ABSTRACT
Undertakings in underground mining are often complicated, particularly in situations where geotechnical conditions are not favorable. This study investigates the collapse of tunnels at the Lega-Dembi gold mine in Southern Ethiopia, an area characterized by weak talc formations. The persistent deformation of tunnels poses a threat to the safety of workers and mining operations. In this study, a numerical method that combines continuum and discontinuum approaches is employed to analyze tunnel failures. Additionally, the study evaluates the effect of geotechnical parameters on tunnel deformation, considering various support systems. The results indicate that a combination of rock bolts and shotcrete is effective in mitigating tunnel deformation. Furthermore, the study identifies the geological strength index and unconfined compressive strength as the most influential parameters on tunnel deformation. The findings also suggest appropriate support systems for managing underground instability and enhancing safety measures in weak geological formations.
ABSTRACT
Sandstone is a kind of bedded rock mass commonly used in engineering. The deformation and strength of bedded sandstone impose some problems during excavation. In this study, triaxial unloading tests were conducted on rock specimens (considering seven different bedding angles (ß)). The results revealed the following key points: (1) At a constant confining pressure, the elastic modulus was gradually increasing when the bedding angle increased. Furthermore, after initial decreasing, the deformation modulus was increased and had a U-shaped distribution. (2) During the unloading of the axial compression, the rate of axial strain variations was initially increased and then decreased while the bedding angle increased (it has exhibited an inverted U-shaped distribution). However, the peak strength, cohesion, and angle of internal friction of rock specimens showed an initial decreasing and then increasing trend. (3) During the loading and unloading stages, the confining pressure reduces the anisotropy of bedded rock masses. (4) In the triaxial unloading test, the failure of rock specimens can be classified into four modes. When there was a large intersection angle between the bedding plane and the unloading direction, failure developed at the bedding planes.
ABSTRACT
Rock mechanics tests are essential for advancing theoretical and practical knowledge in the field. The rock failure mechanism can be studied by analyzing the failure characteristics of rock samples through mechanical tests. However, despite their usefulness, quantitative rock classification systems still possess certain limitations that need to be addressed. The main objective of this paper was to develop a comprehensive quantitative rock classification system based on rock failure characteristics. The rock classification indices, including crack density and crack saturation, were systematically introduced based on rigorous statistical analyses conducted on a diverse set of 200 rock samples. In particular, the crack saturation index serves as a crucial metric that primarily captures and quantifies the extent of actual crack propagation within the rock samples. Moreover, it is important to note that the two evaluation indices, crack density and crack saturation, work in harmony and complement each other, enhancing the overall understanding of rock fragmentation and failure characteristics. By taking into account both crack density and crack saturation, the proposed method effectively categorizes rock fragmentation into five distinct classes, namely "relatively intact", "slightly fragmented", "fragmented", "very fragmented" and "extremely fragmented." The validation process confirmed the efficacy of the proposed classification method in accurately capturing the crack-propagation characteristics of rocks. This outcome is highly significant as it significantly advances ones understanding of rock failure mechanisms and provides valuable insights into the overall characteristics of rocks.
