Brendon Lilly discusses drill, blast and the advantages of using a quality guidance system
Traditional approaches to extracting material from the earth have nearly always involved drilling holes in the ground, filling the holes with some type of explosive and blasting the ground to fracture it sufficiently enough to dig. This does not always produce the desired result and is considered one of the more expensive processes in a mine, especially in coal mines.
Some mines place little emphasis on generating an efficient drill and blast process, which can cause downstream complications. Several different areas contribute to blasting inefficiencies. Irregular hole placement can cause poor fragmentation that results in difficult digging for the loading equipment. Inaccurate depths can result in uneven benches. Holes that are over drilled into the coal seam result in dilution of material into the coal. A misunderstanding of how hard the ground is can lead to a poor choice of explosive product for a particular blast.
Many varying and unique approaches have been explored to improve this process while trying to reduce costs. Approaching this just for drill and blast alone yields some advantages. However, more frequently a broader strategy such as the ‘Mine to Mill’ approach used for hard rock mines is required to improve efficiencies across the mine as a whole.
Hexagon Mining looks at the entire mining process to identify areas where technology can be applied to improve coal production. The drill and blast process is one area where considerable benefits can be gained.
Firstly the design of the pit should be examined. How the material is to be moved and what equipment moves it, needs to be understood. A strip mine operation may utilise throw or cast blasts where the blast moves the material into the adjacent strip. This means the dragline or digging equipment does not have to move the material. The more efficiently this is performed, the less the digging machine has to dig and the faster the production equipment can recover the coal.
Coal seams vary in shape, size, angle and location. It’s difficult to dig to a coal seam that is on a steep angle. The coal horizon in this case may be too steep a grade for digging equipment to position itself correctly. Furthermore, coal seams that are near vertical provide additional challenges, as the seam will need to be exposed from the side. In all these cases, the design of the pit and consideration of how the equipment will extract the coal is a must. The design of the blast and hence the drill pattern is an important step in this process.
Simulation of the blast before it happens has become more common recently. Changes in drillhole placement and blast timing can be performed before the drilling operator drills the holes. This approach allows for design changes before a single hole is drilled. If a high-precision guidance solution, such as Leica J2drill, is used to drill the holes, as-drilled data can feed into blasting simulators to adjust the blast design further before a shot is fired. This compensates for any variation in the drill operations performed to the original plan.
However, varying the loading design at this stage is not best practice and generally not required. The use of a high-precision guidance system to achieve accurate hole placement and accurate depth in the first place, is the ultimate solution.
Once a pattern is designed, it is transferred to the guidance system either via a wireless radio system or manually on-board the machine. The operator is then presented with a virtual pattern on their display. Using a positioning technology such as GNSS (mixture of satellite based positioning technologies such as GPS, Glonass, BeiDou and Galileo), the guidance system is able to position the drilling rig over the hole accurately without any markers on the ground. This eliminates the need for surveyors to mark out the pattern, reducing the risk of injury and resulting in a reduction of light and heavy vehicle interaction on the mine site.
In some cases, the accuracy of a guidance system can be less than 50mm (2in). Angle sensors and sophisticated modelling algorithms compensate for any slope variations and predict how the machine will position over the hole even before it jacks up. For angled holes, especially for pre-split patterns, this is even more critical. During the design phase, the collar elevation of the hole is usually estimated or determined via a survey pickup. If this elevation differs significantly to what the drill surveys, then the X-Y position of the hole needs to be moved to the correct location in order to achieve the correct hole toe position. Drilling to the correct toe position is important for pre-split patterns as this defines the toe of the wall. If this difference in elevation is not catered for, the wall will not be in the right position or instability could result that then requires additional monitoring or control processes.
A quality guidance system, such as the Leica J2drill, will compensate for this difference and adjust the position of the hole so as to achieve the correct toe position. This difference can occur simply because, in the process of prepping the pattern, the dozer took more material than what had been planned. Drill and Blast engineers often compensate for this by performing a survey pickup just after the pattern has been prepped.
Using a high-precision guidance system means a survey pickup is not required. The guidance system performs the pickup in real-time and makes any necessary adjustments automatically, by providing the system with a toe position. This is known as drilling to elevation and the guidance system will recalculate the target or required travel depth to achieve the correct toe elevation. This is important when drilling to the correct horizon and avoiding drilling into the coal seam. Ideally, most coal mines want to have a small amount of hard toe by standing off the coal by a small amount. This protects the coal seam during the blast, avoiding dilution of the overburden material with the coal seam.
Drilling to elevation is also important when the blast design requires a level bench as a result. High-precision guidance systems can automatically compensate for different elevations across the pattern and again adjust the target or travel depth to achieve a constant elevation. Once blasted, this will result in a flat bench. The resulting bench does not need to be flat and other design parameters can be incorporated. A blast design can contain a sump that the guidance system will automatically recognise. The system will indicate a new target depth to drill to without the operator spending time ascertaining whether they are in the correct location or not. Additionally, it is possible to take into account other features, such as bench drainage, etc.
To improve the safety of the operator and avoid mishaps caused by pattern designs that do not fit correctly to the actual environment, warning or hazard zones can be placed into the blast design. This can then be virtually displayed to the operator on-board the equipment and an audible and visual alarm appear if the machine enters a predetermined warning zone.
Most drill guidance systems transfer their as-drilled data back to a central server in real-time. This allows the recorded data to be further analysed, trends established and the ability to respond to real time deviations to plans given. Firstly, the recorded data can be used to identify issues in the pattern that has not gone to plan. For example, holes that have not been drilled far enough and need to be re-drilled, or holes that have been over-drilled and require backfilling. Using a high-precision guidance system correctly allows the operator to do this in real time, getting it right the first time. Drilling operations then do not have to go back to address these issues after the pattern is completed.
The recorded data can then be used to adjust the loading or charge design of the blast. Adjustments to the loading of specific holes, or the timing or initiation sequences may be required as a result of holes drilled to a different depth or in a different location. In the case of the Leica J2drill solution, as-drilled information is stored in a database on a central server that can be retrieved by third-party systems as data becomes available.
Depending on the conditions of the hole (dry or wet) or the hardness of the ground, an appropriate explosive product will be selected to place into the holes. A different product may be required for different holes, or even down the same hole if hard bands exist in the strata. Technology contained in Leica J2drill can help here. Using measurement while drilling (MWD) techniques, the hardness down the hole can be determined and the data used to vary the explosive product in the hole, or pack areas in the hole that do not need to be blasted. The down-the-hole profiling data can also be used to detect coal seams to adjust the mine model, or to perform through-seam blasting. The system can also identify hard bands to change the loading design.
Once the blast design and relevant simulation activities are completed, load sheets for the blast holes can be created. These load sheets show the blast crew what material and how much material is to be added to the hole. The blast crew use a mobile manufacturing unit (MMU), ANFO or Explosives Truck to load the holes. This is performed to the design specified in the load sheets, which can also be displayed on board the machine virtually.
Blast consumables, such as explosive and stemming, are placed into the hole and the amount used can be recorded for that hole. This is called as-charged data. This can then be used later to reconcile the planned versus actual. For example, the number of detonators that are taken from the magazine for a blast can be reconciled against the number actually put into the pattern before firing a blast. Other benefits of these types of systems are the ability to track the drilling consumables. Drilling with a worn out bit will slow down operations and replacing consumables too early will result in lost time and additional replacement costs. Leica J2drill tracks the use of drill consumables on the machine so the mine can optimise the life of each component. Utilising systems that monitor the health of a mine’s equipment are paramount to adhere to budgets.
There are many opportunities to improve the efficiency of the drill and blast process using technology. By using high-precision guidance systems, drill and blast engineers are able to carry out tasks to plan, the first time.
In addition to ‘as-drilled’ and ‘as-charged’ data, other data can be reported from these systems. Leica J2drill automatically detects the activity changes during the hole, which minimises operator-to-screen interaction. This function, as well as the manually entering of reasons for stoppages, enables Leica J2drill to provide time utilisation data to improve practices in drilling operations. By analysing where delays occur, and by putting actions into place to address these occurrences, the drilling process can be further optimised. This too can be equally applied to the equipment involved in the blast process.
Brendon Lilly is manager of Product Management-Operations at Hexagon Mining.