plastic strain increments, shakedown behavior for red clay under intermittent cyclic loading is divided into three categories: plastic shakedown, critical shakedown, and plastic creep. A quantitative shakedown limit criterion is proposed using the Boltzmann function. Shakedown behavior significantly influences the post-cyclic strengths, and the influence diminishes as confining pressure increases. Samples exhibiting plastic creep and plastic shakedown behavior have the lowest and highest strengths, and those with critical shakedown behaviors have medium strengths. Cyclic loading with relatively low-stress amplitude causes a hardening effect, while cyclic loading intermittence or cyclic loading with relatively high-stress amplitude causes a degradation effect, and both effects are mitigated by higher confining pressures.