By Sue Stefko
On the morning of November 2, a strong blast shook homes near the building site of The Clemow retirement residence at 275 Carling. The blasting was part of the excavation for the 5-storey underground parking garage in the 16-storey development. After feeling the intense vibrations, residents from Dow’s Lake, the Glebe Annex and the Glebe began contacting Councillor Shawn Menard’s office to voice their concerns. Menard acted immediately, contacting city staff, the Katasa Group and BurMont construction, trying to resolve the issue.
That blast was 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 so future blasts remain below those warning levels.
Although the size of the blast and vibrations depend on the amount of explosives used, it is more complicated than that – it also depends on the size of the pre-drilled hole the explosives are placed in before detonation, the depth they are placed at, any holes or seams in surrounding rock, the rock density and the blast’s relation to neighbouring properties. All this is calculated before setting 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 so far, only three have triggered warnings.
However, the blasts continue to concern neighbours. Some have expressed worries about their foundations – both from the current work and from cumulative impacts from the multiple developments in the area. Andrew Campbell from Explotech Engineering Ltd., a third-party blasting consulting firm which is monitoring compliance with regulations throughout the process, believes damage to foundations should not be an issue. The levels of vibrations allowed by blasting regulations are based on in-depth, long-term studies, which included near-continuous blasting for over 50 years near a variety of homes 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 be seen before in weaker components of the home, like plaster or drywall, before stronger components, like the foundation. To be able to monitor possible changes to nearby foundations, each building within 150 metres of the site had pre-blast inspections, including photographs and videos to help in documenting any issues from the blasting.
A number of neighbours very close to the site are concerned about rock debris damaging their windows. While the use of large rubber mats minimizes the possibility of projectile rock, Sean Montgomery from BurMont Construction, the manager for the project, says anyone worried about damage to their property should contact Katasa to file a complaint to make sure concerns are addressed.
Some want to see blasting stopped. However, in the absence of blasting, since the site is entirely bedrock (past the first metre or so of soil), other methods such as hoe-ramming would have to be used to extract the rock. This would extend the process until about the end of 2022 instead of next February as currently planned. It would also have a disproportionate impact, with neighbours closest to the site having to deal with near constant jackhammering from several hoe rams onsite. This would obviously benefit those who are too far away to hear the jackhammer as they would not be impacted by the vibrations of the blasts. However, it would be very difficult for those nearest the site, as those who experienced the hoe-ramming for the John Howard Society build at 289 Carling can attest.
As the digging progresses (as of this writing, two of the five floors have been excavated), we expect the level of vibrations will stay the same, but the noise and air pressure from the blast itself will be reduced 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, with construction scheduled to finish in spring 2024.
Sue Stefko is president of the Glebe Annex Community Association.