3.1 Mine Dewatering and Control of Water Pressures in Mine Workings

The hydrogeologic setting in which a mine is located is a key feature in both the development of a mine plan and its successful execution. There are recent examples in Canada where difficulties in controlling groundwater inflows to underground workings led to a multi-year delay in the startup of operations at one mine and in another case excessive inflow to the underground workings was a key contributing factor in early closure of a mine. High inflows hinder access to working faces and elevated pore pressures decrease rock mass stability. Both of these factors are potential safety issues that require control.

Groundwater control at an open pit mine has two principal objectives: (i) to ensure pore pressures behind the pit walls and beneath the pit floor are sufficiently low so that the pore pressures do not induce rock mass instability, and (ii) to ensure trafficability of heavy equipment on the pit floor is not compromised by either water-saturated sediments or standing water. Wet ground conditions have negative impacts on blasting efficiencies in the open pit. There can also be a risk of flooding of the pit due to an unanticipated inflow of water if a high hydraulic conductivity feature connected to a surface water body or an aquifer is intersected on the pit wall or floor. Figure 8 illustrates an open pit mining operation with effective seepage control, evidenced by the dry condition of the pit floor and walls, with only minor seepage apparent on the walls of the pit and at the pit floor.

Figure 8 – Example of an open pit with effective groundwater control. The pit floor is far below the elevation of the pre-development groundwater table.

With regard to rock mass stability, there are two concerns. First, elevated pore pressures reduce the shear strength of the rock mass. A dewatering system is designed so that for a chosen slope of the pit walls, an acceptable factor of safety against failure of the walls is maintained. Second, uplift pressures immediately beneath the pit floor can be similar in magnitude to the downward force exerted by the rock mass. Uplift pressures are a particular concern when a confined aquifer with elevated hydraulic head is located beneath the pit floor. If the rock ruptures due to the unbalanced uplift forces and the underlying aquifer has relatively high hydraulic conductivity, this could create a pathway for a rapid and substantial inflow of groundwater onto the pit floor. High seepage gradients can also lead to permeability enhancement in bedrock if a higher groundwater velocity mobilizes fine-grained materials infilling fractures.

In geotechnical stability assessments, it is important to recognize the difference between the effects of pressure propagation along a higher hydraulic conductivity zone and a volumetric flow effect where the stratigraphy and flow regime also provides a pathway for higher fluid fluxes. A detrimental pore pressure propagation effect on stability in the foundation of a tailings dam, for example, can occur in the absence of significant fluid flow.

Many of the concepts used in the design of a well field for water supply are applicable in the design of dewatering or depressurization systems at mines even though the geometry of internal boundary conditions imposed by an open pit or underground adit differs from those adopted in well field design for evaluating water supply from an aquifer. The time required to achieve a dewatering/depressurization target is an important design consideration, so these analyses are based on models considering transient groundwater flow. Singh and Atkins (1985) provide a compilation of analytical solutions useful in the estimation of groundwater inflows to mine workings.