May 18, 2009

Environmental Engineering

Size of Calgary sinkhole remains a mystery

RICHARD GILBERT

staff writer

The owner of a high-rise construction project in Calgary has been ordered to remove hoarding and buildings, tear up the sidewalk and remove surface material to determine the size and cause of a massive sinkhole.

City of Calgary engineers are working side by side with engineers hired by the owner of Pointe of View Developments.

They are doing visual inspections and are using ground penetrating radar (GPR) to determine how big the hole is.

“We have finished doing a vertical scan of the area around the site and the preliminary report is in,” said Kevin Griffiths Calgary’s manager of building regulations.

“There are areas that will need to be investigated further using GPR. The engineers will go through a process called truthing (finding out what happened) and there is a need to do more invasive types of investigation.”

A massive sinkhole was discovered on April 24 under the sidewalk adjacent to the Gateway-Midtown condo project, but it could not be seen because it is behind shoring.

The investigation should determine the size and cause of the chasm.

“We have had our engineer do an assessment and remediation plan on how to fix the sinkhole and his report is in,” said Griffiths. “We will order the owner to submit their own report and remediation plan, and then we will provide direction to remediate the site.”

Calgary ordered the owners to pinpoint the cause of the sinkhole under the road adjacent to an excavation site.

Under the Municipal Government Act Order, the owner was required to remove hoarding, a job shack above the hoarding and the sidewalk up to the curb.

“We removed all the surface materials and exposed the void from above to do visual checks,” said Griffiths.

“There was some sluffing (dirt falling back into the hole), so we have some sense of the depth, but it is not definitive. We again instructed the owner to continue opening and exposing another void running vertically on his side of the property to establish if it extends onto city property.”

Parts of a nearby street had to be ripped up to determine how big the sinkhole is.

Initially, it was reported that the crater was six metres wide, six metres long and six metres deep.

Due to the complexity of the situation, it has been more difficult than expected to understand the scope of the hole.

“Until we physically open up the hole, we don’t know how deep it is,” he explained.

“The actual void that was detected is slightly larger at the surface than we originally thought. There is another void below that and possibly another one. We have reason to believe it reaches the bottom of the excavation or 70 feet.”

The crater is now thought to be bigger than the original estimate.

“We are working with the owner to establish if there are other voids extending down the south side of the property,” said Griffiths.

“One of the soldier piles that forms part of the shoring system has been compromised. There is no risk of failure because there is nothing on it.”

The city will continue to work with the owner to make recommendations that will ensure the engineering safety of the excavation and the adjacent roads.

The owner is on board with finding out what happened.

“We have submitted a report to the city with the remediation plan. The city will review and advise if they are in agreement,” said Brian Stoddard, president and CEO of Pointe of View Developments.

He said his remediation plan will use gravel, compacted through vibration, to fill the hole.

Two lanes of the nearby street were soon reopened after it was determined that the huge sinkhole didn’t extend under the road.

“Investigation and engineering reports have determined that the sinkhole is isolated to an area immediately adjacent to the construction site up to and including the sidewalk but does not extend beneath the surface of the road,” said a Calgary press release.

However, two lanes will remain closed to maintain a safe distance from the hole.

The excavation site was abandoned in August, after the foundation hole had been excavated.

The previous owner, Resiance, said the project was no longer financially feasible.

According to Stoddard, construction on the six-storey parkade for two high-rise towers resumed about three weeks ago.

The 550-unit development will have a three-storey podium connecting the two towers, with commercial space on the first floor.

Crews are erecting the crane and preparations are being made to pour the first slab of the parkade, which should be completed and reach ground level in nine months.

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