The Ageing and Creep of Dense Granular Materials

Elisabeth Bowman, Cambridge University
Geotechnical Engineering Group


An attempt is made to understand the mechanisms that lead to the phenomenon of 'ageing' in granular materials and 'set up' of displacement piles in sands, via creep, by relating macro-behaviour to micro-structural change. Two categories of parameters that may influence ageing and creep are identified as inherent and induced. The inherent parameters investigated are particle size, shape and strength and relative density. The induced parameters investigated are stress path, loading rate, the influence of water and small perturbations of load during constant stress creep periods.

Shape is regarded as being of particular importance to creep behaviour. A novel method to characterise particle shape or morphology and roughness or texture is described. This uses Fourier descriptor analysis of individual particle outlines captured by scanning electron microscopy.

Experiments were conducted on a range of pluviated dense granular materials in a triaxial apparatus. A stress path was chosen to mimic that felt by an element of soil near to a displacement pile as it is installed. This stress path involved loading of the soil into compression at high mean and shear stress, then unloading into extension at low mean stress, while maintaining a high shear stress ratio. During loading, tests conducted at different rates showed no influence on the strain developed. In unloading, the slower rate tests produced greater shear strains once the condition became extensive. Creep stages in compression and extension showed that all of the materials initially contracted but then dilated over time as the creep strain vector rotated. Stronger, smaller, more rounded and more densely packed particles tended to produce dilation earlier during creep. The presence or absence of water did not influence the creep of a clean sand. Loading rate and stress path was found to influence the initial creep behaviour greatly, however, with time the creep responses became more similar. Applying small deviatoric load cycles during creep tended to accelerate the dilatant tendency of a fine angular sand.

The change in microstructure of pluviated dense sands during one-dimensional creep was investigated, using resin injection and optical microscopy of sections. It was found that, upon application of load, particles aligned to be more perpendicular to the load direction. However with time, the particles rotated in space. A change in the local void ratio distribution was also found. Initially, the particles were relatively evenly spaced. However with time, they grouped or clustered together. A conceptual model of creep was proposed which emphasises the bimodal load-bearing nature of granular materials. The model may help to explain the complex volumetric response of dense sands and why disturbed soils 'age' with no detectable change in relative density.

An alternative hypothesis for the set up of displacement piles was proposed, involving dilatant creep at macro- and micro-level. Kinematically restrained dense sands attempt to dilate under high shear stress during creep, resulting in an increase in mean stress. Particle clustering and interlock within the soil mass leads to a stiffer material with time. The result is an increase in radial stress around the pile and ageing of the soil with time.