The mechanical behaviour of Eglin sand at the micro-scale was studied in this work. Laboratory experiments using unconventional apparatuses were carried out in order to study the contact behaviour of pairs of particles in both compression (normal loading) and shearing (tangential loading) and the strength of grains. Particle fractions were identified according to their colour by visual observation and both their chemical composition and surface morphologies were obtained. The tangential stiffnesses and inter-particle coefficients of frictions for the different fractions found in the sand sample were determined under the range of normal loadings applied (1-9N). The results show some discrepancies between the theoretical models commonly found in literature to describe either the normal or the tangential loading response, which are able to predict the trend of the force-displacement curves but using elastic moduli that are lower than those found in literature, especially in the case of tangential loading. Also, the results of particle crushing tests show quite consistent results (excluding one particle group), probably related to the similar mineralogy of all fractions, which are mainly constituted by silica.

A comprehensive series of tests was carried out using an innovative three-axis inter-particle loading apparatus, investigating the normal and shearing contact behaviour between sand particles of a variety of geological origins. The results show that the normal loading behaviour is very sensitive to the type of sand and its surface condition, and thus to its geological origin and history. For particles characterised by a highly regular shape and low surface roughness, such as those of a common quartz sand, good predictions may be made with models based on the elasticity theory, but at lower loads the particle roughness needs to be accounted for. In shearing the coefficient of inter-particle friction is highly variable, both for a single sand and between different sands, but the average values are controlled predominantly by the surface roughness. Current elasticity-based methods for predicting the tangential stiffness overestimate it considerably, but coaxiality between the forces and displacements indicates that the underlying assumption of elasticity with micro-slipping at the contacts may be correct.

In this paper, an experimental micromechanical study is presented investigating the contact mechanics and tribological behaviour of highly/completely decomposed tuff granules (denoted as H/CDT). The parent material was taken from two locationsnamed the top and bottomfrom a recent landslide in Hong Kong and in this study the tested granules were obtainedfrom the parent material after drying and sieving processes. Basic material characterization was conducted quantifying the particle shape, the surface roughness and the strength of a set of grains. A set of twenty-nine monotonic inter-particle shearing tests were conducted on pairs of granules taken from the top and bottom of the landslide. It was found that the granules had very high friction angles at their contacts, in general greater in comparison to other materials reported in the literature. The slightly greater inter-particle friction for the granules taken from the top of the landslide might be because of their higher roughness in comparison to the ones from the bottom. Additional experiments were conducted to investigate the normal and tangential load-displacement response of the granules subjected to cyclic loading. A good curve fitting for the normal load-displacement 2 response could be obtained by using very low apparent Young's moduli in the Hertzian model. In general, the decomposed tuff granules showed significant plastic response during the first normal load cycle, and this plastic behaviour continued for the subsequent third and fourth cycles. In the cyclic inter-particle shearing tests, the non-linearity and hysteresis increased for larger cyclic displacements, but the effect of the number of shearing cycles on the energy loss was generally small. Finally, a limited discussion is presented on the applicability of a theoretical model on the tangential loaddisplacement behaviour of the granules.

The mechanical behaviour of cemented sands at macro-scale has been widely studied in the past, while there is still a lack of laboratory test data for the micro-mechanical response. Therefore, a series of uniaxial compression and tangential shear tests on artificially cemented sand particles have been conducted using a microscope to observe their behaviour. First, the consistency of cemented particles with different cementing agents is discussed. Three breakage modes are proposed according to the images taken using a microscope camera. A new parameter named ‘local roundness at contact’ is introduced to emphasise the effect of contact morphology on sample strength. Moreover, the effect of bond thickness on the mechanical response of cemented particles has been investigated. Finally, the shear strength parameters of artificially cemented sand samples have been determined for two different sample diameters.

No abstract

No abstract

The long travel distances recorded by most submarine landslides indicate that changes in particle attributes (shape and size) may occur during their movement. Yet, little is known about the magnitude of such changes, and their underlying physical drivers. In order to understand the failure characteristics of submarine landslides, a dynamic image analyzer was used to characterize the particle size(s), aspect ratio, sphericity and convexity of 200 samples recovered from two IODP sites (C0018 and C0021) offshore Nankai (SE Japan). The two IODP sites drilled a series of submarine landslides, and undisturbed slope sediment, previously characterized in detail on 3D seismic data. The results of this work reveal no perceptive differences for particle size and shape between landslide and undisturbed slope strata, suggesting that remobilized sediment, sourced from marine deposits accumulated on the upper slope of Nankai do not change through slope instability. This lack of particle attribute differences between remobilized (landslide) and undisturbed sediment, and between the two IODP Sites, suggests limited interaction between particles during their movement. A fluidization mechanism is therefore proposed whereby the soft and saturated state of deep-sea sediment leads to the development of excess pore water pressure. This mechanism maintains movement and inhibits particle-to-particle contact, limiting subsequent particle shape and size variations in the failed sediment.

Given the urgent informational needs connected with the diffusion of infection with regard to the COVID-19 pandemic, in this article, we propose a sampling design for building a continuous-time surveillance system. Compared with other observational strategies, the proposed method has three important elements of strength and originality: (1) it aims to provide a snapshot of the phenomenon at a single moment in time, and it is designed to be a continuous survey that is repeated in several waves over time, taking different target variables during different stages of the development of the epidemic into account; (2) the statistical optimality properties of the proposed estimators are formally derived and tested with a Monte Carlo experiment; and (3) it is rapidly operational as this property is required by the emergency connected with the diffusion of the virus. The sampling design is thought to be designed with the diffusion of SAR-CoV-2 in Italy during the spring of 2020 in mind. However, it is very general, and we are confident that it can be easily extended to other geographical areas and to possible future epidemic outbreaks. Formal proofs and a Monte Carlo exercise highlight that the estimators are unbiased and have higher efficiency than the simple random sampling scheme.