Exercise 2 – Inflow Prediction to Open Pit Mine

An open pit for extracting ore is to be excavated through a thick section of low permeability till to the base of the fractured bedrock horizon, as shown in the cross section below (the schematic diagram is vertically exaggerated, thus not drawn to true scale). The till, which acts as a confining layer to the underlying more permeable fractured bedrock, is 15 m thick with a hydraulic conductivity of 1×10^–8 m/s. The fractured bedrock horizon is 30 m thick, with an effective hydraulic conductivity of 5×10^–6 m/s and specific storage of 1×10^–5 m^–1. Below the fractured horizon, the bedrock has a hydraulic conductivity of 1×10^–9 m/s. Assume a depressurization system (e.g., horizontal drain holes) will be put in place to maintain the hydraulic head at the toe of the pit wall at an elevation 5 m above the pit floor. The pit will be roughly circular in geometry, with an ultimate diameter of 400 m.

1. Estimate the diffuse inflow to this open pit three years after it reaches full depth, assuming instantaneous development of the pit. Standard practice would be to develop a three-dimensional numerical model that simulates the incremental construction of the open pit through time and the resulting groundwater conditions within the till and fractured bedrock. An analytical solution is useful for providing initial insight to the magnitude of flow into the pit over time. Given the minimal time required to calculate an analytical solution, the results are valuable to a numerical modeler who can compare the values and, while qualitatively compensating for the simplifications of the analytical solution, can investigate the numerical model setup more carefully if the analytical and numerical results are strikingly different. One would expect the initial flow to be much higher in the analytical solution and the steady state value to be roughly the same between solutions.
2. Develop a second estimate of inflow for a case where the water table is near the top of the fractured bedrock, the till is unsaturated, and the fractured bedrock unit responds as an unconfined aquifer with a specific yield (S_y) of 0.03. Explain why the inflow is significantly higher for one case relative to the other.

Figure Exercise 2–1 – Open Pit Mine Schematic for Exercise 2.

Click for solution to exercise 2