Hydraulic Fracturing (HF) in cave mining is commonly used in competent rock under high stresses and seismic conditions, to manage risks associated with induced seismicity and cave propagation. The HF process is done using up holes drilled from the undercut level (e.g. El Teniente -Codelco) or down holes from an upper level (e.g. Newcrest mining). Discrete fractures assumed to be in the order of 20 to 40 m in diameter are then created every 1.5 m or 2.5 m along the drill holes, using water at higher pressures. In comparison, other industry experience such as Petroleum, HF is based on similar principles in terms of the drill hole alignment with the principal stresses and the use of water at high pressures, but more efficient during the process of the creation of the network of fractures. The network of fractures thus created covers a volume rather than discrete fractures and could be as long as 100 m. Currently, petroleum HF process is carried out from the surface down to 3,000 m in depth. Also, the use of additional materials such as "sand" to improve the propagation of fractures is another example of Petroleum practices that can be used in Cave Mining. Another point of discussion relies on the current practices in Cave mining, in which, stress anisotropy is not considered to address important topics such as efficiency of the HF process and spatial distribution of the HF holes, for a better coverage of the rock mass. Hydraulic Fracturing is increasingly becoming an essential enabling tool in deep hard rock cave mining, and some of the current HF practices used in the Petroleum industry offer an opportunity to improve the cave mining HF processes. This paper proposes the use of known rock mechanics principles for the cave mining industry to adopt practices used in the Petroleum to achieve the intended outcomes from HF as currently used in cave mining.
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.
Amongst many mechanical properties, cohesion (c) and angle of internal friction (φp) are probably the most widely used rock and rock-like material strength design parameters. However, unlike Mohr-Coulomb (MC) failure criterion that assumes cohesion and friction angle are intrinsic material properties and are not affected by the applied stress level, the Hoek-Brown (HB) criterion predicts a continuous change of apparent cohesion and friction angle if the induced normal stress on the fracture plane changes. That is, at low values of normal stresses, the instantaneous angle of friction will be relatively large, whereas cohesion will be a small value. As the applied normal stress value on the fracture plane increases (moving ‘up’ the non-linear Hoek-Brown strength envelope), the angle of friction reduces, and the cohesion increases. This is an important result from the HB failure model and allows a more realistic estimate of shear strength to be made at low values of normal stress, preventing potential over-design problems. Nevertheless, the HB model neglects the effect of the intermediate principal stress on material properties. Limited studies on the variation of apparent cohesion and friction angle under polyaxial stresses in concrete are available in the literature. Therefore, this paper aims to investigate the effect of true triaxial stresses on concrete cohesion and friction angle using a polyaxial strength criterion developed by Mogi. The results of concrete show that the intermediate principal stress has a pronounced effect on cohesion degradation and the mobilization of internal friction angle as the ratio of the intermediate to the minor principal stress changes. The results are expected to provide a framework for a more realistic design of underground concrete structures at depth.
On 31 October 2013, production at Leinster's sublevel caving (SLC) operations were suspended following a significant mining-related seismic event that caused unforeseen levels of damage to underground workings. The event occurred when mining was taking place in an area known as the 11 Level Fold, approximately 1,100 m below surface. An assessment of the seismic risk following the 2013 event determined that it was too significant to continue with the SLC operations. Therefore, the SLC operations were not restarted and alternate mining methods would need to be investigated to address the exposure of personnel to the effects of the seismic concentration associated with the 11 Level Fold area. The proposed solution avoids the zones of highest seismic activity by transitioning to a block cave mining method and locating a production footprint at a level where interaction with the previous SLC and the 11 Level Fold can be avoided. This design is based on an advance undercut and excavation of 22 drawbells. Due to a significant regional shear that vertically bisects the orebody, it is expected that a 'chimney' cave will develop, thereby connecting with the previous sublevel cave column. The design will maintain critical infrastructure, such as the existing shaft, and use the current decline infrastructure to access the new production level. This work shows how high seismic risk can be managed by defining seismic exclusion zones and employing a mine design and mining method that best accounts for the geotechnical environment.
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