The design of a mine water control system depends on many factors including:
• hydrogeological conditions
• dewatering or depressurisation requirements
• start, duration and rates of pumping at different stages of mining
• interaction of pumping infrastructure with mine planning and operations
• availability of drilling and pumping equipment
• contractor experience
• capital and operating costs
• production schedule for life of mine
The proposed mine site is located in Western Australia along a steep ridge. The ridge is made of steeply -dipping banded iron formations (BIFs), or iron-rich sedimentary rock, interlayered with felsic sedimentary and igneous rocks that host the iron ore mineralisation. The sub-vertical, high permeable BIFs act as a groundwater storage system. Mining will take place below the water table. The pit water control system must deal with the stored groundwater, rainfall recharge and the lateral groundwater recharge through un-mineralised BIFs, palaeochannels and open faults that cross-cut the area.
Based on the geometry of the formation and the design of the open pits that develop wider and deeper with time along the ridge, in-pit dewatering boreholes, along with interception boreholes located outside the pit perimeters, were recommended for the project. In-pit dewatering boreholes can interfere with the mining operations and affect the mine’s efficiency. In order to minimise potential interference and support decision-making on the water control strategy for the project, an analysis was undertaken for several in-pit water control options. This analysis considered the location and orientation of potential boreholes, their longevity and replacement requirements, the estimated cost for each option, and the operational considerations during installation and operation of the system based on regional drilling experience and regional mine water control implementation. Next followed a qualitative risk assessment related to the planned dewatering system involving all stakeholders (project mining manager, project engineer, master driller, open pit mining engineers). The objective of the risk assessment was to categorise the possible implications and uncertainty associated with the design, provide detailed control measures for each identified risk and define residual risk after the control measures were implemented.
Using the above approach, SRK was able to present a recommended water management strategy for the project that met the requirements for dewatering and the mining operations in the most cost effective manner.
