This thesis presents a study into the effect of rock mass stress change, resulting from open pit mineral extraction, on the hydrogeological characteristics of the host rock, with particular focus on hydraulic conductivity change in fracture-dominated groundwater flow systems. The study investigates the potential significance of hydrogeological change and whether such effects should be considered during environmental impact assessment for new mineral workings.
Stress change resulting from lithostatic unloading due to mineral extraction results in the development of a near field excavation damage zone (EDZ) and an excavation disturbed zone (EdZ) that extends further into the host rock. In this study the boundary of the EdZ is defined as the point at which rock mass horizontal and vertical stresses are within 5% of pre-excavation gravitational stress. The majority of open pit mineral workings are established in formations where groundwater flow predominantly occurs via discontinuity networks consisting of interconnected fractures, joints and faults. Stress change, and associated extensional strain, may result in rock mass displacement and discontinuity dilation with effect on groundwater flow and contaminant transport capacity. The lateral and vertical boundaries of EdZ development around open pit excavations have not previously been well-defined and previous investigations into discontinuity dilation due to stress change have tended to focus on individual discontinuities rather than discrete fracture networks at field scale.
The EDZ extents around open pit excavations tend to be highly localised with limited hydrogeological significance. Investigation of EdZ extents and associated discontinuity dilation has been undertaken through development of a series of discrete fracture network (DFN) models with progressively increasing DFN complexity. Geomechanical modelling is undertaken in two and three dimensions using Rockfield’s Elfen code. Open pit excavation is simulated at three successively increased depths and under two different in-situ stress conditions. The DFN configurations are designed to be generally representative of discontinuity networks observed at multiple excavations in northern England. The results of the geomechanical modelling programme are reported as vertical and lateral EdZ extents and discontinuity dilation -v- distance relationships. Discontinuity dilation has been translated into equivalent porous media (EPM) hydraulic conductivity change through application of the cubic law and then applied to hydrogeological case studies through development of groundwater flow and contaminant transport models.
Modelling results indicate that the EdZ around open pit excavations extends for several hundred metres behind an excavation face and below an excavation floor. The EdZ extent increases with increasing depth of excavation. The presence of pre-existing discontinuities reduces EdZ extents with the magnitude of reduction influenced by discontinuity intensity, orientation and interaction with excavation surfaces. A normalised approach to EdZ estimation has been developed by reference to excavation dimensions. Rock mass displacements of up to 10cm occur at the excavation face and floor with discontinuity dilation of up to 1cm. Cumulative discontinuity dilation accounts for between 30%-50% of total vertical or lateral rock mass displacement. Discontinuity dilation magnitude -v- distance from the excavation floor or face has been described by an inverse power law function that captures the effect of discontinuity intensity variation. Discontinuity dilation occurs over a shorter distance than rock mass displacement, resulting in the separation of a dilation zone from a displacement zone, within the EdZ boundary. The dilation zone is defined in this study as the hydrogeological significant excavation disturbed zone (HS-EdZ).
Groundwater flow and contaminant transport modelling demonstrates that the effects of lithostatic unloading can result in a reduction in hydraulic gradient around open pit excavations with potential change in groundwater inflow to the excavations. The development of enhanced permeability zones behind excavation floor and face establishes preferential flowpaths for contaminant migration from open or restored mineral workings with implications for the after use of open pit excavations. The study concludes with recommendations regarding conditions in which consideration of lithostatic unloading effects should form part of hydrological impact assessments for new open pit mineral workings.
