Authors: Webster, S; Snyman, L; Francois, N; Saadatfar, M

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Webster, S, Snyman, L, Francois, N & Saadatfar, M 2022, 'X-ray computer tomography and ground conditions at Northparkes cave edges to further the understanding of the caving mechanisms of strain and hydraulic conductivity', in Y Potvin (ed.), Caving 2022: Proceedings of the Fifth International Conference on Block and Sublevel Caving, Australian Centre for Geomechanics, Perth, pp. 635-650,

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It is generally understood that rock mass strain increases from the intact, undisturbed zone towards a block cave edge damaged region and into the mobilised zone. The ability to observe this underground is restricted by a lack of safe observation points. However, from this information, the current understanding of cave growth is commonly described using the Duplancic & Brady (1999) caving model. Here, ground conditions transition from pseudo-continuous intact rock to seismogenic, degraded, and loosened rock, and finally, rock mobilised within the cave. The characteristics of the damaged cave zones are highly variable and impact the ongoing caveability, nearby mining activities and hydraulic conductivity. Numerical models can now model these pieces by coupling the interactions of the cave muck pile particle flow, geomechanical response to caving, and hydrological effects. The relationship between each of the components is non-linear and in terms of the hydrology, more definition is required. To improve the efficacy of the hydrological models, the authors are undertaking an experimental project to better define the relationship between hydrological properties and strain. The first triaxial experiment has been performed on the E26 core coupled with X-ray microcomputer tomography (XCT) imaging throughout the loading stages. The images presented here enable 3D visualisation of the complex failure mechanisms and the relationship between strain, fracture dilation, and fracture connectivity. At the same time, Northparkes has mined adjacent and into historic block caves where the development mines against the existing cave edge and in some instances, exposed the mobile caved zone. This allowed observations of the ground conditions transitioning from the pseudo-continuous intact ground through to the mobilised zone and the failure mechanisms encountered. This paper presents underground observations of the variable failure mechanisms encountered from the cave operations. These observations are reconciled against the geomechanical model results to show how strain presents in the mine. The modelling, failure mechanisms, and experiment imaging are brought together to better illustrate the coupling and non-linear relationship between strain, fracture mechanics, and hydraulic conductivity.

Keywords: failure mechanism, strain, permeability, field observations

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