Determining coal directional mechanical properties using true triaxial testing facility

Knowledge of coal mechanical properties and strength is critical in modelling and understanding pillar stability, gateroads stability, gas drainage borehole integrity as well as coal responses to hydraulic fracturing stimulation. However, due to the complexity of coal structures and difficulties in obtaining decent coal specimens, measurements of coal mechanical properties have been limited to the application of traditional triaxial and UCS tests, which in turn has shown adverse influence on the design confidence and reliability in practice. In addition, coal is an anisotropic material and such conventional testing techniques are clearly not capable of directly capturing coal anisotropic features. In this work, a true tri-axial testing facility was used to quantify coal strength and its anisotropic characteristics. Eight 50 mm side cube coal blocks were prepared and three types of tests were implemented. The proposed testing procedure measured successfully the mean values of coal young’s moduli in three different x, y and z (vertical) directions as 1,025 MPa, 1,887 MPa, and 2,543 MPa, respectively, which gives the ratio of 1.00: 1.84: 2.48. The mean Poisson’s ratio is also measured as 0.098, 0.038, and 0.091 in x, y and z directions. Coal strength follows the Hoek-Brown criterion reasonably well, and the m value is found to be 23.9. These findings suggest that the implementation of true-triaxial testing techniques for coal mechanical properties can effectively capture its anisotropic characteristics, which could enhance analysis confidence for future designs

Brittle-ductile transition in coal pillar failures

Factor of Safety (FoS) is the most widely used empirical criterion in coal pillar design. The increased vertical stresses associated with deep mining are resulting in pillar sizes much larger than those used in the development of the empirical strength formula. It is not uncommon to see pillars with over 50 m solid widths based on the established design criteria, which reduces overall reserve recovery and development productivity. Previous research revealed that pillars will transform from brittle to ductile failure with strain-hardening behaviour when their w/h ratio exceeds a certain critical value. However, the actual failure mechanics of pillars with different w/h ratios have largely been understudied. This paper discusses whether a coal pillar with a critical w/h ratio and reduced FoS can achieve functionality to allow more flexibility in pillar design for mine layout optimization. Using the more advanced laboratory testing and monitoring equipment that is now available, the failure mechanics of coal samples with different dimensions are investigated through uniaxial compression tests. The w/h value at which in situ pillar behaviour would be expected to transform from brittle to ductile is then considered and discussed further, including the research program that would enable the findings to be safely applied in deep underground mines

Investigation of shear strength and breakdown of mine waste rock

To investigate the shear strength of mine waste rock, large-scale laboratory direct shear tests were carried out on Breccia, Weathered Shale, Breccia on Weathered Shale, and Weathered Shale on compacted clay, under applied normal stresses of 250 kPa, 500 kPa or 1000 kPa. The Breccia, Weathered Shale and Breccia on Weathered Shale samples were loosely-placed and tested dry, representing the bulk of the waste rock dump volume in the field. The Weathered Shale on compacted clay was tested under both dry and wet (the worst case) conditions to represent the interface between Weathered Shale and compacted clay liners within waste rock dumps. The peak shear and normal stresses were corrected for area reduction and plotted to provide the shear strength envelopes, from which shear strength parameters were recommended. To assess the potential for breakdown of the waste rock on wetting, particle size distribution curves were obtained by dry and wet sieving. Also, slake durability indices were obtained for Breccia and Weathered Shale by carrying out slake durability tests. Overall, the results indicated negligible potential for breakdown of the Breccia and Weathered Shale on wetting

The Brazilian disc test under a non-uniform contact pressure along its thickness

Using a three-dimensional analytical approach, the sensitivity of the Brazilian test to its standard testing recommendations was investigated. It was concluded that the tensile stress induced in a Brazilian disc is significantly affected by the distribution of the applied load along its thickness rather than its circumferential condition. Under a non-uniform contact pressure along the BTS thickness, it was evident that both the numerical value and the location of the maximum tensile stress varied as a function of the geometrical aspect ratio of the disc specimen. For test conditions in which load distribution in the contact region along the thickness does not follow the standards or the uncertainty of its exact nature is large, e.g. in testing of super hard materials with relatively high stiffness and hardness greater than the contact testing platens, great care should be taken in regard to the interpretation of the Brazilian test result

Measurement of shear strength and interface parameters by multi-stage large-scale firect/interface shear and pull-out tests

It is essential to measure the shear strength of soils and interface parameters between soils and geosynthetics for the safety design and stability analysis of geosynthetic-reinforced soil structures. These parameters recommended for engineering projects are normally measured by laboratory single-stage direct/interface shear and pull-out tests. The conventional single-stage tests are carried out on at least three representative specimens under three different normal stresses. However, a large quantity of specimens will be required for large-scale tests, with tedious sample preparation procedures, so large-scale single-stage testing will be very labour intensive, time consuming and expensive. Given that the multi-stage testing method is able to measure the shear strength parameters by testing only one representative specimen, this paper investigates the feasibility, reliability and applicability of the multi-stage testing method in large-scale direct/interface shear and pull-out tests. Two compacted soils and a geogrid were tested using both single-stage and multi-stage tests. It was found that the shear strengths obtained from the multi-stage tests were slightly lower that those obtained from the single-stage tests, and the inferred apparent cohesion and friction angle matched closely. In addition, the limitations of the multi-stage testing method were highlighted, and the measured direct shear strength of the soils, the interface shear strength and pull-out shear strength between the soils and the geogrid were also compared and discussed in this paper

Crack damage evolution in coal at elevated temperatures

This work studies the behaviour of coal samples at various thermal environments from sub-zero temperatures of -100 to 300 °C by monitoring their fracture speed, ultimate strength, and crack damage evolution mode. High-speed recordings captured at 140,000 frames per second identified three different fracture evolution modes as a function of the applied ambient temperature. As the temperature increases, the fracture behaviour was observed to be through intact rock bridges resulting in reduced crack speeds and smaller fragments. Interestingly, at sub-zero temperatures, a different failure pattern was observed through the formation of a single fracture plane resulting in larger fragments with sharper edges

A niche application of permanent CO2 disposal from coal-fired power stations via mineralisation in proximal mafic and ultramafic deposits

Australia\u27s coal-fired power stations accounted for approximately 23% of the country\u27s total Green House Gas (GHG) emissions from all power stations in the 2021-2022 period. These emissions encompass both Scope 1 and Scope 2 categories. Such a significant contribution has led to continuous criticism of the coal industry within Australia, as the global energy landscape shifts towards a broader range of low to zero-emission energy sources. This transition calls for the coal industry to explore innovative methods to capture and remove Green House Gas emissions, particularly CO2, which is a major contributor to global warming. Current CO2 disposal methodologies are predominantly focused on injecting CO2 into deep aquifers and depleted oil and gas reservoirs. In this paper, we propose an alternative approach for the permanent disposal of CO2 emissions from coal-fired power stations. This approach involves the underground injection of CO2 into ultramafic and mafic deposits located within economical distances from these stations in Southeast Australia. The exothermic reactions between CO2 and magnesium- and calcium-rich minerals in ultramafic and mafic rocks lead to the permanent conversion of CO2 into stable carbonate minerals. This process eliminates the need for extensive monitoring and control systems to detect any unintended CO2 seepage on the surface and is expected to lower the running costs of CO2 disposal

Kaolin Clay Reinforced with a Granular Column Containing Crushed Waste Glass or Traditional Construction Sands

Installation of granular columns is a cost-effective and versatile in situ technique to improve the shear strength, settlement, and drainage behavior of weak soils. It involves backfilling vertical boreholes in the ground with granular materials stiffer than the native soil, such as stone or compacted sand. However, the massive use and overexploitation of sand and natural aggregates have depleted their reserves in recent decades, causing damage to the environment, creating sand shortages, and skyrocketing their price. Hence, it is essential to develop a sustainable alternative to natural aggregates to construct granular columns. The ever-increasing stockpiles of waste glass could be a potential replacement for natural sand in several geotechnical construction applications, noting that both materials have a similar chemical composition. Using crushed waste glass (CWG) as an alternative to traditional natural and manufactured (quarried) sands in granular columns could offer a multipronged benefit by recycling nonbiodegradable waste (glass) and by conserving a depleting natural resource (sand). Using a large direct shear (LDS) machine, this study investigated the shear strength behavior of kaolin (to represent a typical weak soil) reinforced with a central granular column. Three different materials were separately used to backfill the column, including natural sand (NS), manufactured sand (MS), and CWG. The results revealed that the geocomposites containing the CWG column have the highest peak friction angle and relatively greater shear strength under high normal stresses, favoring the potential use of CWG as a green alternative to traditional sands in backfilling granular columns, ultimately supporting resource conservation, waste recycling, and the paradigm shift toward a circular economy

Characterization of the fracture mode in asphalt at varying temperatures

ABSTRACT: Cracking is a primary mode of distress in asphalt pavements that is generally caused due to repeated traffic loadings, exposure to temperature fluctuations, aging or reflection of cracks in underlying layers. Such cracking can readily lead to higher maintenance and rehabilitation costs for pavement infrastructure, hence negatively affecting the economy both directly and indirectly. To prevent excessive cracking, it is important to understand the cracking characteristics of asphalt mixtures for implementation in road, airport and port pavements. This study aims to investigate the effect of loading and temperature on the cracking behaviour of asphalt using the Indirect Tensile Test (IDT), along with high-speed photography analysis techniques. The results indicate that cracking can occur prior to the asphalt reaching its peak strength in the IDT test. Furthermore, it was observed that increasing temperature can cause a decrease in the peak strength of the asphalt samples and change its fracturing behaviour as well

Determining coal directional mechanical properties using true triaxial testing facility

Knowledge of coal mechanical properties and strength is critical in modelling and understanding pillar stability, gateroads stability, gas drainage borehole integrity as well as coal responses to hydraulic fracturing stimulation. However, due to the complexity of coal structures and difficulties in obtaining decent coal specimens, measurements of coal mechanical properties have been limited to the application of traditional triaxial and UCS tests, which in turn has shown adverse influence on the design confidence and reliability in practice. In addition, coal is an anisotropic material and such conventional testing techniques are clearly not capable of directly capturing coal anisotropic features. In this work, a true tri-axial testing facility was used to quantify coal strength and its anisotropic characteristics. Eight 50 mm side cube coal blocks were prepared and three types of tests were implemented. The proposed testing procedure measured successfully the mean values of coal young’s moduli in three different x, y and z (vertical) directions as 1,025 MPa, 1,887 MPa, and 2,543 MPa, respectively, which gives the ratio of 1.00: 1.84: 2.48. The mean Poisson’s ratio is also measured as 0.098, 0.038, and 0.091 in x, y and z directions. Coal strength follows the Hoek-Brown criterion reasonably well, and the m value is found to be 23.9. These findings suggest that the implementation of true-triaxial testing techniques for coal mechanical properties can effectively capture its anisotropic characteristics, which could enhance analysis confidence for future designs