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Structural geology: the fabric of the mine

SRK News | Issue 54: Rock Engineering and Slope Stability

A4   |   Letter

Geotechnical studies in mining operations rely on the ability of geologists to use measurements and observations from drilled or exposed rock to predict lithology and structural weaknesses in areas that may impact the stability of the mine, but are not yet exposed. This requires investigating and understanding the structural patterns that define the rock mass, including their continuity and cross-cutting relationships. The structural patterns not only include the primary fault systems or the associated minor faults, but also the fracture patterns associated with fault systems or located in the rock mass between fault systems.

On many projects, a set of discontinuities (faults, fractures, veins or bedding) can be so frequent and repetitive that predicting the pattern is far more confident than predicting the location or continuity of individual structures. The repetitive structures impart a structural fabric to the rock mass that influences the strength and therefore the behaviour of the rock mass in a more predictable manner. In porphyry systems, for example, structural, alteration and geotechnical domains are commonly characterised by fracture-vein systems influenced by fault patterns. In epithermal vein systems, the domains are commonly constrained by fault-vein systems sub-parallel to the orebody. The trend of the fabric can change rapidly or gradually, and knowing the trend variations can in some cases be valuable to the engineers. The figure above shows a 3D modelled structural fabric of bedding-parallel fault systems with variable dip that impacts the location of multi-bench rock failures. The fabric model orientation has then been painted onto the planned pit surface. The fabric trends are broken by faults but controlled by folding patterns interpreted by the structural geologists. Structural fabric patterns can be used to predict how rock stability and risk varies across a mining excavation.

Wayne Barnett:

SRK Africa