Influence of pillar wide and room's height on the acoustic emission phenomena during crack evolution

The present study probes the effects of pillar wide and room's height on the failure mechanism of model using experimental test and numerical simulation. For this purpose, firstly gypsum specimens with dimension of 15*15*5 cm containing two neighboring rooms and one pillar were prepared. Pillar widths were 3 cm, 6 cm and 9 cm. In each pillar width, the roof height was 1 cm, 2 cm, 3 cm, 4 cm, 5 cm, and 6 cm. Totally 9 physical specimens were tested. Concurrent with experimental tests, 18 numerical models were simulated. The results show that the pillar wide and roof height affect the failure pattern and compressive strength of models. Two different failure patterns were observed in the model. In the first stage, two tensile fractures were developed from room edge and propagated parallel to loading axis till coalescence with model boundary. In the second stage, mixed tensile-shear cracks were developed through the rock pillar. It is shown that with the augment of stress-strain curve until the peak, the AE hits of numerical specimens presents three stages. In the first stage, AE hits is almost zero; this step is in the elastic stage, and there isn't any crack in the sample. In the second stage, AE hits is in a slowly increasing stage, which is due to the micro cracks in the model continuous expansion. In the third stage, the AE hits increase rapidly, because the rock almost enters its bearing limit and the crack propagation speed is very fast. Generally, the maximum value of AE is near the peak value. When the roof height is the same, different pillar wide led to a significant change in the maximum AE hits. The experimental test and numerical simulation show the similar outcomes. Investigation of the influence of pillar dimension on the failure mechanism of rock pillar shows that in constant pillar wide, the mixed mode failure change to pure tensile failure by increasing the pillar height.In constant pillar height, the mixed mode failure change to pure tensile failure by decreasing the pillar wide.Delay failure in rock pillar changes to rapidly failure by increasng the ratio of pillar height to pillar wide.In constant pillar height, acoustic emission phenomena was increased by decreasing the pillar wide while in constant pillar wide, acoustic emission phenomena was increased by increasing the pillar height.