Abstract
One element that is essential to consider in underground mining engineering applications is the possibility of pillar failure, which can result in deadly geological disasters, including earthquakes and surface subsidence. Pillars are commonly present under an inclined state and are significantly dependent upon combined compression and shear loading. However, many scholars regard the pure uniaxial compression strength (UCS) of rock as the main evaluation index of pillar strength, which is inconsistent with the field practice. Hence, the present study developed a novel combined compression and shear test (C-CAST) system, which was applied in the investigative acoustic emission (AE) experiments to characterize the failure mechanism and micro-fracture behavior of granite specimens at different inclination angles. The experimental results presented the exponential decrease of UCS of inclined specimens with increase in the shear stress component. Changes in the inclination angle with a range of 0°–10° produced a splitting-shear failure fracture mode from the initial splitting failure. In comparison, an increase in the inclination angle from 10° to 20° produced a single shear failure fracture mode from the initial combined splitting-shear failure. The specimens exhibited nonlinearly reduced microcrack initiation (CI) and damage (CD) thresholds following an increase in the inclination angle, suggesting the dependence of the microcrack initiation and propagation on the shear stress component. The ratio of CI and CD thresholds to inclined UCS varies within a certain range, indicating that the ratio may be an inherent property of granite specimens and is not affected by external load conditions. Additionally, the rock fracture behavior was largely dependent upon the mechanism of shear stress component, as validated by the microcrack initiation and growth. Finally, a modified empirical formula for pillar strength is proposed to investigate the actual strength of inclined pillar. Results of a case study show that the modified formula can be better used to evaluate the stability of inclined pillars.
Highlights
One element that is essential to consider in underground mining engineering applications is the possibility of pillar failure, which can result in deadly geological disasters, including earthquakes and surface subsidence
Many engineering applications have reported the importance of inclination angle as this significantly affects the pillar under combined compression and shear loading due to its relation to failure mode as shown in Fig. 1. Pariseau[14] and F oroughi[15] previously reported the effect of the external shear loading on coal pillar stability estimation in inclined coal seams. Jessu[16] indicated the dependence of the pillar design on the inclination of ore body
S heorey[21] improved the calculation formula of coal pillar strength by introducing buried depth and width-to-height ratio. Ebrahim[22] reported that weathering and moisture greatly reduce the strength of coal pillars. Esterhuizen[23] stated that the destruction of the pillar is related to the spalling of the rock and the discontinuous shear planes of different angles in the pillar
Summary
One element that is essential to consider in underground mining engineering applications is the possibility of pillar failure, which can result in deadly geological disasters, including earthquakes and surface subsidence. S uorineni[19] challenged the use of the empirical pillar strength formulae to investigate the applicability of the conventional uniaxial compressive strength (UCS) given that this parameter only highlights the rock strength under pure compression In comparison, their numerical simulation results indicated lower pillar strength due to increase in the pillar inclination. Xu and D ai[25] applied a dynamic impact test, which implemented the modified split Hopkinson pressure bar (SHPB) system, to which the dynamic response and failure mechanism results in brittle failure of rocks were due to application of combined compression-shear loading. Characterization of the specimen properties and fracture behavior following the application of combined compression and shear at low strain rates remains a challenge
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