An analysis of the effect of parameter unpredictability on the propagation of hydraulic fractures is presented. Hydraulic fracturing (HF) is becoming an established technique in mining for rock mass preconditioning. Initial applications were to improve the caving ability of ore; it is now often used for stress control. We propose the term ‘stress amelioration’ for this application, and show how it is essentially being used to control strain energy density. A simplified model of HF as an elliptical opening is used to investigate the effect on crack tip strain energy density of unpredictability in fracture extent and aperture. By reference to a recently proposed characterisation of unpredictability we apply interval analysis to the problem, and show that it is unpredictability in fracture aperture that has greatest influence on predicting strain energy density.
Mining operations are continually advancing deeper and thus into higher stress regimes, and so there is an increasing need for stress control techniques to protect vital infrastructure against violent rock bursting events in deep underground excavations.
The large stress concentrations induced around underground openings are of particular concern for cave mining operations. As shown in the leftmost diagram of Fig. 1, a caved zone comprises a volume of highly fractured, low-stiffness rock that with continued extraction may eventually become a void. As the rightmost diagram of Fig. 1 shows, such a low-stiffness inclusion can induce large stress concentrations in vital locations.
The importance of reducing such stress concentrations has been recognised since the 1950s (Roux et al. 1957; Sellers 2011), and although initial approaches involved specialised blasting techniques to change the state of induced stress, it is hydraulic fracturing (HF) that has become particularly prevalent of late. Despite the importance to mining operations of controlling the state of induced stress, there seems to be vagueness in the definition – and hence practical assessment of the efficacy of – ‘stress control’. In addition, there seems to have been little attention directed at the effect of parameter unpredictability (say, rock mass elastic modulus) on the outcome of a stress control campaign, with the controlling parameters often being considered as fixed values.
In this paper, we investigate both of these problematic issues: definition vagueness and the effect of parameter unpredictability on design.
