Scaling of the structure and strength of soil aggregates

In this study we provide experimental evidence to support concepts concerning the scaling of soil strength and pore structure over a range of aggregate sizes. The aggregates were obtained by the repetitive fracture of larger soil aggregates. This ensured that the smaller aggregates studied were the building blocks of the larger aggregates. Sieving tilled soil to obtain a range of aggregate sizes introduces artifacts due to stresses imparted during tillage and due to the different effects of wet/dry cycles on aggregates of different sizes.

Aggregate strength was measured by the indirect tension test. The measurements of pore structure are (a) specific volume, (b) the pore size distribution by mercury porosimetry, (c) the pore size distribution measured on thin-sections, and (d) the proportion of the fracture surface consisting of pre-existing cracks. In the latter approach, incipient cracks in soil that link up during fracture were stained using methylene blue dye. Scaling of all pore structure properties is assessed using fractals.

For the three soils tested, aggregate strength increased with decreasing aggregate size. A log-log relationship between aggregate size and strength is found from which both friability and a fractal dimension are determined. Measurements of pore structure scaling suggest that fewer pores are available for crack propagation with decreasing aggregate size. Specific volume and the proportion of pre-existing cracks on fracture surfaces both decreased as aggregate size decreased. Analysis of thin-sections and measurements of pore-size distributions using mercury porosimetry suggest that only the few largest cracks form incipient interaggregate fracture surfaces and control the strength.

The results are consistent with the concept that aggregate strength scaling is dependent upon pore structure scaling. A comparison of aggregate strength with the amount of pre-existing cracks on the fracture surfaces shows that fracture occurs because of crack propagation. Further research examining the mechanisms of crack propagation in soil is required to determine a physical relationship between pore structure and strength scaling in soil aggregates.