Research Scholar, Indian Institute of Technology Bombay
Performance and predictive analysis of geotechnical properties at high temperatures
The success of any geotechnical engineering project depends on the nature and behaviour of geotechnical properties. Although several studies have been focussed on the characterisation of these properties at room temperature, studies analysing the effect of environmental conditions such as heat is relatively nascent. Such studies are extremely vital for defining the efficacy and performance of processes such as underground coal gasification (UCG), nuclear waste disposal and restoration of fire-damaged structures. As seen in the case of every other physical material, rocks along with their constituent minerals expand when exposed to high temperatures. However, this thermal expansion is anisotropic in nature due to the difference in the thermal expansion behaviour between minerals. Additionally, anisotropy exists between different crystallographic axes of a particular mineral. The combined effect of such anisotropies results in a the formation of thermal stresses within the microstructure of the rock. Change in the microcrack network occurs when the thermal stresses exceed the stress threshold, thereby resulting in large scale variation in physico-mechanical properties of the rock. Since the magnitude of change depends entirely on the mineralogy and morphology of the rock, it is imperative to study the behaviour of the physico-mechanical properties of the rocks that are inherent to the aforementioned processes. Our study focusses on analysing and quantifying the changes that occur in the strata in a UCG process. Based on the results of the experimental analysis, predictive modelling has been performed in order to estimate the important geomechanical parameters from the physical properties. This would help in reducing the time and cost of experimental analysis.
Abstract: Water content has a pronounced influence on the properties of rock materials, which is responsible for many rock engineering hazards, such as landslides and karst collapse. Meanwhile, water injection is also used for the prevention of some engineering disasters like rock-bursts. To comprehensively investigate the effect of water content on mechanical properties of rocks, laboratory tests were carried out on sandstone specimens with different water contents in both saturation and drying processes. The Nuclear Magnetic Resonance technique was applied to study the water distribution in specimens with variation of water contents. The servo-controlled rock mechanics testing machine and Split Hopkinson Pressure Bar technique were used to conduct both compressive and tensile tests on sandstone specimens with different water contents. From the laboratory tests, reductions of the compressive and tensile strength of sandstone under static and dynamic states in different saturation processes were observed. In the drying process, all of the saturated specimens could basically regain their mechanical properties and recover its strength as in the dry state. However, for partially saturated specimens in the saturation and drying processes, the tensile strength of specimens with the same water content was different, which could be related to different water distributions in specimens.
Pub.: 28 Apr '16, Pinned: 28 Jul '17
Abstract: The cracked chevron notched Brazilian disc (CCNBD) specimen has been suggested by the International Society for Rock Mechanics to measure the mode I fracture toughness of rocks, and has been widely adopted in laboratory tests. Nevertheless, a certain discrepancy has been observed in results when compared with those derived from methods using straight through cracked specimens, which might be due to the fact that the fracture profiles of rock specimens cannot match the straight through crack front as assumed in the measuring principle. In this study, the progressive fracturing of the CCNBD specimen is numerically investigated using the discrete element method (DEM), aiming to evaluate the impact of the realistic cracking profiles on the mode I fracture toughness measurements. The obtained results validate the curved fracture fronts throughout the fracture process, as reported in the literature. The fracture toughness is subsequently determined via the proposed G-method originated from Griffith’s energy theory, in which the evolution of the realistic fracture profile as well as the accumulated fracture energy is quantified by DEM simulation. A comparison between the numerical tests and the experimental results derived from both the CCNBD and the semi-circular bend (SCB) specimens verifies that the G-method incorporating realistic fracture profiles can contribute to narrowing down the gap between the fracture toughness values measured via the CCNBD and the SCB method.
Pub.: 23 Apr '16, Pinned: 28 Jul '17
Abstract: Direct tensile strength and fracture toughness of rock and concrete, important properties for many applications, are cumbersome to measure directly. In this study, granite is chosen as an example to show how the tensile strength and fracture toughness can be measured from small three-point-bend samples of a single size but with different notches. An existing fracture mechanics model has been extended to include the stable fictitious crack growth before peak loads, which is then linked to the granite grain size. Both tensile strength and fracture toughness of granite can be estimated by the maximum load measurements from those notched three-point-bend samples. In total, 72 three-point-bend granite samples with different notches have been tested, and the estimated tensile strength and fracture toughness are compared with those available in the literature. The modified fracture mechanics model is then used to predict the fracture behaviour of smaller samples of the same granite. The theoretical prediction is confirmed by the experimental results of those smaller samples. Finally, the fracture model and its relation with the American Society for Testing and Materials (ASTM) standard on fracture toughness are discussed.
Pub.: 03 Oct '16, Pinned: 28 Jul '17
Abstract: Understanding the aspect of enhanced energy extraction through an approach of rock mechanics requires a detailed interpretation of rock fracture mechanism. Common examples are hydraulic fracturing of gas shales and geothermal energy systems. Resolving rock fracture behavioural patterns and its properties such as fracture toughness and energy-release rate during fracturing is important for the successful implementation of such projects. These properties are a function of different environmental factors such as temperature, humidity, water vapour, pressure, and strain rate. In this study, the effects of various strain rates on the fracture toughness as well as the energy-release rate of gas shales were investigated. The three-point bending method was applied using notched semicircular bending shale specimens that were prepared as per the international standards. The fracture toughness and the energy-release rates were measured for three different modes, namely, mode I, mixed mode (I–II) and mode II. In addition, X-ray diffraction analysis was carried out to identify the composition of the selected shales. Finally, scanning electron microscope (SEM) analyses were performed in order to acquire an insight into the effects of strain rate on fractures at microstructural scales. The experimental results indicate that the fracture toughness and the energy-release rate for all the three modes are a function of strain rate. At lower strain rates, the fracture toughness and the strain-energy-release rates for all the modes are comparable but vary significantly at higher strain rates. At high strain rates, the strength and stiffness of the shale increases, which in turn increases the fracture toughness and, eventually, the energy-release rate of the shale. For all the strain rates, mode I requires the minimum application of energy, while mode II requires maximum energy for the onset of crack growth. The energy-release rate in mode I is maximum, in comparison with the two other modes. The findings of this investigation will be useful in achieving a better and comprehensive understanding of some aspects such as initiation, propagation and failure of shales during hydraulic fracturing for the extraction of hydrocarbons.
Pub.: 28 Dec '16, Pinned: 28 Jul '17
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