A peek into the origin of creep in sand

Abstract

This paper presents the results of an experimental study of the particle scale mechanisms that underpin creep, on-going deformations under constant external load, in dry non-cemented sand under 1D oedometric compression loading at 2500 kPa. Traditional observations on the boundary of the sample are complemented with simultaneous measurements of the 3D kinematics of both the entire grain assembly and details of grain-scale mechanisms using synchrotron based X-ray tomography at two different spatial resolutions. Both the continuum response and the local grain scale response are captured using two spatial resolutions, i.e. and respectively. The results, for the first time, illustrate that small displacements measured at the boundary can be the result of rather pronounced fracturing at the individual grain scale.

Evolution of fabric anisotropy of granular soils: x-ray tomography measurements and theoretical modelling

“…Using the labelled image as the definition of the grains in a given tomography volume, the displacement and rotation of each grain between images is measured with Discrete Digital Image/Volume Correlation, as first used in [95] and implemented in the spam-ddic script (see for example [96]). Concisely, this means that using the labelled image of each grain as a mask, the grey level from the reference image is extracted and an iterative procedure is used to seek the best transformation of the deformed image that minimises a sum-of-squares residual function.…”

Un-jamming due to energetic instability: statics to dynamics

“…It was recently extended to include friction, softness and cohesion [2,12,13,69,70,[73][74][75], but it does not have a fully tensorial form [76,77], and doubts about its well-posed-ness are still discussed [78][79][80]. Modern experimental techniques [25,71,81,82], also with focus on low confining stress [83], shed new light on classical works on the response to local perturbations [84], jamming and un-jamming [23,[84][85][86][87], in particular by shear [10,49,60,65,[88][89][90], and transient fabric/micro-structure evolution [31,49,55,[91][92][93][94]. One of the classical experimental techniques involves photoelastic materials that allow to visualize stress [95][96][97], as complemented by a huge amount of particle simulations, e.g., see Ref.…”

“…In many non-Newtonian systems, complex fluids [1], colloidal suspensions, review [2][3][4], and especially granular matter [5] in its flowing state [6], the transport coefficients depend on various state-variables such as the density and the granular temperature [7]. This interdependence and the presence of energy dissipation is at the origin of many interesting phenomena: clustering [8], shear-band formation [9], jamming/un-jamming [10], dilatancy [11], shear-thickening [3,4,12,13] or shear-jamming [10,14], plastic deformations [15][16][17][18][19][20][21], related also to creep/relaxation [11,[22][23][24][25], and many others. The research on granular matter in the last decades-to a good fraction inspired by works of Bob Behringer and co-workers-will be briefly reviewed next.…”

Contact evolution in granular materials with inherently anisotropic fabric

“…In the first scan the solid phase is identified with a global normalised threshold (based on the peaks of the attenuation field histogram), and the individual lentils are labelled using a watershed algorithm, with manual postprocessing to ensure 100 % fidelity. Given that the scans are performed at small increments of axial shortening, the position of each particle is followed accurately, using a novel discrete digital image correlation technique based on [6,7], allowing a consistent labelling of the particles in all subsequent scans. The tracking in this case is total (i.e., always keeping the 0%-shortening image as the reference configuration), and subsequent labelled images are generated by applying the unique measured transformation to each particle in the initial labelled image, and used as markers for the re-segmentation of the deformed binary image.…”