Sinkhole due to old construction pit, says RTG
By MATTHEW PEARSON, OTTAWA CITIZEN March 21, 2014
The cave-in near Sandy Hill last month is being blamed on an old construction pit the builders of the city’s rail system didn’t know about, but the incident hasn’t shaken the confidence of the top bureaucrat overseeing the project.
Deputy city manager Nancy Schepers says the Rideau Transit Group’s quick response to the Feb. 20 incident — which included containing the site, filling the hole to prevent secondary damage, restoring water service to affected residents nearby and contacting the necessary city departments — actually reassured her.
“If anything, this makes me feel more confident because they had an incident and their processes worked. We all have processes in terms of, ‘how do you respond to an incident?’ and we really don’t want to test them.”
“There was a test and, quite frankly, they did it perfectly,” Schepers said.
Her comments come a month after a section of road about eight metres by eight metres collapsed just south of Laurier. Waller is closed there for the rail project and isn’t travelled, though the city closed the nearby intersection briefly while the cause of the collapse was investigated. Nobody was injured.
Leaking pipes, typically water mains, are the most frequent cause of sinkholes, as slow trickles of water wear away at underground soil and then the surface falls in all at once.
But the only pipe running through the area is a small sewer and there was no indication that there was anything wrong with it until the collapsing ground broke it.
Now, according to a report sent to city council Friday, RTG says its investigation of the cave-in found that the “tunnel crossed beneath a previously excavated construction pit” that was underneath the known city utilities, and likely extended from bedrock to the surface.
The shape of the old pit closely matched the shape of the six-metre deep sinkhole that opened up, and material from the pit turned out to be “indicative of poor quality, un-compacted backfill,” the report said.
Although it’s possible that similar surprises might arise in the future, the report said the city carried out an extensive geotechnical investigation before awarding the construction contact and RTG conducted further testing once it was selected to build the $2.1-billion LRT project.
The Waller sinkhole is considered a localized anomaly that wasn’t caught by the borehole investigations. RTG had drilled a borehole within 15 metres of the sinkhole, and along the underground ramp leading to the tunnel, and the soil conditions in those locations were consistent.
“There was no indication that anything was going to change,” said Peter Lauch, RTG’s technical director.
Lauch said RTG is reviewing its borehole logs and drawings to see where any other such anomaly could potentially occur and might, in some instances, drill additional boreholes, such as those recently completed on Laurier Avenue, to confirm the strength of the ground.
“This was an anomaly that couldn’t have been predicted unless you happen to have a borehole right at that location,” Schepers said, adding it would be pure speculation to guess why that underground pit was there in the first place.
“Who knows when that happened. We certainly weren’t aware of it, and clearly RTG was also not aware of it,” she said.
While tunnelling work on at the eastern portal of the LRT tunnel has been held up now for four weeks, work on the western and central shafts has continued. In fact, it’s ahead of schedule, with tunnelling equipment on the western portal soon to reach the future location of Lyon station, the westernmost stop downtown.
Tunnelling at the eastern end began again on Friday and will soon reach bedrock, which is a much more predictable material.
Lauch said a “sequential excavation method” is being used to do the tunnelling work. “You move forward every day and you assess on a daily basis what transpired with that tunnelling to make sure it’s consistent with what you assume,” he explained.
Unlike what it has done previously, RTG will dig out the bottom layer of the tunnel underneath Laurier Avenue first and then go over it a second time to dig out the top layer, making sure to leave at least two metres of bedrock between the top of the tunnel and the surface of the ground.
Cost over-runs caused by the delay are still being compiled, but Lauch said the incident — as well as damage to city utilities — is covered by insurance.
The incident did not cause damage to nearby buildings, he said.
Before work began, RTG installed specialized vibration monitors along the line and has so far received no negative results, meaning none of the monitors has sent out a warning that the level of movement exceeds a previously-determined value.
Expected to be completed by spring of 2018, the Confederation Line will run from Tunney’s Pasture station in the west to Blair Road in the east. Thirteen stations will dot the 12.5-kilometre route, including three located underground in the new, 2.5-km downtown tunnel.
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