Présentations Colloques

Session 7.04: Groundwater and man-made underground works
Timms Wendy
Conceptual models of risks to shallow aquifers from underground mining through geological fault structures
Groundwater risks associated with underground excavations through fault zones has received little attention compared to geomechanical and safety risks. Several plausible conceptual hydrogeological models for geological faults of mining operations are considered in this presentation, to inform the design of site based monitoring and adaptive management. Key lessons from several underground metal and coal mines in Australia are highlighted. ****Favorable conditions where faults can form barriers to groundwater include the following- partially penetrating hydrogeological facies, filled with clay gouge, tight due to favorable high stresses, lacking a fractured damage zone, vertical or horizontal fault offsets that do not connect aquifer zones, and a geologically stable. However, in other conditions, groundwater level declines may be recorded at a considerable distance along the strike of the fault zone, without affecting groundwater levels in zones with limited hydraulic connection. ****The likelihood of groundwater impacts increases if fault structures are reactivated by significant tensile strains that are induced by mining. For example, horizontal displacements of 65 mm have been surveyed at the ground surface over 680 m away from longwall mines. Improvements in monitoring technology and evaluation of far-field effects over at least 2 km from operations are ongoing. However, quantifying the effects of differential movement of rock masses on preferential groundwater flow and subsurface storage volume is more challenging, particularly where the rock surface does not outcrop. ****Monitoring and investigations of mining impacts must also account for multiple hydraulic stresses such as changing land use, water use and climatic variability. Where mining through geological faults does contribute to changes in shallow aquifers, localized loss of base flow to streams could occur and or if preferential flow occurs to mine depth, increased dewatering could be managed. In situations where the long term consequences for shallow aquifers are uncertain, further research and adaptive management is needed. **