A weaker than usual seam of rock along the edge of the quarry is thought to have contributed to vibrations felt in the community on August 5, 2020. Excavation unveiled a small fractured seam amongst good rock. Vibrations can travel freely through this type of fracture. Almost all of this seam has now been removed with an excavation, limiting future issues.
Following the blast, seismographic data, including ground vibration and air blast measurements recorded on the monitor at the Exshaw Water Treatment Plant, were reviewed and compared to previous blasts. While the blast was well within safe limits for structural and personal safety, it was higher than previously experienced readings in the area.
Data showed ground vibration, not air blast overpressure, was the source. Peak Particle Velocity is the highest instantaneous vibration that will be felt from a blast and is measured in millimeters per second. The result on August 5 was a PPV of 11.1 mm/s; previous blasts in that area were all under 3 mm/s. The nearest regulation for reference is a Canmore Bylaw that states a maximum of 25mm/s at any structure.
“There has been one blast in the area since. We positioned it further to the west to ensure the energy had a chance to disperse. We recorded 2.61mm/s vibration. This is in line with previous experiences in the area and we intend to use this technique for all blasts in this zone moving forward,” said Dylan Treadwell, Quarry Manager.
At no point was this blast uncontrolled. The rock did what was expected; falling within the mining lease.
“This blasting is key in the development of the long-term haul road. Once the work is done in this corner, it is finished for the life of the quarry,” Dylan said. “I regret that we had a blast that exceeded our usual predictions and caused concern within the community. We had no reason to believe that it would be any different than expected.”