The evolution and future of overburden dewatering at Highland Valley Copper

CIM Bulletin, Vol. 98, No. 1086, 2005

P.A. Witt

Highland Valley Copper has been mining its Valley pit since 1982. This pit requires large quantities of water-bearing overburden to be stripped away to expose the porphyry copper deposit underneath it. In order to dewater the aquifers, the mine began extracting groundwater in 1985 through the implementation of strategically placed deepwells containing submersible pump sets. Initially, dewatering focused on aquifers close to the topographic surface and pumping flow rate targets were set in reaction to observed piezometric levels. Later, it would be necessary to accelerate dewatering rates in response to a change in the mine plan in 1999. This change called for accelerated stripping on the east side of the Valley pit, requiring the basal aquifer to be dewatered faster. The capacity for pumping this aquifer, therefore, had to be increased substantially within a short period. To accomplish this as economically as possible, a more proactive approach to forecasting required flow rates and, by extension, planning and budgeting was required. It was not until 2001 that a groundwater model specific to the Valley pit was developed. Prior to that, future dewatering flow rates were calculated by examining previous dewatering data (flow rates and piezometer readings, etc.) to estimate the following years’ requirement for number of wells and their locations. This method is acceptable for shorter term forecasting, but is less reliable for planning into the longer term. Using the model, consultants were able to provide pumping volumes to the mine’s engineering department just before the end of 2001. These values were immediately used to determine the number and location of new wells that would be required around the Valley pit. At the same time, improved methods of drilling and commissioning wells were necessary as well as the acquisition of highly specialized pump technology to move water from increasingly substantial depths. The groundwater model indicated that basal wells would have needed to be established at a faster rate, to deeper levels, and to larger completion sizes. To address this, a contractor dual-rotary type drill rig was brought on site for testing to supplement the existing cable tool drill rig fleet in late 2000. The well was drilled successfully in approximately five weeks. The benefits of this method of drilling were: substantially improved penetration rates over cable tool drills; the assurance of deeper completions (to 335 m at bedrock); and finished diameters of two to four inches greater than before. The ability to drill deeper wells at larger completion diameters created opportunities to enhance existing pumping systems. It was now possible to install more powerful pumps at significantly deeper settings. This was critical in order to achieve overall pumping rates on time and cost-effectively. The current standard is to install 200 to 250 hp, 200-mm diameter submersible pump sets with variable speed drives and data loggers. Other aspects of pump systems were also improved, especially with instrumentation. This facilitated better maintenance of individual wells and optimization of output and performance. As a result of better modelling and use of new technologies, Highland Valley Copper has managed to increase pumping yields to levels of 25,000 L/min. while average depths have nearly doubled from what they were only a few years ago. Future dewatering requirements are significant and are vital to achieve mine plan targets. It will be necessary to utilize recent experience to forecast targets and flow rates in operating conditions that will continue to become more demanding and complex. This is especially true as the Valley pit continues to intersect an increasing number of aquifer units at greater depths.