By Sue Stefko
(Appeared in the Glebe Report, December 2021)
On the morning of November 2nd, a strong blast shook homes near the building site of The Clemow retirement residence at 275 Carling. The blasting was occurring as part of the excavation for the 5-storey underground parking garage that is part of the 16-storey development. On feeling the intense vibrations, residents from Dow’s Lake, the Glebe Annex and the Glebe began contacting Councillor Menard’s office to voice their concerns. Councillor Menard took action immediately, contacting city staff, the Katasa Group, and BurMont construction, trying to resolve the issue.
That blast was indeed strong enough to trigger an instantaneous warning message from the vibration monitors surrounding the construction site. While those blasts still fell into what is considered permissible by the city, the construction crew modified their practices in order that future blasts remain below those warning levels.
Although the size of the blast/vibrations depends on the amount of explosive used, it is more complicated than that, as it also depends on the size of the pre-drilled hole it’s placed inside prior to detonation, the depth that it is placed at, any holes or seams in surrounding rock, the rock density, the blast’s relation to neighbouring properties, etc. All this is calculated when considering how to set up the charge, and sometimes the crew doesn’t get it right. But, all things considered, the majority of blasts do go according to plan – of the more than 110 blasts that took place so far, only three of them triggered warnings.
However, some of the blasts continue to concern neighbours. Some have expressed worries about their foundations – both from the work currently taking place, as well as from cumulative impacts from the multiple developments in the area. Andrew Campbell from Explotech Engineering Ltd., a third-party blasting consultant which is monitoring compliance with regulations throughout the process, explained that this is not the case. The levels of vibrations allowed by the blasting regulations are based off in-depth and long-term studies, which included near-continuous blasting for over 50 years, and took place near a variety of home and foundations of different ages. This means that if the construction crew follows the regulations, there should not be damage to any nearby structures. He added that if there was damage, it would first be seen in the weaker components of the home, such as plaster or drywall, before stronger components like the foundation would be affected. To be able to monitor any possible changes to nearby foundations, each building within 150 metres of the site had pre-blast inspections, including photographs and videos to document any potential issues arising from the blasting.
A number of neighbours very close to the site are concerned about rock debris damaging their windows, etc. While the use of large rubber mats minimizes the possibility of projectile rock, Sean Montgomery from BurMont Construction, the construction manager for the project, added that anyone worried about damage to their property should contact Katasa to file a complaint to make sure any concerns are addressed.
Some want to see blasting stopped. However, in the absence of blasting, since the site is entirely bedrock (at least past the first metre or so of soil), other methods, such as hoe-ramming, would be used to extract the rock. This would extend the process until about the end of 2022, instead of February, as currently planned. It would also have a disproportionate impact, with neighbours closest to the site having to deal with near constant jack-hammering from several hoe rams onsite. (This would obviously benefit those who are too far away to hear the jack-hammering, as they would not be impacted by the vibrations of the blasts. However, it would be very difficult to live through for those nearest the site, as those who experienced the hoe-ramming for the John Howard Society build at 289 Carling can attest to.)
As the digging progresses (as of the time of writing, two of the five floors have been excavated so far), we expect that the level of vibrations will stay the same, but the noise and air pressure from the blast itself will be minimized as the blasts take place further and further underground. After the excavation wraps up this winter, the rest of the project is expected to take another two years – to spring 2024.