Publication: Differential weathering effects on macro-micro failure characteristics of sandstone in semi-arid region
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Date
2022-08-01
Authors
Sultan Shah, Kausar
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Abstract
The significance of rock failure can be found from the fact that microfracture genesis and coalescence in the rock mass results in macroscale fractures. Rock may fail due to an increase in local stress, natural fractures, and weathering inducing micro crack genesis, coalescence and propagation. Understanding the effect of loading and weathering grade on the failure behavior of rock is critical for a wide range of engineering applications. Therefore, this research work aims to elucidate the micro and macroscale failure behaviour of sandstone under quasi-static loadings from weathering zones. The microscale failure response of various weathering grade sandstones was studied under wet and dry cycles. Each sample was tested for microstructures and microfracture characteristics using image analysis. Furthermore, the obtained micrographs were also used to create microstructure-based models, which were then simulated in ANSYS software. The findings indicate that wet and dry cycles have insignificant impact on particle shape and size. However, the variation in particle shape and size implies that this is a result of the prevailing interaction of rock and water. The microscale simulation revealed that as the density of microstructures increases, the chances of primary fracture deviation from the loading axis increases. The effect of loading rates (i.e., 0.001 kN/s, 0.01 kN/s, 0.1 kN/s, and 1 kN/s) on the mechanical behavior and fracture response of sandstones with varying weathering grades were then examined. The results revealed that the tensile strength of sandstone is equally sensitive to loading rate and weathering grade. Brazilian tensile strength (BTS) test has shown strong relationship with sandstone fracture characteristics. Additionally, the fracture characteristics of sandstone are randomly related to the loading rate and are depending on the weathering grade. The findings indicate that fracture deviation area (FDA), fracture angle (FA), and fracture maximum deviation distance (FMDD) was unclearly associated with particle shape or size. In contrast, fracture length (FL) increases with an increase in particle size. The impacts of cemented and stained natural fractures on stress-induced sandstone failure mechanisms and mechanical behaviour was performed under unconfined compressive strength (UCS) tests. ANSYS software was used for 3D fracture modelling and simulation. The numerical simulation findings demonstrated that UCS of sandstone declined with increasing fracture angles of 0°, 30°, 45°, 50° to 60°, respectively. The specimens with 0° and 30° fractures show multiple fracture patterns, while 90° fractures exhibit axial splitting. Similarly, a 50° fracture angle specimen fails in shearing.