The safety of our surrounding communities is of the utmost importance to Lafarge. Our neighbours and employees expect there to be limited vibrations felt by the community from blasting operations. With each blast, we purposefully aim to be well below threshold values to make sure this is the case.
The blasting being currently undertaken is key in the development of the long-term haul road. We are trying to recess it behind a ridgeline in order to reduce visual, dust and noise impacts on the community. Once the work is done in this corner, it is completely finished for the life of the quarry.
Since the blast on August 5 was felt more strongly than usual, we are exploring the vibrations that occurred in the communities of Exshaw and Lac Des Arcs. We want to provide residents with updated information, including seismographic data. As many of you know, we have our seismograph at the Exshaw water treatment plant that records all ground vibration and air overpressure. It is a sensitive monitor that gives us results within two hours of each blast. Below, you will find data that we have reviewed and an explanation of its relevance.
Peak Particle Velocity (PPV)
Peak Particle Velocity is the highest instantaneous vibration that will be felt as the result of a blast and is measured in millimeters per second. The result of the blast as recorded at the Exshaw Water Treatment Plant on August 5 was a PPV of 11.1 mm/s longitudinal. The below chart is the actual vibration data recorded showing instantaneous points for transverse, vertical and longitudinal vibration.
The chart is based on the United States Bureau of Mines (USBM) Report of Investigation 8507 – Structure Response and Damage Produced by Ground Vibration From Surface Mine Blasting. This is the widely-accepted industry standard for PPV and is what regulatory limits are usually based on.
In a previous posting, I mentioned that we had fired 3 blasts adjacent to the one fired on August 5 in the last few months. The PPV results for these blasts were:
- July 23: no register (<1.75mm/s)
- July 3: 2.91mm/s
- June 10: 2.76mm/s.
In the previous 18 months that I have been with Lafarge Exshaw Cement Plant, the highest reading was 4.19mm/s. This was also recorded on this ridge.
For reference, some guideline limits for similar industries are as follows. It is important to note that these are conservative limits and no damage has ever been scientifically observed below these points. It is also important to note that these are only guidelines; the only regulated values are the NPC-119 of Ontario and the Canmore Bylaw.
- Canmore Bylaw 32-96: PPV 25mm/s at any structure
- USBM RI8507: PPV 19mm/s
- Environment Canada Environmental Code of Practice for Metal Mines: PPV 12.5mm/s
- Best Practice Guide for Urban Blasting Operations, Western Canada Chapter of the International Society of Explosives Engineers for Level 4 Urban Blasting, PPV as determined by USBM Chart. Usually taken at 19mm/s
- NPC-119 Ontario – PPV 12.5mm/s.
In our last update, I referred to the results being well within the safe limits. These are the limits that I am referring to. We do not have a regulated maximum for PPV, however we run under a self-imposed design target of 2 mm/s, with a maximum of 5mm/s. This is an ambitious target that is stricter than most regulations. We put in these measures because it is extremely important for us to control the blasting as much as reasonably possible for the safety and comfort of our neighbors. This goal also gives us an added safety buffer should the vibration be higher than predicted.
While our designed safety factor meant we did not exceed the widely accepted guidelines, I regret that this blast exceeded our usual targets and caused concern within the community. We had no reason to believe that this blast would be any different than expected and I will be examining the mountain closely as it is excavated to identify any geological changes that could have contributed to the increased propagation of vibration so we can make corrections moving forward.
The airblast overpressure is a linear measurement of the pressure waves that travel in the atmosphere as a result of a blast. It is measured in dB(L). The result of the blast, recorded at the Exshaw Water Treatment Plant, was 125.7 dB(L) at a frequency of 3.9Hz. This data is shown in the information below.
Similar to PPV, we do not have a regulated limit for airblast overpressure. The standard that we use for airblast is the USBM RI8485 – Structure Response and Damage Produced by airblast From Surface Mining. This standard states that safe levels of airblast were found to be 134 dB(L) at 0.1Hz, 133dBL at 2Hz and 129 dB(L) at 6Hz. If we extrapolate this linearly, it is clear that we were below safe levels with this blast. These limits are backed by the Best Practice Guide for Urban Blasting Operations, Western Canada Chapter of the International Society of Explosives Engineers for Level 4 Urban Blasting, which recommends a limit of 134 dB(L).
There have been several instances in previous blasts of very similar and slightly higher recorded airblast overpressure at the Exshaw Water Treatment Plant. Airblast is also greatly affected by a low cloud ceiling as the pressure waves cannot disperse. Since August 5 was a clear day and the airblast overpressure reading was in line with our previous results, I can be certain that the experience of the Exshaw and Lac Des Arcs communities was primarily as a result of ground vibration, not airblast overpressure.
I want to reinforce that at no point was this blast uncontrolled. The blasted rock did what was expected and all rock fell within our exclusion zone and mining lease.
Every shot is designed with a delay pattern in order to reduce the Maximum Instantaneous Charge (MIC), which is the greatest amount of explosives going off at one time. If all holes went off together, the vibration waves would be compounded and the result would be a large ground vibration. I have double checked and all holes fired at individual times, as was designed. This means that the vibration felt was most likely the result of the instantaneous charge of one hole. I have heard concerns that the blast was too large; however this outcome would have been possible with one hole or one thousand holes.
Conclusion and Future
I hope that this information eases some of the concerns residents have rightfully expressed. Please be assured that myself and the blasting crew take their jobs very seriously and do not take matters such as this lightly. We will be looking into every element over the coming weeks to ensure we have a solid foundation to move forward and we will continue to share updates with you. We must always learn from these events. I have learnt that the communication to the communities on the location and proximity of blasting needs to be improved and we will be looking into ways of doing this. I am happy to take suggestions from the community as to how you would like to see this happen.
If you would like to receive email updates on blasting operations, please contact firstname.lastname@example.org. I am available to be contacted through Janet Brygger should you have any more questions. I am always happy to talk about all things blasting.
Lafarge Exshaw Cement Plant