Water inflow into the pit is a crucial problem in open pit mine operation when it is constructed below the water table. In this work, inflow rate into the pit mine is simulated by using MODFLOW codes. The model is assumed to be an equivalent porous medium and created in step excavation method. The result from the simulated model is verified by comparing with the analytical solution (Thiem-Dupuit assumption). During model creation, it is observed that Grid size plays an important role in inflow prediction. Model started with very finer size of grid and gradually increased the size to determine the effect of grid size on inflow rate. With the larger grid size, MODFLOW can predict inflow rate much better than the smaller one. At the same time, model boundary up to the ROI (Radius of Influence) gives better results. The inflow rate achieved by the simulated model is very close to the analytical solution (Error rate obtained approximately 0.3%). Several precautions are made to achieve a highly efficient result. Sensitivity analysis is also done for the simulated model with factorial design. A full factorial design is made by considering head, conductivity, drawdown, pit length and grid size as influencing factors. Among all these five factors, conductivity acts as the most significant factor, responsible to influence the inflow rate most.

1. Introduction

Computer simulation model increases the ability of understanding hydrogeology of an open-pit mine. It also aids fast analyzing various geological properties of underground and open-pit mine. Model helps to reconstruct known groundwater head distributions and flow rates. Model identifies hydrogeologic parameters, which can influence groundwater flow path and helps to predict the future flow paths. When simulating hydraulic flow into an open pit mine in relatively flat landed region, water is supposed to flow into the pit from all directions. To create this situation, a higher head value is assigned at the boundary and a relatively smaller head value is assigned at the pit seepage face to make the flow towards the pit maintaining a standard linear gradient. Fig. 1 represents the 2D mesh of the model area and AA' line passes through the region.

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