Eliminating Drill and Blast Flyrock Hazards
Author: Alfred Tsang, Global Product Manager, Operations Technology
As Drill & Blast professionals, we oversee an incredible amount of hazardous processes on a mine site, all while trying to do so according to a busy production schedule. This puts an incredible amount of pressure on us to do everything on time, and most importantly, to do it safely. Despite the production pressures, it is incredibly important to never let safety come second. Of all the hazards that are involved in drilling and blasting, flyrock is one that can be highly unpredictable with a disastrous outcome.
Understanding the problem
The most common definition of flyrock is any rock fragment that has been projected from the blast, and lands outside of the blast exclusion zone. However, it can be argued that even rock projectiles that land at a considerable distance from the main blasted muckpile footprint are symptomatic of improper control. Sources of flyrock are most commonly from inadequate confinement of explosives along the free face, hole collar cratering, stemming ejection, and from specialised blasting situations like secondary blasting.
Drill and Blast Engineers, Designers, Drillers, QA/Blast Crews and Shotfirers all play a part in minimising the likelihood of flyrock hazards. To effectively control flyrock hazards, here are some things we must consider:
- Design assumptions
- Conformance to design
- Geology
Design Assumptions
A safe design considers:
- Depth of cover (distance from hole to nearest free face distance)
- Stemming quantity and quality
- Known or visible geological properties
- Site factors of safety
- Explosive type
To circle back to the earlier definition, flyrock is any rock fragment that has landed outside exclusion zones during blasting. So, what is your exclusion zone distance based on? Hopefully the answer is not just, “what we’ve always used”.
There are some common empirical models available for calculating predicted rock projectile distances. A common thread is that they are based on the depth of cover and explosive weight. The resultant calculated distance multiplied by an appropriate factor of safety should be a starting point for your exclusion zone (higher FOS for personnel exclusion vs equipment). While we do not necessarily calculate the required exclusion zone for every pattern, we should design the hole layouts with the depth of cover as a minimum to ensure the exclusion zone distance is still applicable. The appropriate depth of cover is also dependent on the size of the hole and the type of explosives used. Conceptually, the depth of cover describes the face burden distance or the stemming height. After each blast we reassess and adjust these values for each subsequent pattern as required. So how do we rapidly check the design holes are placed appropriately?
Built into the DataBlast suite of tools is a Burden Check feature that lets users rapidly assess holes that are designed without face burden distance.
This approach only considers a homogeneous rock mass in front of the face holes. Visual inspections should be a routine part of the D&B professionals pre-design considerations. These inspections are fundamental to highlighting irregularities, jointing and fractures on the apparent rock mass, read this article from Aron Muniz for more details (D&B best practices part two).
Conformance to design
The pattern design is correct, with holes placed with the right depth of cover based on the results of the DataBlast face burden check. However, even with a correct pattern design, the D&B team must ensure that the design is implemented within a tolerable level of variance. The levels of variance to design position/angle/bearing is something each site should study.
By this stage the holes have been drilled, you obviously cannot move the hole anymore, except if you decide to abandon and redrill. Are your assumptions, for example 5 m of free face cover, still correct? Could the drilling have placed it closer to the free face than you realise? Variances of even 0.5 m for the collar position can have large effects on the potential for increased flyrock distance. Moreover, angular deviations can dramatically change the resulting position of the drilled toe, leading to decreased burden or concentration of explosive energy at the face. If the drillers can’t drill the free-face holes as per plan, drilling 101 says to pull back away from the face and never to drill closer to the face. Are we training operators to understand the implications if they don’t?
With the integrated tools available in DataBlast, updating as-drilled hole positions is seamless. It is inarguably necessary to recheck the real depth of cover. A quick display is available in DataBlast to render an area of influence around each drilled hole. Overlaying the topographical scan then allows you to find protrusions or, insufficient depth of cover.
In addition, DataBlast will automatically update this quick display if there is as-loaded data of explosives. This allows users to check if loading of explosives has also altered the depth of cover assumptions, i.e. insufficient stemming. When this occurs, actions need to be taken to remedy the situation or prepare to push out exclusion zones.
Geology
Briefly mentioned before, D&B Engineers must also consider the structures of the geology. There are some common sources of information for this, visual inspections, free face scans and photogrammetry. Utilising face scans and photogrammetry is the modern method to gather structural data on your rock mass. This valuable information should be used to consider increasing depth of cover if necessary, based on the frequency of jointing and fractures. Is the geology team capable of providing D&B with this data?
When trying to understand the effect of geology on the potential to increase risk for flyrock, there is a logical concept to keep in mind. This is that explosive energy will want to travel the path of least resistance. So, are there geological structures evident in the free face that will allow explosive gases to cause face bursts? In these situations, even with the typical depth of cover, flyrock likelihood will increase. Consider increasing depth of cover in your design, use novel charging methods or increasing exclusion zones in around your blast.
While there are a few things to consider when minimising flyrock hazards during blasts, with the right procedures in place this should become second nature. Tools like DataBlast can help users to layout appropriate holes and check for the right design depth of cover and utilise real hole data to check for conformance, highlighting problem areas.
Aside from what’s been discussed, is there something you think is even more important?
