Tribological and Mechanical Characterization of Rocks and Soil-Rock Interfaces

Abstract

Geomaterials exhibit complex mechanical behavior across multiple scales, influenced by their inherent microstructure, external loading conditions and geological processes. Recent studies have placed particular focus in scaling-down the problem of geological interfaces at the level of micrometer-to-millimeter. These recent works have emphasized that by scaling-down the problem, realistically high contact stresses can be achieved, which are representative of in-situ conditions, while studies of micrometer-level can provide insights on the tribological behavior of geological materials. Understanding the micro-mechanical response of geo-materials, which necessitates measurement of forces and displacements at the micronewton and micrometer, respectively, is essential for predicting the macroscopic performance of various systems. Such a level of micrometer of displacements is critical to be explored in various problems involving fault mechanics, micro-seismicity, the constitutive modeling of fractured rocks, proppant-rock interactions and particle-to-particle contact response. In modeling the contact problem of interfaces, at a micromechanical standpoint, some of the important parameters to be studied include the slip displacement and contact stiffness, which have a dependency on material hardness and modulus. The present research transposes the investigation of geomaterial behavior to the micrometer level, incorporating the effects of wear to examine the influence of loading conditions on the micromechanical response of a wide spectrum of natural and analog soils/rocks. This approach provides a deeper understanding of the fundamental mechanical behavior of geomaterials under realistic loading scenarios and delivers direct input contact parameters for Discrete Element Method (DEM) modeling, derived from micro-scale testing.

The thesis focuses on the micromechanical behavior of geomaterials using custom-built inter-particle micromechanical testing equipment and developing novel methods in sample preparation and testing conditions. In addition to these tests, complementary methods such as micro-indentation and statistical analysis are employed. The primary objective of the tests is to investigate fundamental properties, including the normal and tangential mechanical response, the tribological behavior, time-dependent deformation, surface damage, and fracture properties, by establishing correlations between micromechanical test results with the structural features of soils and rocks at the small scale. Geological materials are inherently highly heterogeneous. To prevent the lack of engineering universality in parameters derived from studying homogeneous materials alone, the research content demonstrates a progressive investigation evolving from simulated rocks to actual rocks and finally to organic soils. The rock materials examined in this study are primarily categorized into three types: (i) igneous rock, (ii) sedimentary rock, and (iii) metamorphic rock. Additionally, artificial materials are utilized as analog rock and particles, while natural particles are also examined, which, in some of the experiments are subjected to laboratory-controlled coatings. Given that geological processes often involve complex stress- and strain-induced phenomena, the study examines the influence of the magnitudes of normal load, loading rate, multiple loading modes, loading history, and surface texture, structure, and coatings/thin films. The study aims to investigate microscale mechanical responses not only at the grain-to-grain level but also on block samples to provide a comprehensive understanding of the constitutive behavior of materials and various interface types. The experimental results, stemming from the microscale mechanical tests, are analyzed using analytical expressions (e.g., contact mechanics models), and selectively, the extracted mechanical properties from the small-scale experiments, are compared and/or integrated with the results from macroscopic tests to obtain multi-scale insights into the problem of micro-to-macro scale response of granular and fractured systems.

The spatial variability of the properties of granular materials, natural rocks, and analog porous rock was investigated through micro-indentation tests to analyze the mechanical phase distribution and surface characteristics of these materials. Indentation tests were conducted on a variety of rock samples using an instrumented experimental procedure based on a grid-based methodology. The influence of loading history was examined by applying repeated loads at a single point. The results indicate that the spatial variability of hardness and modulus is significant, and the location of the indenter penetration plays a critical role in the measured properties. Moreover, cluster analysis, Weibull statistics, and contour mapping were employed to evaluate the elastic properties and the relationship between modulus, hardness, and the distinct phases of the material. Additionally, this study compares various methods for calculating fracture toughness. Micro-indentation testing directly measures and characterizes the physical properties of materials, while micromechanical testing providing interfacial interactions between the two contacts. Micro-mechanical tests were conducted to investigate the contact stiffness, friction, and wear mechanisms at the interfaces of geomaterial contacts. By applying different loading modes to the samples to induce full-slip or partial-slip states, the experimental data reveal that the loading mode significantly influences the tribological response of interfaces. Compared to particle-particle systems, the presence of an iron coating markedly alters the mechanical response in particle-rock interactions. Furthermore, repeated monotonic and cyclic loading tests performed on various materials demonstrated variations in surface morphology, stiffness changes, and material strengthening behavior, providing valuable insights into their fatigue behavior. Additionally, time-dependent deformations in particle-block and block-block systems were analyzed under varying magnitudes of normal load to evaluate creep rates and strain development. This analysis offers a deeper understanding of the long-term deformation behavior of geomaterials.

This study uses advanced tribological and indentation experiments, integrated with analytical modeling, to directly quantify the key influencing factors on the micro-scale mechanical response. The research establishes fundamental links between loading history, resultant surface damage, and consequent micro-mechanical property alterations. These findings provide novel insights into the microstructural processes governing in-situ macro-scale geomaterial behavior, delivering a physical basis for predicting field-scale performance and informing reliable geotechnical engineering design and modeling.

Date of Award	2 Sept 2025
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Kostas SENETAKIS (Supervisor)