Flat boundaries and their effect on sand testing

A study of the effect of the use of flat boundaries on the stressing of a sample of an idealized granular material with no applied shear is presented. Discrete element method (DEM) data of 1D compression were analysed and the local strain field inside the sample was investigated as the sample was stressed. A best-fit strain was seen to best describe the material behaviour free from boundary effects. The individual particle displacements were probed, providing insight into the behaviour of particles adjacent to the boundaries. In addition, the porosity and force distribution inside the sample were observed, allowing for estimates of the width of a boundary region to be made. This region, non-representative of far-field material behaviour, will affect the behaviour of a granular sample in DEM or laboratory tests, with local porosity differences leading to a change in the transport properties of the sample, and force distribution changes leading to a bias in the location of grain cracking or crushing events for sufficiently high stress levels. Nevertheless, the largest effect of the boundary region was a severe underestimation of the stiffness of a granular material.