Experimental study of accumulative deformation behaviour and shakedown limit of saturated red mudstone fill material

Red mudstone compacted at optimal water content has been verified to be appropriate as fill material of high-speed railway subgrade. But the accumulative deformation behaviour of compacted red mudstone at saturated condition has not been investigated very well. In this study, a series of cyclic triaxial tests was carried out to study the effect of confining pressure and dynamic load on the accumulative deformation of saturated red mudstone fill (SRMF) material. Results indicate that the equivalent Young's modulus E-eq decreases rapidly to a stable state with increasing the cyclic number. The stable E-eq is strongly correlated to the stress state, and it increases with increasing the confining pressure and decreases exponentially with increasing the cyclic stress ratio. Based on shakedown theory, three types of accumulative deformation modes, namely, plastic shakedown, plastic creep, and incremental collapse, were identified. The stress state causing the unstable deformation was named as the plastic shakedown limit. The stress state that causes a sudden increase in strain rate was defined as the plastic creep limit. The cyclic stress ratios corresponding to the plastic shakedown and plastic creep limit were smaller than the shear strength at static state, and decreased exponentially as the confining pressure increases. With particular emphasis on the effects of rainfall, the drainage facilities and improved red mudstone fill material were suggested in the design of surface layer of subgrade to enhance their stability.