Experimental Micromechanics of Sand Grains including Coated and Bonded Geomaterials


Student thesis: Doctoral Thesis

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Awarding Institution
Award date11 Aug 2020


Over the years, the mechanical behaviour of geological materials at the macro-scale has been thoroughly examined using laboratory experiments, numerical and analytical studies. The importance of micromechanical studies has been recognized, and advanced numerical analyses have been developed to investigate the micro and multi-scale aspects of soil behaviour. The discrete element method (DEM) is one of the efficient numerical tools, especially in applications related to geological materials. Since the grain-scale behaviour of geological materials is required for accurate simulations of the macro-scale behaviour in numerical analyses such as DEM, the research at the grain-scale using micromechanical methods became necessary. With the advancements in technology, many sophisticated experimental apparatuses have been developed to match these needs. Because of the significant morphological and mechanical characteristics of natural soils, micromechanical studies are in general challenging.

A few idealizations of applied loading conditions and surface characteristics of geological materials are needed to be incorporated while physically simulating the in-situ conditions of geological materials in laboratory experiments. The sand grains in nature are seldom devoid of any visible debris on their surfaces. Generally, sand-silt and sand-clay mixtures are available in-situ. It is expected that these microparticles interfering at the contact of sand grains cause nonconformities in their mechanical behaviour. It is necessary to distinctly investigate these changes in soil behaviour using laboratory experiments and eventually developing concerned contact models. These microparticles at the contacts could be clays and silts which often do not impose any adhesion between the contacting grains, or they could be bonding agents such as cement. The loading conditions to be applied and the parameters to be explored would depend on the type of microparticles at the contact region.

This research aims at understanding the grain-scale behaviour of sand grains due to the presence of silts, clays, or bonding agents in their contact zones. Three types of specimens were created and examined in the laboratory as: artificially coated sands, gouge simulant filling, and artificial bonding. The silt and clay microparticles were coated on sand grains at different concentration using customized methods. The methods of sample preparation were standardized using energy-dispersive X-ray spectroscopy (EDS) analysis, scanning electron microscope (SEM) images, and surface roughness measurements. The contact response of sand grains due to the influence of clays and silts was compared with the corresponding behaviour of uncoated sand grains in terms of load-displacement curves, stiffness variation, and microscopic observations of surface damage. The behaviour of gouge materials under two different surface conditions (rough and smooth) of bounding surfaces were studied and the results showed a dominant influence of gouge material type on the frictional behaviour of gouges. Also, the gouge materials at different saturation levels were used to obtain further insights on their behaviour.

The micromechanical experiments were performed using two apparatus; one existing apparatus which was used, majorly, in the study of uncoated and coated grains and a newly developed apparatus, which was used, majorly, for the study of cemented grains. The newly developed apparatus was implemented based on significant upgrades of a previously designed system. In the study, both monotonic and cyclic tests were performed in the normal (vertical) and tangential (horizontal) directions of the contacted surfaces so that to provide insights into the complete micromechanical behaviour of variously coated/cemented grains.