Abstract
Rock fracture propagation is a major hazard for mining and tunnel excavation in fractured rock masses or coal seams. A longwall mining panel with a typical dimension of 200m (width)×1000m (length)×3m (height) can be considered as an open edge crack. The fracturing processes in the vicinity of the edge crack (or the longwall panel) particularly in the roof and floor are critically important for the safety of mining operation because fracturing can lead to water inrush and dynamic loading on the working face. It’s therefore important to understand and predict the pre-existing edge crack initiation and propagation in rock masses. This paper describes a study investigating the mechanisms and pathways of rock fracture under uniaxial compression. In this study, a rock-like material which consists of model gypsum, water and diatomaceous earth at a mass ratio of 165:75:2 was used. The uniaxial compression strength of the material decreased with the increase of the length of pre-existing edge crack. During the tests, wing (tensile) cracks were first observed at the tip of the pre-existing edge crack. This was followed by secondary cracks as the loading increased. The final failure of the specimens however was dominated by tensile cracks throughout the specimens. Due to the sudden crack initiations in the specimens, the loading stress in the specimen varies stepwise, and acoustic emission (AE) energy and amplitude showed abrupt changes when crack initiated. When the crack initiation occurred, the loading stress of the specimens showed a notable retreat in the stress-strain curve, and the recorded AE energy and amplitude showed a sharp spike. These findings from this experimental study have been applied to the underground longwall mining to explain the failure mechanisms in the floor of the mining panel. The fracturing process associated with the pre-existing edge crack resembles the formation of flow channels for water inrush during longwall mining.
Highlights
IntroductionRock fracture propagation is one of the key concerns in many rock engineering problems (Shen et al 2016, 2020; Feng et al 2019b) because it could lead to rock mass instability
Rock fracture propagation is one of the key concerns in many rock engineering problems (Shen et al 2016, 2020; Feng et al 2019b) because it could lead to rock mass instability and rapid increase of water inflow
The rock fracturing processes in the roof and floor of a longwall panel can be investigated in laboratory using specimens containing edge cracks under uniaxial compression
Summary
Rock fracture propagation is one of the key concerns in many rock engineering problems (Shen et al 2016, 2020; Feng et al 2019b) because it could lead to rock mass instability. Due to its relatively thin height, it may be considered as an open edge crack under the compressive stress (overburden stress) With this simplification, the rock fracturing processes in the roof and floor of a longwall panel can be investigated in laboratory using specimens containing edge cracks under uniaxial compression. Luis et al (2013), Shen et al (2011) and Tang (2007) used numerical simulation software to simulate the failure process of rock with pre-existing cracks, and revealed the propagation mechanism of Mode I (the opening mode), Mode II (the edge-sliding mode), Mode III (the tearing mode). It has not been found that any of the previous studies were focused on using the edge cracks to investigate the seam roof and floor fracturing in coal mining. The results will be directly relevant to the seam floor water inrush and its prevention
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