Investigation of the change in soil fabric during cone penetration in silt using 2D measurements

Abstract

Interpretation of CPTU testing in silt is nontrivial because of the partially drained conditions that are likely to occur during penetration. A better understanding of the pore pressure generation/dissipation is needed in order to obtain reliable design parameters. Following a previous study using X-ray computed tomography (micro-CT) with volumetric digital image correlation (3D-DIC) that clearly showed the formation of distinct dilation and compression areas around the cone; the present work takes a closer look at those areas in order to link volumetric behavior to changes in soil fabric. High-resolution 2D backscattered electron images of polished thin sections prepared from frozen samples at the end of penetration are used. The images have a spatial resolution of 0.4 lm/pixel that allow a clear identification of grains and pore spaces. Image processing techniques are developed to quantify local porosity and obtain the statistical distribution of the particle orientation for the zones around the cone tip and shaft. It is shown that the formation of compaction regions is related to the ability of the grains to rearrange and align along a well-defined preferred orientation forming a more closed-fabric characterized by high anisotropy values, while zones of dilation are associated with a more open packing with grains randomly oriented and with large voids within. These observations suggested that for a saturated soil, water will move from a compressive zone to a neighboring dilative zone, creating a short drainage path. By shedding light on the link between soil fabric and drainage patterns, this study contributes toward a better understanding of the measured macro-response during CPTU tests on silt.

Étude du comportement de l’interface sable-pieu sous chargement axial monotone et cyclique à l’échelle granulaire

“…Dans les zones où le sable se dilate, les grains subissent des déplacements plus élevés et leur orientation est relativement aléatoire. Des résultats similaires ont été obtenus par Paniagua et al (2018) pour des essais de pénétration de cône (CPT) dans un sol argileux. Les auteurs ont montré que les zones de compression et de dilatation étaient corrélées avec deux types de fabriques (induites par la forme plate et allongée des grains) : une « fabrique ouverte » où les grains ont une orientation aléatoire, créant ainsi de larges pores connectés entre eux et une « fabrique fermée » où les grains s'alignent selon une orientation bien définie formant une structure plus compacte.…”

Fabric evolution and crack propagation in salt during consolidation and cyclic compression tests

“…However, salt is a monomineralic rock that is difficult to image based on the heterogeneity of density in a specimen. In the present study, we thus base our fabric analysis on microscopy images, a technique that provided significant insights on fabric evolution and ensued deformation in a variety of geomaterials (Katsman et al, 2009;Paniagua et al, 2018;Vangla et al, 2018), including salt crust (Dai et al, 2016).…”