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Shoring wall collapse at B.C. condo development raises questions

Engineering experts say a number of factors may have led to the dramatic collapse of a shoring wall at the development site of a high-rise condo in Coquitlam.

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A full investigation will likely take months, but engineering experts and others in the building industry say a number of factors may have led to the dramatic collapse of a shoring wall at the development site of a high-rise condo in Coquitlam.

Perry Adebar, a University of B.C. professor of structural engineering, said he spent Friday discussing the “dramatic video” that has been circulating on social media with his class and other structural and geotechnical engineers.

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“I’m a concrete structures expert … so when I saw the video, I thought, ‘Wow, that’s unbelievable.’ And then I talked to geotechnical engineers (who) are the ones that design these retaining walls and was surprised to learn that it’s quite common that they don’t put reinforcing steel in them,” said Adebar.

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The wall is designed to help protect the sides of an excavation while the building foundation is built in the hole.

Vancouver developer Amacon confirmed in a statement that a section of a shoring retention wall failed at its project on Foster Avenue and North Road, where it is building a 40-plus-storey condo tower.

This resulted “in soil falling into the excavated site and a cavity in the soil in the area adjacent to the property.”

There were no injuries.

Adebar said that with collapses like this, there are usually a number of contributing factors, rather than one single reason.

He said observers are focused on what might have triggered the collapse and why the wall crashed so quickly and spectacularly. The short video shows a crack that grows larger until a big piece of the wall breaks off and tumbles down, followed by a rush of soil until “large chunks of essentially plain concrete come falling off the wall,” he said.

Adebar said the wall was made of shotcrete, which is a form of concrete that is sprayed onto surfaces from a nozzle, as opposed to being poured into forms.

The “square dots or spaces on the shoring wall in the video are tiebacks where the concrete is supported back into the soil” from behind, he added.

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“When you look at the video, one thing all the engineers quickly noticed is that the tiebacks all seem to still be in position even when the soil has come out so you think, right off, that’s probably not what failed.”

development
Dirt is poured into an open pit foundation for a high-rise building under construction Friday to shore-up the walls after a cavern opened-up on the Northside of the foundation pouring tonnes of soil and rocks into the pit Wednesday. (Photo by Jason Payne/ PNG) Photo by Jason Payne /PNG

It seemed there was already damage on the wall at the beginning of the video and the contractor was able to get people out, he said.

He thinks the speed at which the wall crashes is a result of it not being reinforced concrete.

Defects in the shotcrete, such as insufficient thickness or poor consolidation in and around the tieback anchor plates, are an obvious first place to look for possible causes, he said.

“That’s got to be at the top of the list of potential causes,” he said. “Shotcrete being that it’s like this onsite-injected concrete, the variability is very high. If the person doing the job isn’t going a great job, it can very quickly not be good quality.”

Others added that a change in the environment, such as water underground that shifts its course of direction, can lead to a build-up of pressure that stresses the wall.

“It rarely happens, but it does happen,” said Anne McMullin, CEO of the Urban Development Institute, which represents the development and building industry across B.C.

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development
Dirt is poured into an open pit foundation for a high-rise building under construction Friday to shore-up the walls after a cavern opened-up on the Northside of the foundation pouring tonnes of soil and rocks into the pit Wednesday. (Photo by Jason Payne/ PNG) Photo by Jason Payne /PNG

Adebar, however, said in this case that the soil that crashed to the bottom of the excavation pit appeared in the video to be quite dry.

“That suggests there wasn’t a big buildup of water behind that caused it, unless it started fracturing a day ago and all the water leaked out. That’s not impossible. I think we really need to know: When did they first notice the problem? Because they vacated, maybe it was there for a while.”

An investigation will look at what the engineer specified for spacings between tiebacks and shotcrete thickness, and what was actually built, said Adebar.

Stepan Vdovine, Amacon’s vice-president of executive operations, confirmed in a statement Friday that all the work on the site was done with the proper permits and was approved and monitored by geotechnical and engineering consultants.

His statement added “no immediate infrastructure damage is apparent.”

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