An investigation of crushing and compaction bands in granular material

George Marketos, Cambridge University
Geotechnical Engineering Group


An investigation into the breakage and crushing of grains inside a granular material has been conducted. Grain crushing is one of the deformation micromechanisms for such materials, its extent directly affecting their stress-strain response and permeability. It is observed at moderate stress-levels for weak-grained materials such as calcareous soils and at higher stress-levels for hard-grained materials, often attained locally in common engineering applications.

The present study made extensive use of Discrete Element Method (DEM) computer simulations, where a crushing model was implemented. The probability density function describing the inter-grain force distribution was thence calculated and combined with the grain crushing strength distribution using probability and statistics arguments. This led to the development of a method that successfully related the grain strength to the extent of crushing observed macroscopically in simulations, providing an example of how similar methods might link the of micromechanics and continuum mechanics of granular media.

The location of the grain breakage events inside the simulations was also studied. In some cases crushing localisation was observed, inside compaction bands which were perpendicular to the principal straining direction. Compaction band formation was associated with sample instability. Such bands are important as flow barriers, sources of sand production or causes of borehole instability in hydrocarbon extraction, but the material conditions that favour their formation have remained relatively unknown. This study identified the number of bonds broken prior to a grain crushing event as an additional requisite for discrete compaction band formation, itself linked to the ratio of bond strength to grain strength. The initiation and propagation of such a band is also reported here. Furthermore, the propagation of a crushing front through the sample, sometimes termed a diffuse compaction band, was observed in some simulations.

Finally, an extensive investigation into the effect of the use of flat boundaries for stressing a granular material was conducted. This was seen to lead to non-representative strains as inferred by boundary displacement values, the best option being a best-fit strain calculation. The extent of the boundary region was also quantified, and was seen to extend much further than previously thought, raising questions as to the validity of commonly followed guidelines for the size of granular samples used in experiments.