Evaluation of air blast parameters in block cave mining using particle flow code

Air blast, a sudden mass movement of air, can occur in underground mining system where caving develops an extensive mass of unsupported rock spanning a large void. Air blast can result in injury to mine personnel, damage to equipment or disrupts mine operation. Evaluation of air blast parameters is, therefore, an essential part to develop strategies to mitigate the hazard. The properties of a muckpile or a caved zone are significant factors affecting the magnitude of air blast in particular on the undercut and extraction levels. This research investigates the effect of muckpile properties on air flow using the numerical code, PFC2D. The critical parameters such as thickness, block size and porosity (swell factor) of the muckpile have been studied to quantify how much they could change the magnitude of air pressures and velocities while the air flows through the muckpile. It was found that the porosity of the muckpile is the most effective parameter on the magnitude of air blast and by designing a thick layer of blasted rock with low porosity in the caved zone, the intensity of the air blast can be significantly reduced. The findings of this study can be used to design air blast plugs or bulkheads in order to isolate any potential air blast from the active workings, or to quantify the minimum thickness of the muckpile above extraction levels to manage air blast hazards

Investigating Effects of Fracture Aperture and Orientation on the Behaviour of Weak Rock Using Discrete Element Method

The effects of the fracture aperture and orientation on the behaviour of weak rock were numerically investigated using discrete element method (DEM). In this study, the mechanical behaviour of the intact and fractured rock specimens was simulated by adopting the discontinuum based software PFC3D. The rock specimens with various fracture apertures and orientations were replicated, and the effects of these two fracture characteristics were studied through triaxial tests. The flat-joint model was employed for simulating the stress-strain behaviour of intact rock and had the ability to reproduce the cementation effect. The smooth-joint contact model was utilised to simulate the sliding effect of the fractures. The effects of five different fracture orientations were investigated in the combination of three different fracture aperture categories, namely very tight, open, and moderately wide. It can be concluded that the strength of the fractured weak rock specimens reduces as the fracture aperture width increases. The amount of alternation in strength and deformability that were contributed by fracture apertures differed with the orientations of the fracture. With the fracture orientation that was parallel to the deviatoric loading, the effect of fracture aperture on the strength and deformability of the specimens was less evident

Modelling and analyses of rock bridge fracture and step-path failure in open-pit mine rock slope

Rock Bridge fracturing and coalescence with pre-existing discontinuities in rock mass due to the initiation, propagation and interaction of these fractures refers to instability mode of step-path failure. Step-path failure is a typical type of instable mode of man-made and natural rock slopes. The continuum finite element method was applied to work on deeper insight into the propagation of tensile cracks which developing in the intact rock bridges that can finally coalesce to form step-path failure. In this paper, based on the intact rock fracturing hypothesis, two selected slope simulations from the Handlebar Hill open - pit mine near Mt. Isa in Queensland, Australia, modeled the process of fracturing and step-path failure through different pre-existing discontinuities. The empirical models of Bobet and Einstein (1998) and the progressively cracks development are observed within crack initiation, propagation and coalescence in the intact rock bridges. Proposed slope models of the mine included four joint-net distributions through the rock masses considering the geometry of structures (dip angles, spacing, lengths and orientation) illustrated the extension cracks from the flaw tips and propagated to the slope surface. Modes of intact rock bridges fracturing (shear, tensile and a combination of shear and tensile) have been observed. Tensile fracture is usually generated when the rock bridge angle is sub-vertical. Shear fracture can be initiated in less steep rock bridge angles. A combination of shear and tensile failure is normally generated in slopes with. Slope with explicit large-scale structures of steeper dip angles increased the yielding. Larger structures show much higher potential for yielding as the tensile stresses increasing. Major joint plane spacing resulted in less potential for relative deformations between neighboring structures and consequently reduced slope instability. The changes of length and spacing have more influence on slope stability than a change in the dip angle of the structures. © 2019, Springer Nature Switzerland AG