- Title
- Experimental and theoretical investigation of chemo-hydro-mechanical behaviour of hard soil-soft rock
- Creator
- Ou, Ke
- Relation
- University of Newcastle Research Higher Degree Thesis
- Resource Type
- thesis
- Date
- 2021
- Description
- Research Doctorate - Doctor of Philosophy (PhD)
- Description
- The thesis describes a comprehensive experimental study aimed at evaluating the influence of the pore fluid chemistry on the mechanical behaviour of Ashfield shale, a low porosity rock from the Sydney Basin in New South Wales. Particular emphasis has been placed on the effects of the pore fluid constituents on rock microstructure (compacted and natural), which in turn controls the (macroscopic) volumetric behaviour of Ashfield shale. Despite the importance of the chemo-mechanical phenomena in clayey rocks, empiricism is still and commonly present in engineering practice when dealing with this type of geomaterials. Therefore, this experimental research is an attempt to understand in a comprehensive way the (geo)chemical interactions (osmotic processes, cation exchange and acid/alkaline contamination) in clayey rocks and their consequences on engineering properties. The chemo-mechanical behaviour of Ashfield shale was not only studied at the macroscopic scale but also at the microstructural level. Both compacted and natural Ashfield shale specimens were used for this purpose. Two isotropic cells (with a maximum confined pressure of 15.5 MPa) and auxiliary devices were designed, developed and carefully calibrated for evaluating the influence of pore fluid osmotic potential and alkalinity on the mechanical behaviour of the rock specimens (compressibility, stiffness and water permeability). A free stress state was selected to perform immersion tests and batch tests on natural and powder specimens. The physicochemical properties such as mineralogy, cation exchange capacity (CEC), specific surface area (SSA) and cation retention behaviour were evaluated. Moreover, free swelling tests and constant volume tests were performed to study the swelling potential of Ashfield shale saturated with different pore fluid solutions. The uniaxial compression tests and Brazilian splitting tests were conducted on natural specimens to study the environmental effects on uniaxial compression strength (UCS) and tensile strength. Oedometer tests, permeability test and high- pressure consolidation tests were then carried out to investigate the compressibility and permeability under one-dimensional conditions. Both quantitative and qualitative tests were performed. These include nitrogen adsorption (NA) tests, mercury intrusion porosimetry (MIP) tests, and Field Emission Scanning Electron Microscope-Energy Dispersive Spectrometer (FESEM-EDX) observations. Moreover, specimens were imaged before and after exposure to different brine solutionsVI using position relocation technique via FESEM under the Variable Pressure mode to study chemical or physical alternation. The testing results clearly show that the physicochemical properties and mechanical behaviour of Ashfield shale are strongly influenced by initial structure, pore fluid osmotic potential, acidity and alkalinity. For salt solutions (NaCl, KCl and CaCl2), an increase in concentration results in lower SSA, swelling potential and compressibility, higher CEC, UCS and tensile strength, stiffness and permeability. The consolidation parameters are also affected by the osmotic potential. Compared with NaCl, the trend is more evident in KCl and CaCl2, since they result in a strong bond (K-linkage) and Van der Waal’s forces, respectively. For HCl solutions, cementitious mineral (siderite) dissolution and H+ adsorption result in cementing bonds’ weakening and fissure initiation, which is accompanied by a decrease in CEC, swelling potential for compacted specimens, UCS and tensile strength, but an increase in swelling potential for natural specimens due to cementing expansion. For NaOH solutions, clay mineral (kaolinite) and cementitious mineral (siderite) dissolution and isomorphous substitution also lead to the formation of cracks, which is associated with a decrease in CEC, swelling potential for compacted specimens, UCS, tensile strength and compressibility, but an increase in CEC, SSA, swelling potential and permeability. On the other hand, batch tests show that cation retention behaviour increases with an increase in concentration. However, this also strongly depends on the ion concentration and acid or alkaline (pH) conditions. From a microstructural viewpoint, rock anisotropy was clearly identified and more significant modifications occur in specimens cut perpendicular to the bedding planes. Moreover, a progressive change in the pore size density function of the rock with the osmotic potential of salt solutions is observed. Salt crystallization phenomena usually occur in specimens inundated in high concentration (2.0 N) solutions which block parts of micropores. The main modification of pore size density (PSD) function occurs in micropores of natural specimens and mesopores of compacted specimens. While acid and alkaline solutions, the mineral dissolution and cementing bonds’ weakening leads to matrix fabric deterioration and porous rock microstructure, resulting in an entire increase of the PSD function. These phenomena were more intuitively observed by SEM-EDX analysis.
- Subject
- chemo-hydro-mechanical; hard soil soft rock; pore fluid chemistry; Ashfield shale
- Identifier
- http://hdl.handle.net/1959.13/1433450
- Identifier
- uon:39257
- Rights
- Copyright 2021 Ke Ou
- Language
- eng
- Full Text
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| Thumbnail | File | Description | Size | Format | |||
|---|---|---|---|---|---|---|---|
| View Details Download | ATTACHMENT01 | Thesis | 19 MB | Adobe Acrobat PDF | View Details Download | ||
| View Details Download | ATTACHMENT02 | Abstract | 337 KB | Adobe Acrobat PDF | View Details Download |