Cranfield-Brooks, Keelan (2024) Mechanical Behaviour of Hawkesbury Sandstone Exposed to Different Heating Conditions. Research Master thesis, Victoria University.

Abstract

Subsurface tunnels have become a critical solution for urban transportation in areas where road upgrade options are constrained by space or infrastructure limitations. However, the confined nature of tunnels and vehicles powered by combustion engines pose significant fire risks. Such fires, resulting from accidents, could compromise the integrity of the surrounding rock structure. Despite extensive research on fire effects in various settings, limited knowledge exists on fire's specific mechanical and chemical impacts on Hawkesbury sandstone, a rock type commonly encountered in Australian tunnel projects. This research addresses this knowledge gap by investigating the effects of fire on Hawkesbury sandstone's mechanical behaviour and chemical properties. The study aimed to evaluate sandstone's mechanical and chemical responses subjected to controlled heating conditions that simulate real-world tunnel fire scenarios. Victoria University’s NATA-accredited structural fire testing facility provided a unique platform for replicating these conditions. Experiments included heating of specimens following the Hydrocarbon Curve (HC) and Modified Hydrocarbon Curve (MHC), which reach peak temperatures of 1,100°C and 1,300°C, respectively, within five minutes, and specimens heating linearly at the rates of 2, 5, 10 and 20°C/min for comparison. The cylindrical sandstone specimens after heating were subjected to uniaxial compression strength (UCS) and splitting tensile strength tests to quantify mechanical changes. X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and digital image correlation (DIC) analyses were conducted to examine mineral transformations and the damage evolution behaviour. The results demonstrated a consistent reduction in both UCS and tensile strength across all heating scenarios, with more pronounced degradation observed under the rapid heating conditions of the HC and MHC. These conditions also led to increased axial strain capacity, although discrepancies were noted between the axial strain measured by the testing apparatus and that observed in DIC analysis. Mineralogical analysis via XRD revealed enhanced crystallinity and an increased quartz content, accompanied by a reduction in clay minerals. SEM analysis was limited to elemental changes. This structural evolution correlated with the mechanical changes observed. In conclusion, the study provides critical insights into the implications of tunnel fires on Hawkesbury sandstone. By establishing the mechanical and mineralogical transformations of varying heating rates, this research contributes to the understanding necessary for geotechnical risk assessments and tunnel design considerations in fire-prone environments.

Search Google Scholar

Repository staff login