5.2 Underground Workings

A decline tunnel, vertical shaft, horizontal drift and stopes (an open mine void where ore has been removed) act as hydraulic sinks due to the atmospheric pressure boundary condition they impose in the region below the water table. During the planning stage, insight is required to the anticipated flow of groundwater into a tunnel so that this volume of water can be incorporated within the water management plan, and when necessary, measures implemented to control those inflows during and following adit construction. Field observations indicate two flow regimes often characterize the inflow rate; one a diffuse flow through the rock mass with inflow locations determined locally by open joints in the rock mass, and the other (if it exists) a larger focused flow associated with a damaged rock zone along transmissive fault zones that cross the alignment of the tunnel.

The component of diffuse inflow can be estimated to first order from an analytical solution adopting a simply representation of the hydrogeologic setting. The steady state inflow per unit length of the tunnel (Q₀) can be estimated as shown in Equation 2 (Goodman et. al., 1965).

(2)

where:

By way of example, a 4 m diameter tunnel 200 m below the water table, in relatively permeable bedrock with a hydraulic conductivity of 10^-6 m/s would have a predicted diffuse inflow of about 20 L/s over a 100 m length of the tunnel. This steady state calculation provides insight to the strong dependence of the diffuse inflow component on the bulk hydraulic conductivity of the rock mass. If the hydraulic conductivity of the bedrock were lower by two orders of magnitude, a reasonable value for sparsely fractured bedrock, the diffuse inflow would be on the order of 0.2 L/s over that same distance. This small flow rate is easily managed by a pumping system. Substantive inflows to underground workings are usually associated with the intersection of discrete fault zones of high transmissivity. If those fault zones are in hydraulic communication with a surface water body, or intersect a near-surface aquifer of high hydraulic conductivity, substantial inflows can be expected.

Applying this equation requires the assumption that there would be a negligible impact of the inflow on the elevation of the water table. Alternative formulations have been presented for the estimation of the transient flow response to emplacement of a tunnel that does lower the water table elevation. The equation above is based on modeling groundwater inflow to a single tunnel and therefore is not directly applicable to the complex adit geometry typical of underground mine workings. Underground mines with a large footprint, and many tens to hundreds of kilometers of workings, can require groundwater extraction rates on the order of 10,000 to 20,000 L/s or higher if located in areas with permeable bedrock.

At sites where appreciable groundwater inflows are anticipated, groundwater control is focused on lowering the water table to reduce inflow volumes and maintaining the lower water levels over time. Cost-effective dewatering designs require a sound understanding of the hydrogeologic setting. Dewatering of underground workings is often undertaken with a combination of vertical wells and collection sumps within the mine workings. At older mines, boreholes might also target abandoned mine works that have been allowed to flood. A potentially important constraint in mine dewatering arises if the groundwater that is produced requires treatment before release to the environment.

At underground operations no longer in operation, after the dewatering systems are shut down, the regional water table slowly rebounds. In historic mining districts where groundwater withdrawals might have continued over periods of 80 to 100 years, the water table could still be rebounding many decades after mine closure. Water table rebound is discussed further in Section 9.