Factors that Influence the Terminal Grading of Sands

Earlier research found that for a carbonate sand shearing to extremely large strains allowed the soil to reach a stable grading; findings also reported by others for a limited range of sands, typically sedimentary and often weak grained. This paper describes similar tests on a soil of very different geological origin, a weathered soil, that confirms similar patterns of behaviour. Tests investigating factors affecting the final grading for both soils indicated small effects of shearing velocity but significant effects of the initial grading, highlighting that the final fractal dimension is not a constant

Quantitative Description of Grain Contacts in a Locked Sand

Quantifying the fabric of intact soil is of great importance in both geomechanics and geology. A unique and interesting example of fabric can be found in “locked sands”. These geologically old sands are characterized by significant grain interlocking and a low cement content. They can be sampled with minimal fabric disturbance. This study analyzes images acquired by x-ray microtomography of resin impregnated samples of a natural sand, Reigate Silver Sand part of the Folkestone Bed formation from southeast England. 2D and 3D image analyses were carried out to identify the grain-grain contacts and quantify individual contact areas. In contrast to earlier studies that have focused on the coordination number, this work demonstrates that for non-punctual contacts a measure of fabric that considers the contact area may be more appropriate

The role of particle mineralogy in mixtures of sands

Several recent studies on mixtures of sands of different granulometries and/or mineralogies have focused on the key factors that might lead the behaviour to change from transitional to not transitional, where a transitional behaviour is characterised by non-convergent compression paths and critical state lines that might be non-unique. The authors present a review of mixtures of different soils showing a complex pattern of compression and shearing behaviour in which transitional behaviour can be caused by relatively small varia- tions to the proportion or nature of soil particles. Laboratory investigations, carried out by means of oedometer tests, have confirmed the role of the mineralogy of the matrix composed by larger grains. This determines the mode of behaviour so that, if there is a strong and stiff matrix made of quartz sand particles, which are either larger than or at least of similar size to the other component, then non-convergent compression paths are likely to occur, no matter whether particle breakage occurs or not

Crossing Over from Attractive to Repulsive Interactions in a Tunneling Bosonic Josephson Junction

We explore the interplay between tunneling and interatomic interactions in the dynamics of a bosonic Josephson junction. We tune the scattering length of an atomic $^{39}$K Bose-Einstein condensate confined in a double-well trap to investigate regimes inaccessible to other superconducting or superfluid systems. In the limit of small-amplitude oscillations, we study the transition from Rabi to plasma oscillations by crossing over from attractive to repulsive interatomic interactions. We observe a critical slowing down in the oscillation frequency by increasing the strength of an attractive interaction up to the point of a quantum phase transition. With sufficiently large initial oscillation amplitude and repulsive interactions the system enters the macroscopic quantum self-trapping regime, where we observe coherent undamped oscillations with a self-sustained average imbalance of the relative well population. The exquisite agreement between theory and experiments enables the observation of a broad range of many body coherent dynamical regimes driven by tunable tunneling energy, interactions and external forces, with applications spanning from atomtronics to quantum metrology.Comment: 10 pages, 8 figures, supplemental materials are include

Experimental investigation into the primary fabric of stress transmitting particles

Understanding the stress distribution amongst the constituent grains is fundamental to predict the response of soil and advance science-based, rather than purely empirical, constitutive models. Photoelastic experiments and discrete element method simulations have provided evidence that, upon loading, discrete force chains form in granular materials. These force chains are made up of particles transmitting relatively large stresses and they are aligned in the direction of the major principal stress. A few qualitative studies have identified the presence of these force chains in sands but direct measurements of force chains have not been previously documented and tracking stress transmission in assemblies of real soil grains remains a challenging task. The present study makes use of three dimensional micro CT images to investigate the evolution of the internal topology of a sand subjected to triaxial compression loading. The analysis of the contact normal and branch vector orientations has shown the realignment of the contact normals in the direction of the major principal stress as a clear indication of the formation of force chains in the post-peak regime. Here the extent of the non-colinearity of the branch and contact normal vectors is explored. Using the micro CT data contact force networks within and outside of shear bands are compared