Authors: Rimmelin, R

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Rimmelin, R 2022, 'Modelling of fracture initiation near a cave back and its implications for hydraulic fracturing concurrent to cave mining', in Y Potvin (ed.), Caving 2022: Proceedings of the Fifth International Conference on Block and Sublevel Caving, Australian Centre for Geomechanics, Perth, pp. 1189-1200,

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Rock mass pre-conditioning has been applied in the mining industry since it was first trialled at Northparkes in 1997, to continue its wide application in large cave mines such as Cadia East and El Teniente mines. The most known application is based on hydraulic fracturing techniques transferred from the experience of the oil and gas industry to create additional fractures in the rock mass prior to initiating undercutting. In the mining industry, the aim is to reduce the rock mass quality to obtain an improved cave performance and other benefits such as better seismic response and reduced fragmentation. In addition to pre-conditioning, there are other experiences of hydraulic fracturing concurrent to an existing cave back. In this context, documented experiences from Northparkes, Grasberg and Cadia East mines were carried out by applying hydraulic fracturing to induce caving after a stalled or slowed cave propagation. Applications concurrent to cave mining do not follow the same rules as applied prior to undercutting (pre-conditioning) for fracture initiation and propagation. Furthermore, these have not been clearly explained in terms of the supporting fundamental physics. The present work proposes an elastic closed form solution to estimate fracture initiation demand when the hydraulic fracturing is interacting concurrent to a cave back. This analytical model was able to explain the reduced demand of breakdown pressure due to the modified stress field near a cave back (near field stress). In addition to this result, there are preliminary implications for cave engineering applications to support cases of cave induction but also new potential applications for directional caving as conceptualised in the present work. This development is part of a current ongoing research that will also consider aspects related to fracture propagation.

Keywords: hydraulic fracturing, pre-conditioning, post-conditioning, fracture initiation, caving mechanics

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