Three-dimensional quantification of the morphology and intragranular void ratio of a shelly carbonate sand

Abstract

Shelly carbonate sands represent an extreme soil type in terms of their mechanical behavior which derives from the bioclastic nature of the constituent grains. In their uncemented form, these deposits exhibit very high compressibility, which has posed a number of geotechnical engineering problems; in most cases related to the reduction in the bearing capacities of both shallow and deep foundations. Remarkable features of these carbonate sands include the complex shape and the structural weakness of the grains and the high inter and intra granular porosity. Previous studies, have quoted the interlocking of the angular shelly particles to be at the origin of their high friction angles and high initial void ratio, however, up until now, no scientific micro-scale examination has been carried out. This paper presents a non-invasive image based investigation into the grain morphology of a carbonate sand from the Persian Gulf. This sand has a median grain size of 570μm and a high CaCO3 content in the form of aragonite and calcite. Three-dimensional images from x-ray computed tomography (3DXRCT) with a size of 6μm were used. The presence of various skeletal bodies such as shells of small organisms with distinct densities and composition poses real challenges for an accurate segmentation. Image processing algorithms were developed in order to identify the individual sand grains and quantify their properties. Earlier work on silica sands has highlighted the importance of 3D non-invasive techniques in providing an accurate distribution of the grain sizes when compared to more traditional techniques such as sieving analysis and 2D microscopy.