[5] The key considerations for establishing a network of blasthole wells dedicated to shallow dewatering involve conducting an evaluation of water storage distribution in the final benches of the phase of mining, identifying specific dewatering areas, drilling, designing, and constructing blastholes. Furthermore, there are operational considerations associated with this methodology that are discussed in the following sections of this technical article.
Evaluation of Water Distribution in Wet Blasts
[6] The removal of surface and groundwater water levels at the bottom of the pit before carrying out blasting or excavations can have several benefits. For example, shallow dewatering at the pit bottom enables the planning and execution of blasting operations, as issues related to excessive water presence can be avoided (Agboola et al., 2020). These issues may include a decrease in the effectiveness of explosives, the generation of mud, and the presence of nitrates in groundwater which is often attributed to the leaching of compounds from explosives used in blasting activities (Lai et al., 2021). The primary motivation for removing shallow water in the final benches of a phase are to ensure stable blast holes and improve rock fragmentation. This can be critical for executing the mine plan over the final benches, which are often the most economically valuable region of a pushback.
[7] The prospection of target areas for shallow dewatering requires an assessment of blast hole patterns and the possible presence of existing water (Figure 1). The analysis can be conducted by considering the recent history of blast hole drilling in preceding benches or through an evaluation of recent blasting pattern operations that are carried out as operational activities progress towards the end-of-phase open pit mining (Figure 2). Generally, there is a preference for selecting areas where the groundwater level is highest within the active bench.
[8] On the other hand, if there were no data available regarding blasting patterns and the presence of water in them, it is possible to complement the evaluation by analyzing the hydraulic properties of Geological Units, the Rock Quality Index (RQD), or through the identification of major fractured structures (Figure 3). In this regard, geological analyses provide valuable information about the composition and permeability of the units in the target areas, and the use of RQD models can provide insights for the assessment of the level of fracturing in the rocks within the geological units. Additionally, the presence of major structures can produce enhanced fracturing that remains below the sub-drill in the pit floor and can lead to preferential water movement. Such areas could be considered as good targets for water pumping (Figure 3).
Identification of Potential Dewatering Areas
[9] Once the analysis of blasting patterns, geology units, major structures, and/or rock quality has been completed, it is necessary to identify potential areas for implementing shallow dewatering using blast holes. These areas can be identified by the preceding evaluation methods by grouping the highest groundwater levels identified in blasting patterns or by identifying geological units with high permeability, in combination with an analysis of historic dewatering or monitoring pilot holes.
[10] This process involves an assessment of all the available data collected during the previous analysis. Grouping the highest groundwater levels detected in blasting patterns can pinpoint critical areas that require special attention for shallow dewatering (Figure 2). Additionally, the identification of geological units with higher permeability, and the detection of regions with a high level of fracturing can be utilized for targeted groundwater drainage and removal (Figure 3). Moreover, while areas abundant in clay may be less permeable, they could still accumulate water and necessitate targeted dewatering efforts, especially if they are present in critical zones (Figure 3).
[11] The combination of these factors provides a solid foundation for selecting priority areas to implement shallow dewatering for end-of-phase open pit mining using blasthole wells. This approach ensures efficient and focused management of operations, optimizing the effectiveness of the dewatering process in the identified areas.
Drilling of Shallow Dewatering Wells
[12] Once the process of identifying target areas for developing shallow dewatering using blastholes is completed, there are two viable options. One option is to carry out a drilling program in the identified areas of interest using a blasthole drilling rig that allows drilling holes of different diameters (typically between 10 5/8 to 16 inches), employing a combination of methods including rotary drilling and Down-The-Hole (DTH) drilling. The maximum depth these rigs can reach varies between 60 to 135 feet, depending on the manufacturer and drill rig model. Automated drilling functions when available, also allow for precision and customization of new wells that are fully aligned with the specific requirements of the mining. This approach ensures precision and customization in establishing new wells for shallow dewatering, aligning with the specific requirements of the mining site.
[13] Another option is to utilize existing blasthole patterns to design and construct shallow dewatering wells. In this context, it is essential to be aware of drilling depths and diameters, the well condition (dry/water), and the distance to the water level. With this information, it is possible to plan the design of the shallow dewatering blasthole wells based on the field requirements for casing diameter riser pipes and properly sized submersible pumps. This alternative capitalizes on the blastholes already in place, optimizing resource utilization and potentially expediting the dewatering process.
[14] The choice between conducting a drilling program or constructing shallow wells in existing blasthole patterns depends on site-specific factors, resource availability, and the desired level of customization. However, both options present practical pathways to implement the shallow dewatering strategy, contributing to the overall success of mining operations during the concluding phases of extraction and pit floor deepening. The process of shallow dewatering using blastholes involves a continuous assessment of results regarding water distribution and decision-making that is necessary to carry out the drilling, design, and construction of shallow wells to achieve dewatering during the end-of-phase open pit mining operations (Figure 4).
Design and Construction of Shallow Dewatering Wells
[15] The design and final construction of shallow dewatering can play a critical role in optimizing water removal during the concluding phases of mining operations. The process requires a sequence of carefully coordinated steps, each contributing to the efficiency and effectiveness of the overall shallow dewatering strategy. Key activities in the design and construction of shallow dewatering wells utilizing blastholes encompass drawing designs (Figure 5), sizing pumps, configuring the dewatering system (including elements like dewatering lines, sumps, and/or tanks), and considerations pertaining to the lifespan of the dewatering assets.
[16] The simplicity of utilizing blastholes for shallow dewatering wells is a notable advantage, allowing for inexpensive, rapid, and efficient implementation of the method. The straightforward nature of the process enables the installation of at least 1 to 3 blasthole wells within a day, contingent upon the availability of resources. This speed of deployment is particularly advantageous during the concluding phases of mining operations when rapid water removal is essential to the mine operations. The simple design and construction steps (see details in Table 1 and Figure 6) contribute to the efficiency of this method, making it a practical choice for initiating shallow dewatering quickly.
Table 1: Key Steps in the construction of shallow dewatering wells using blastholes.
|
Step
|
Activity
|
|
1
|
Pump Sizing: Choose pump size based on water levels, production, and/or aquifer transmissivity. Typically, small pumps with rates ranging between 10 to 70 gpm can be implemented.
|
|
2
|
PVC Casing, Riser Pipes, Submersible Pump, and VWP sensor: Install 6-inch blank (0 to 10 f.b.g.s.) and slotted PVC casing in blastholes (10 f.b.g.s. to total blasthole depth). Cut PVC casing near the surface if needed. Install 2-5 riser pipes depending on blasthole total depth. Connect the first riser pipe to the submersible pump. Install a Vibrating Wired Piezometer (VWP) sensor to monitor water levels.
|
|
3
|
Well Head and Dewatering Line: Connect well head to the last riser pipe. Use a wooden support structure for wellhead. Perform short term pump testing and install dewatering line to well head end pipe spool.
|
|
4
|
Pumping Challenges: Establish a long-term pump capacity based on testing of water production, and initial water levels decline inside the blasthole. Optimize pumping rates to maintain constant drawdown and capture area over time.
|
|
5
|
Operational Considerations: All available blasthole wells can be connected to a dewatering line leading to a sump or nearby tank to increase efficiency. Larger-capacity surface pumps can be used to remove water from these storage facilities out of the active bench -.
|
|
6
|
Operational Lifespan: Ranges from a few weeks to a few months, depending on the rate of mining and pit advancement.
|
[17] The simplicity of employing blastholes for shallow dewatering wells is complemented by the adaptability to swiftly adapt to the needs of end-of-phase open pit mining activities. The lifespan of these blasthole wells, ranging from a few weeks to a couple of months, highlights the importance of a quick and efficient removal process. The method allows for keeping pace with the dynamic nature of mining operations, where the need for water removal can fluctuate rapidly during the final benches of a mine phase. The ability to install, operate, and dismantle these blasthole wells with speed aligns seamlessly with the evolving demands of the mining, ensuring that the dewatering strategy remains responsive to changes in water levels and operational requirements. This agility is a key advantage, facilitating a timely and effective response to the variable conditions encountered in the dynamic environment of mining activities.
Challenges and Operational Considerations
[18] Understanding and addressing the challenges and operational considerations associated with shallow dewatering using blasthole wells is fundamental to the success of this innovative approach in mining operations. The dynamic nature of water levels within blastholes presents an ongoing challenge (i.e., blast holes stability and rock fragmentation) requiring an accurate assessment to maintain consistent and targeted dewatering efforts. Safety considerations must be carefully monitored to ensure responsible mining practices in a relatively small area of the mine and avoiding potential alterations to groundwater flow and changes in water quality composition.
[19] Efficient resource allocation and timing are crucial to balance the rapid installation of blasthole wells for rapid water removal in response to mining. Coordinating dewatering activities with blasting operations is critical, necessitating seamless integration to optimize both processes safely. The risk of water-related hazards, such as reduced explosive effectiveness and stability of blast holes, underscores the importance of continuous assessment and timely decision-making. Additionally, the need to optimize pumping rates for sustained drawdown and water capture area emphasizes the importance of cautious monitoring and adjustment.
[20] Furthermore, the short operational lifespan of blastholes demands agility in response to rapid operational changes in end-of-phase open pit mining, highlighting the dynamic and adaptive nature required for successful implement blasthole shallow dewatering. Addressing these challenges proactively through strategic planning communication and continuous monitoring is fundamental to ensuring the efficiency and effectiveness of shallow dewatering during the concluding phases of mining operations (Figure 7).