I have started a series of Drill and Blast Best Practices, beginning with my previous article about Bench Preparation and how an often-neglected step can greatly improve the safety and quality of a blast, if well performed. In this article, I will talk about some important variables related to the drill pattern design. But before that, there is another step that should also be discussed, the bench assessment. This includes field inspections and file record analysis.
A good drill and blast pattern design does not start on a blank sheet over a desk, on a computer, nor even inside an office. It begins with a good bench assessment out on the field. Drill and blast engineers and supervisors must visually evaluate factors such as the general conditions of the crest and toe, presence and dip of fractures and joints, loose blocks, humidity, signs of surface or underground water passages, intrusion of materials of different lithologies, stability conditions, among others.
An efficient drill pattern design should start with this sort of evaluation, as it can directly affect the quality of the fragmentation and the stability of the resulting bench. However, and more importantly, identifying these factors is critical because they all represent potential safety hazards for drill and blast teams. We should always bear in mind that the drilling process induces vibrations in the rock mass. This can cause landslides which represent a huge risk for drilling crews and rigs. Drilling also usually happens during night shifts when it is much more difficult to see, understand and realize everything happening in the surroundings.
Bench Historical Records
The analysis of the historical data of the bench is another source of identification for these factors. Mineral deposits vary substantially, therefore, each mining block is unique. But the analysis of blasts performed in previous blocks close to the current one can provide useful information about the structural and geological conditions of the bench and its surrounding rock mass.
After the field assessments are complete, this is the moment when the office work truly starts. But the analysis of previous blasts of the same bench is not limited to the factors listed so far. This procedure can show us what was done right or wrong in former operations, to ideally avoid repeating past mistakes and, also to help us with outcome predictions. We want to know what was done and what were the results. In order to have this, it is fundamental to have all the blast related records archived together. The drill and blast design parameters and results in terms of fragmentation, vibration, dilution, and environment, will form what I call the ‘Bench Database’.
Gathering all this information to build Bench Databases can turn into a nightmare when we have multiple platforms doing each part of the drill and blast process individually. Often the drilling project is done by a person with specific software, while the explosive charging and timing plans are made by a contractor through another platform that does not even interface with the first one. Additionally, fragmentation results, vibration monitoring analysis, pile formation, dilution reconciliation reports, can also be stored elsewhere by other people. Even photos and video recordings are important at this stage. Having a software solution that is capable to perform and store data at each step of the workflow, from the design stage until the results stage, is highly advantageous. Even more so if it is a multi-user platform, as it can then also be used by all involved personnel. This is an essential part of a proper drill and blast management.
Another fundamental parameter for a good drill pattern project that deserves acknowledgement, is topography. Having comprehensive, reliable, and updated data from the surveying team and being able to design the drilling over it, is very important. It is common to see patterns being defined over flat surfaces or outdated survey positions. This may be an alternative when information is simply not available but it is clearly not the most efficient. Bench surfaces are irregular, period. The reference level across a pattern can vary from a couple of centimeters to even meters. And this must be considered for the hole depths calculation, otherwise, the resulting toe will mirror the irregularities. This, in turn, can compromise digging rates and, ultimately, lead to poor road conditions and decrease overall mine productivity. A proper design of the excavation or target toe surface is also indispensable to ensure good results. It should follow the pit’s mine plan and consider the rainwater drainage if applicable.
Surveying done with drones or laser scanners can also provide faces’ information, which can be used by drill and blast engineers to evaluate the real burden of the holes in the first row. A proper burden check will help them place holes within the minimum and maximum distance limits. Holes too close to the face may lead to flying rocks, while holes too far can compromise fragmentation and increase vibration levels.
Drill Pattern Design and Results Simulation
All of this so far is just a part of creating an effective drill pattern design. Other fundamentals are the analysis of the bit diameters available for drilling, pattern geometry calculation, determination of angles, azimuths, and sub-drillings, as well as calculating the final depths of the holes to be executed.
Previous grade control modelling results are also important to be considered because different materials may have varying Blastability Indexes. This requires the application of different powder factors and thus varied pattern geometries or drilling diameters may be used. This happens very often, especially for metalliferous ores, and will also impact the later stage of timing design, when dilution needs to be minimized for a better selective mining.
We know from the Project Management Theory that several unforeseen things may and do happen during the execution of any project. It is not different when it comes to drilling in mining. In this way, it is very likely that some as-built holes’ collars, depths, angles, and azimuths will divert from the project. Nevertheless, it is recommendable to evaluate and simulate some parameters prior to the execution of the drilling – even when only design data is available. The simulated calculation of the explosive charges, as well as the study of the timing sequence between each drill hole, are just some of them. Other examples are the analysis of timing contour lines and maximum instantaneous charges or decks per delay. The fragmentation and effective control of vibrations induced by the blast depend directly on these variables. Although preliminary at this stage, these simulations remain important to ensure that the drilling design is on track.
Once all these steps have been properly completed, the project can be considered ready to be approved. The necessary information and instructions for the drilling must then be generated and forwarded to the field teams. This itself brings a whole new set of best practices that will be addressed in the next article.