Abstract
Size limitations of testing equipment often impliy that samples of coarse granular materials must be scalped or scaled, to reduce the size of the constitutive particles, before they can be tested either by triaxial or direct shear in the laboratory. The objective of the investigation is to evaluate the particle shapes in a natural sample of colluvial sediments, to identify potential correlation(s) between shape and size, that could impact shear strength of scaled samples. The material investigated is derived from eroded ancient sedimentary rocks from the Pilbara region of Australia. The fragments have a particle shape ranging from slabs to sub-equant blocks. The observation indicates that there is an increase in the tendency for slabshapes in larger particles. Therefore, scaling inevitably alters the characteristic shapes of the material particles as it implies substituting larger (slabs) particles by smaller (sub-equant) particles. Changes in particle shape distribution may induce changes in material fabric and shear strength and therefore may need to be considered when scaling samples.
Highlights
The mechanical behaviour of granular materials is affected by the shape of the constituent particles
Any of the common techniques for material scaling change the original particle size distribution and, potentially, the particle shape distribution. This is significant, because changes in particle shape distribution may conduce to a change in material fabric and shear strength
The colluvium derived from the Banded Iron Formation (BIF) outcrops forms densityseparated deposits in incised channels, with denser ironrich fragments in the bottom of the channel, overlain by chert-rich fragments
Summary
The sample is an overburden material composed of eroded and transported particles (colluvium) derived from a sedimentary Precambrian-aged Banded Iron Formation (BIF), with province-wide distribution, located at the Pilbara region of Australia [4]. Thin layering of banding of less than 1 mm to few centimetres can be observed in saw cut and polished fragments (Figure 2). Parting along fractures sub-perpendicular to the lamination can occasionally be recognized. The evaluation of the relationship between particle size and lamination composition was not part of this research. 30X magnification of polished fragments (grey grains are hematite and magnetite; red and orange material is chert.). Twelve different size fractions (Table 1) covering 95% of the mass of the material were defined for analysis
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