Over the past 50 years, experimental studies have repeatedly demonstrated that the mechanical behaviour of sand is sensitive to the material fabric, that is, the arrangement of the grains. Up until now there have been relatively few attempts to describe this fabric quantitatively. Much of our understanding of the link between the particle movements and interactions and the macro-scale response of granular materials, including sand, comes from discrete-element modelling and experiments on ‘analogue' sands with simple, idealised shapes. This paper investigates methods of quantifying the directional fabric of a real sand and its evolution under loading. Statistical analyses of the distribution of fabric directional data in terms of particle, contact normal, branch vector and void orientations were carried out at different stages of shearing deformation. The data show that the initial particle orientation fabric that develops during the deposition of the material tends to persist during shearing, while in the post-peak regime the contact normals seem to be reoriented along the direction of the major principal stress. Different patterns were observed within the shear band, as both the particles and the contact normal vectors appeared to rotate along the shear plane.

Shelly carbonate sands proliferate in regions of the world where construction of offshore structures is in high demand. These structurally weak sands have high intra-granular voids and complex angular grain shapes. To improve the understanding of the mechanical properties of the material, a detailed morphological quantification is required. This paper presents a three-dimensional characterisation of the morphology of shelly carbonate sands based on analyses of X-ray computed tomography images. Two sands from the Persian Gulf with distinct grading were investigated. An adaptive watershed segmentation technique was developed to identify the individual grains for subsequent grain-scale analysis, which overcomes the challenges posed by the intricate microstructure of these sands. Non-invasive measurements of size, intra-granular void and various shape parameters were carried out, and statistical analyses were conducted, to characterise the grains. The results help to better understand the mechanisms of grain interlocking, and the role of grain angularity and intra-granular void ratio on the mechanical behaviour of shelly carbonate sands.

The packing density of a soil is typically described in terms of the scalar parameters void ratio, specific volume or porosity. Within a critical-state soil mechanics framework, the response of a sand is considered to depend on its ‘state', that is, the combination of void ratio and effective stress. When soil is considered from a particle-scale perspective, the packing density is often described using the coordination number (CN), that is, the average number of contacts per particle. Using particles with simple geometries, discrete-element method (DEM) simulations, photoelastic tests and experiments on analogue soils (e.g. glass ballotini) have all demonstrated a correlation between CN and mechanical response. This study investigates whether these correlations apply to real soils using direct measurement of the fabric via micro-computed tomography (micro-CT). A variety of scalar fabric descriptors was measured during the triaxial compression of both intact and reconstituted Reigate sand samples. While each of the parameters considered provides useful information, the contact index (CI), a parameter that takes the contact area into account, was found to be a more sensitive metric in assessing the effects of fabric than coordination number, branch vector length or void size.

Internal erosion involves the migration of particles through a geotechnical structure. Internal erosion poses a significant hazard to embankment dams and flood embankments. The fundamental mechanisms operate at the particle scale and a thorough understanding of these mechanisms can inform the filter design and specification process and reduce the hazard that internal erosion is known to pose to many engineered embankment structures. Engineers have long acknowledged the importance of the grain scale interactions, but until recently, explanations of the mechanisms have been purely hypothetical, as direct observation of the internal structure of filters was not possible. Recent research has used the discrete-element method to establish a particle-scale basis for Kézdi's filter internal stability criterion. The discrete-element method can provide significant useful data on soil microstructure, so a discrete-element method model is inherently ideal. This study therefore examines a number of real sand samples with varying degrees of internal stability at the particle scale using high-resolution micro-computed tomography. The correlation between coordination number and internal stability is confirmed, with the coordination number values being significantly higher for the real material.