Effect of variable confining pressure on the long-term cyclic behaviour of calcareous sand: experimental observation and microstructural interpretation

Subgrade soils are exposed to simultaneous cyclic deviatoric stress and variable confining pressure (VCP) caused by traffic loading. However, the effect of VCP on cyclic behaviour is still debated and no attempt was made to investigate its effect on calcareous sand. In this study, high-cyclic triaxial tests with a wide range of mean effective stress, cyclic stress ratio (ζ), and VCP amplitude were conducted to understand the effect VCP on the long-term cyclic behaviour of calcareous sand. The results demonstrate that VCP has a considerable effect on the accumulated axial strain (εq^acc) and shakedown behaviour of calcareous sand, both of which depend greatly on the stress levels. At a low ζ, the effect of VCP on the εq^acc and plastic shakedown behaviour is insignificant, where the application of VCP tends to slightly promote the εq^acc. However, at a high ζ, εaccq decreases significantly with an increase in the VCP amplitude. For different ζ, the damping ratio increases and resilient modulus decreases significantly as VCP amplitude increases. Moreover, microscopic observations of pore size distribution were obtained through nuclear magnetic resonance tests, and were analysed to discover the microstructural mechanism highlighting how VCP affects cyclic behaviour.