Creep, ageing and microstructural change in dense granular materials

Understanding the mechanisms behind time-dependent behaviour of granular materials is important in assessing ageing of dynamically improved sands and set-up of displacement piles in sand. A series of triaxial tests was carried out to investigate the creep of dense granular materials. In addition to shear strain development, a complex volumetric strain response with rotation of the creep strain vector over time was measured and the effect of the material characteristics was examined. The change in microstructure of dense sands during one-dimensional creep was also investigated using resin injection and optical microscopy of sections. Upon application of load, particles aligned to be more perpendicular to the load direction. However, over time, the particles rotated in space. A change in the local void ratio distribution was also found. Initially, the particles appeared relatively evenly spaced. However, with time, they grouped or clustered together. A conceptual model, accounting for these changes in microstructure, is proposed. The model emphasises the importance of the bimodal load-bearing nature of granular materials, suggesting that creep begins in frictional slippage of weakly loaded particles, which allows strongly loaded columns of particles to gradually adjust. The model may help to explain the complex volumetric creep response of dense soils and why dynamically densified soils 'age' with no detectable change in relative density.