Dewatering in an open pit simply means removal groundwater from a mine. When a mine extends below the water table, groundwater, due to gravity infiltrate the mine working and at this point mechanized form of removal of the water will be needed to continue operation.
There are several methods used in dewatering a mine which also includes calculations needed to design the dewatering system.

Conditions to be met before Dewatering System

To start with analysis on the ground water flow is to be done to know the water table, how the seepage formation looks like, the aquifer point and water direction orientation. All this would be achieved using piezometer and other instrument used for measuring water depth and pressure. Putting in consideration is also the pit limit which will also serve as a function to the surface excavation method engaged. The geologic and soil formation and seepage must be determined and the boundaries must be determined. It should also be noted that the source of seepage can be reduced which takes assumption that the aquifer is homogenous, isotropic formation of uniform thickness and the dewatering system to one or two parallel lines or circle of wells, source of seepage, and soil formation can in mist cases be solved or at least.

Explanation on a typical design of TRENCHING and SUMPING system of dewatering in an Open Pit mining field

Trenching is one of the basic and best practice of dewatering in the mine. This method uses knowledge and calculation derived after possible water table, geological condition, the travel direction of the underground water, water seepage or recharge rate has been determined. However, any depth of excavation that exceed the water table would be seen as potential seepage point or flooding point as the case dictates. To further prevents slimes, and achieve quality dewatering system, the sump pump, the rate of flow into the excavation would have been estimated and calculated from fluid mechanics point of view. Where an excavation extends into rock, and there is a substantial inflow of seepage, perimeter drains are installed at the foundation level outside of the frame work of the structure. The perimeter drainage system will be connected to a pump sealed off from the rest of the area to be connected, and the seepage water is pumped out. After construction, the drainage system is grouted to further prevent return of the seepage water through openings and discontinuities. To relieve the excavated face of operation from hydrostatic pressure in the rock mass endangering the stability, 4 inch diameter horizontal drain holes into the rock at approximately 10 foot center. For large seepage inflow, supplementary vertical holes for deep well pumps at 50 – 100 foot intervals may be desirable for temporary lowering of the ground water level to provide suitable condition for concrete placement.