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Stress Transmission in a Granular System

Nadimi Shahraki, S. (2017). Stress Transmission in a Granular System. (Unpublished Doctoral thesis, City, University of London)


A sample of soil under external loads shows nonlinear behaviour. These external loads are propagated through grain-to-grain contacts. Consequently, the grains are being subjected to both tensile and compressive stresses according to their shape, position, and number of contacts. Thus, the nonlinear mechanical behaviour of soil may be described by investigating inter-particle stress

The direct measurement of stress is a challenging task, both experimentally and numerically. In this study, stress-transmitting grains in a sand specimen are identified using an image-based approach. The methodology consists of measuring the geometrical data of the individual grains and following their evolution. On the numerical side, a more realistic description of soil behaviour is provided by developing a computational approach that quantifies internal stresses in each individual grain, termed micro Finite Element (μFE) model. The fabric of a natural sand obtained from the micro computed tomography (μCT) is virtualised to simulate the mechanical response of the material. The grain-to-grain interactions under loading are modelled in a framework of combined discrete-finite element method. Each individual grain is represented by a collection of nodes and elements and modelled as a continuum body that can deform according to a prescribed constitutive properties with appropriate friction contact conditions.

The insights that can be gained into the stress transmission mechanisms and yield initiation within the grains are shown in a case study of an intact sand subjected to 1D compression. This includes stress and displacement field, inertia tensor, and active contact area. The contact behaviour used in the model is validated against existing theories for a single sphere and an assembly of spheres under triaxial loading. Then, single grain tests are conducted experimentally and numerically in order to better understand the influence of grain morphology on stress transmission. This study shows the strong dependency of contact behaviour on grain morphology. In addition, the effect of surface roughness is investigated showing the role of asperity abrasion under low normal loading.

The evaluation of the μFE model has yielded results that compare well to experimental data obtained from a triaxial test in a μCT scanner. The stress field within each grain in the granular media is studied, contributing new insights beyond the commonly reported force chains. The ‘stress chain’ concept is considered as an alternative way to reflect grain breakage that may initiate on the weak force network and compromise the stability of the assembly. It is thus suggested that the ‘stress chain’ concept can be richer than ‘force chain’ and contains information about grain shape, mechanical properties and local fabric.

Publication Type: Thesis (Doctoral)
Subjects: T Technology > TA Engineering (General). Civil engineering (General)
Departments: School of Science & Technology > Engineering
Doctoral Theses
School of Science & Technology > School of Science & Technology Doctoral Theses
Text - Accepted Version
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