HYDROGEOLOGICAL CHARACTER OF FAULT ZONES - OBSERVATIONS FROM EXPLORATION, MINING, AND TUNNELING

The hydrogeological characteristics of fault zones can be observed at mining sites, tunnels, and other rock engineering excavations. This is because excavations in rock induce abnormally high hydraulic gradients, the effects of which are then observed as hydraulic head distributions at and near the excavations and water inflows along damage zones associated with faults and broken ground. The conduit/barrier nature of faults is often interpreted, but rarely quantified from these observations. There are large data sets, mostly unpublished, from mines and mineral exploration drilling which include fault zone descriptions of various kinds, such as excavation outcrop mappings, geologic and geotechnical core logs, geophysical imaging and hydrogeological test results. These data indicate that the correlation of fracture frequency and other discontinuity data with hydraulic conductivity (or transmissivity) of fault zones or fracture zones is usually weak – with some exceptions. By merging the published knowledge from civil engineering projects, we believe that significant improvements can be made in the understanding of the hydro-geology of faults in the mining sector.

To clarify the objectives and data required for a quantitative analysis of the hydrogeology of faults we present a review of published knowledge and a meta-analysis of data from selected excavation project sites. Unfortunately most studies lack data at all required scales of the project, and conceptual models are built using assumptions around the limited-scale data and applied to the entire project. We use our data to explore these assumptions. In particular, we comment on the usefulness of various data sets (typically collected during mining exploration and design studies) such as fracture counts, fracture properties, geomechanical properties, in the context of other factors that influence conductivity, such as stress variations, structural patterns/geometry, structural continuity and fault properties. We show that there are different hydrogeological responses to rock mass characteristics and structural styles.