Abstract:

The far-field stress tensor is a key input for rock mechanics assessments, including numerical methods. With the global mining industry trend of deeper operations and higher production rates, the need to better understand far-field stress becomes critical, particularly regarding mining-induced seismicity. Factors that can influence the direction and magnitude of far-field stress include depth, topography, and the tectonic environment in which the planned mining will occur.

A number of approaches have been developed to estimate far-field stress, including hydraulic fracturing, over-coring, borehole breakout analysis, flat jack, acoustic emission techniques, borehole slotting, and core disking. It will be noted that all approaches infer the far-field stress tensor from the rock mass response to stress-induced loading (i.e., strain). The profound effects of rock mass strength anisotropy associated with foliation fabric is well documented, yet is often ignored or overlooked within the mining rock mechanics community. However, understanding its affects is critical when undertaking far-field stress estimates within foliated rock masses.

This paper considers four drill holes and associated borehole breakouts from a deep mine (~2 kms) hosted in a rock mass with well-developed foliation fabric. In order to review the effects of rock strength anisotropy on borehole breakout extent and location, the rock mass response has been modelled in FLAC3D using the Improved Unified Constitutive Model (IUCM). Results show that attempting to infer far-field stress orientation through the review of borehole breakout analysis alone can lead to highly misleading results when undertaken in anisotropic rock mass conditions.

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Julian Watson