5.3 Open Pits

When the floor of an open pit is excavated below the water table, groundwater enters the pit through seepage faces that form on the pit walls and as upward flow at the pit floor. There are commonly two components to the inflow; diffuse inflow widely distributed through the general rock mass and focused flow where permeable fractures intersect the pit wall (Figure 17). In addition to physical management of the inflowing water, consideration must also be given to the influence of pore pressures behind the pit wall slopes on stability of those slopes. Both issues reflect distinct concerns, but are linked through similar control measures. In this subsection, aspects of seepage interception to control water inflow are first discussed, and then attention is directed to consider aspects of pit slope depressurization.

Figure 17 – Photo on left shows a seepage face near the base of an open pit, as indicated by general wetness of the rock surface. Photo on the right shows water emerging from discrete fractures farther along this same bench.

Groundwater seepage into an open pit initiates a transient hydraulic response in the surrounding bedrock and surficial units that is similar in a number of respects to radial flow toward a pumping well. Where the surrounding bedrock has low hydraulic conductivity, inflow rates will be small and water management can be handled using sumps on the pit floor. If the bedrock has moderate to high hydraulic conductivity, considerable attention is directed to the design and operation of a multi-well network to intercept groundwater before it enters the open pit. If overburden units above the bedrock surface are permeable, it might also be necessary to operate a dewatering system to intercept flows toward the pit in those units.

Groundwater inflow to an open pit can be estimated to first order using analytical solutions based on a simple representation of the hydrogeologic setting and the assumption of radial flow toward a pit penetrating a homogeneous permeable horizon. Marinelli and Niccoli (2000) provide several analytical solutions for estimating steady state diffuse inflow rates to an open pit. Their solutions allow for calculation of flow rates through the pit wall, and the upward flow component through the pit floor. The influence of a pit lake on inflow rates to a closed pit is also incorporated in their solutions.

Insight to the time-varying inflow rate Q(t) can be derived from Equation 3.

(3)

where:

This equation neglects the effects of the regional groundwater flow system, as it solely accounts for water entering the open pit by release of water from storage in the drawdown zone around the pit. By way of example, if the permeable rock mass had a hydraulic conductivity of 10^-5 m/s and a specific storage of 10^-5 m^-1, with an aquifer thickness of 30 m and a design drawdown of 20 m for an open pit with a radius of 100 m, the inflow 1 month after “instantaneous placement” of the open pit would be approximately 650 m³/d. After 6 months, the inflow would be predicted to decline to 550 m³/d. The scenario envisioned here has the floor of the open pit penetrating 20 m into a 30 m thick permeable horizon located at the pit floor.

Dewatering rates vary considerable from mine site to site. Where active dewatering systems using wells are in place, extraction rates might be as low as 500 – 1000 m³/d and range up to 80,000 m³/d, or higher when dewatering limestone units in a regional system with karst development. Similarly, the number of wells in operation at a given time varies for site to site; dewatering requirements might be achieved with as few as 3 to 5 wells, while in other cases 20 or more dewatering wells could be in operation. It is normal practice to use vertical or slightly inclined wells. In environments such as the oil sands open pit mines in northern Canada, horizontal boreholes completed with long well screens have proven to be effective in meeting dewatering requirements in a cost-effective manner.

A key question that arises in the design of an open pit dewatering system is the lead time in advance of mining to a particular depth below ground surface that groundwater wells need to be operational so that the depressurization target is achieved in advance of the requirements specified by the mine plan. For example, based on the projected rate of drawdown that can be achieved (e.g., m/month), it might be necessary to have operating wells in place 2 years prior to mining reaching that area of an expanding pit. The lead time required will depend upon the pumping rates achievable at each well, the bulk hydraulic conductivity of the geologic unit and its storage properties (specific storage if a confined aquifer system, specific yield if unconfined), local rates of groundwater recharge, and the regional groundwater flow component.

Rock mass depressurization has the potential to impact groundwater flow in a region well beyond the immediate vicinity of an open pit. A common concern expressed is the extent to which dewatering activities could influence baseflow contributions to nearby streams, or reduce the flow rate at springs used by local communities for potable water supplies. These issues are often addressed using numerical simulation models as this approach allows consideration of the many complex hydrogeologic factors that interact to determine the potential magnitude of an impact. For a simple representation of the rock mass as a homogeneous porous medium without any faults acting as barriers or conduits, and no boundary affects such as the interaction of the cone of depression with a source of groundwater recharge, a preliminary estimate of the time (t) for a head response to be detected at a spring can be estimated as Equation 4.

(4)

where:

For example, a spring 1000 m distance from the pumping center, in a moderate permeability bedrock with a hydraulic conductivity of 10^-7 m/s and a specific storage of 10^-5 m^-1, a response time of approximately 3 years is predicted, or 4 months if the bulk hydraulic conductivity were one order of magnitude higher. In a region with karstic limestone, the response time might be measured in hours.

It is often the case that the rock mass in which an open pit is developed is structurally complex. This complexity can reflect, for example, the influence of fault-offset and/or folded stratigraphic units, or through-going faults and dykes. Both factors can lead to the rock mass behaving as a compartmentalized groundwater flow system. Design of an effective dewatering system requires this complexity be understood, at least at a basic level.

Approaches to well-field design can range from applying operational experience and simple analytical methods to the use of detailed three-dimensional groundwater flow models to predict drawdown. It is common to see this latter method be adopted when the bedrock flow system is in a complex geologic setting and/or there are questions concerning potential impacts of the dewatering system on baseflow reductions in nearby streams. The initial designs of many dewatering systems anticipate the eventual need to add additional extraction wells as site experience is gained and the pit is deepened. Pumping wells are either located around the perimeter of the open pit, on mine benches mid-slope on the pit walls, on the floor of the pit, or in some combination of these locations. As the pit is deepened, replacement wells usually need to be installed, which indicates a preference for wells located on the perimeter of the pit where possible.

The geotechnical design of a pit wall slope (overall slope angles, bench widths, see Figure 8) is tied to an evaluation of the requirements for rock mass depressurization. A decision to adopt a steeper pit wall slope often requires a higher degree of pore pressure reduction to achieve a design factor of safety. There is particular interest in prediction of the elevation of the seepage face where it intersects the pit wall. The tradeoff between the design slope angle and depressurization requirements involves consideration of benefits, costs, and risks (e.g., Sperling et al., 1992). A sound understanding of the hydrogeological regime is a key element in design. Changes in recharge rates between wet and dry seasons can exert an important influence on the design of the depressurization system if the change in recharge rate causes significant fluctuations in the water table seasonally.

Another not a common circumstance, hydraulic gradient control might be required in a situation where active mining in an open pit is occurring adjacent to a second pit that has been closed and allowed to fill with water. As the active pit is deepened, the hydraulic gradient across the rock mass between the flooded pit and the active pit will increase, leading to the possibility of particle entrainment and piping by the moving groundwater. This, in turn, introduces a risk of slope instability. Seepage interception wells can be used to manage the distribution of the hydraulic gradient within the rock mass separating the pits.

Beale and Read (2013) provide a comprehensive guide to site investigation, conceptual and numerical model formulation, and the implementation of pit slope depressurization systems, and a discussion of numerous case histories. Challenges specific to mine sites where weak bedrock units are present, such as shales or siltstones, are discussed in Martin and Stacey (2017). Design of the pit wall slopes is commonly based on an assumption of either two-dimensional sectional models for each design sector of the open pit, or a three-dimensional groundwater model when two-dimensional representations of the groundwater flow system are not appropriate. It is more common to see numerical models developed on the basis of continuum approximations of the hydraulic properties of the rock mass rather than the adoption of discrete fracture network models. A current research focus is on better understanding the role of hydromechanical coupling of the groundwater regime, changes in the in-situ stress distribution as mining progresses, and rock mass deformation influencing fracture openings, permeability and the evolution of the pore pressure distribution in the pit walls.

For depressurization of pit wall slopes in the deeper parts of an open pit, where the rock mass hydraulic conductivity might not be sufficient for effective use of pumping wells, the installation of numerous sub-horizontal drains provides an alternative control measure. The drains can be installed at the pit face, or from a drainage gallery driven through the rock behind the ultimate pit face, with drain holes installed from the gallery. The drains yield water by gravity flow, with flow rates that vary with the local connectivity of fracture networks or faults penetrated by the borehole. The effective design of a horizontal drain program depends upon a sound understanding of the structural fabric and jointing in the bedrock. Beale and others (2013) provide an extended discussion of guidelines for the design and implementation of horizontal drain programs.