The train is coming: City says LRT tunnel excavation on schedule
Matthew Pearson, Ottawa Citizen
Published on: April 3, 2016 | Last Updated: April 3, 2016 1:07 PM EDT
Unless you’re living on top of it, the work going on beneath the downtown core on Ottawa’s Confederation line LRT tunnel could easily go unnoticed.
Much of what’s happening, as machines mine a 2.5-kilometre tunnel 15 metres underground, remains far from the view of a curious public.
Yet, as Matthew Pearson reports, there’s lots going on down there.
There are three downtown stations planned for the LRT. Excavation at the Lyon station was complete some months ago. Work is nearly done at Parliament station, but progress at Rideau station continues to inch along.
Why is the Rideau station excavation taking longer?
It was to be expected, said Gary Craig, the city’s head of LRT design and construction.
“There’s nothing that’s happened to date that would cause us any concerns at all with respect to the work in Rideau station,” he told Postmedia.
Digging in that area’s soft ground calls for a different tunnel design, different temporary support structures and even the hiring of a special adviser.
“That was the station where the lower productions were expected to happen,” said Humberto Ferrer, the deputy project director for OLRT Constructors, the group that is building the $2.1-billion Confederation line, which opens in 2018.
“Lower productions” is Ferrer’s way of saying it would be slower, and that engineers like him knew as much at the outset.
“It seems that it is taking longer, but we started much, much later at Rideau,” he added.
About six months later, in December 2014, to be specific, and the station cavern itself, once completed, will be larger in length and volume than the other two downtown stations, he explained. Rideau will be about 50,000 cubic metres (compared to 38,000 cubic metres), while the cavern — the entire area that must be dug out in order to build the station — will be 180 metres long (compared to 165 metres).
And yet, Ferrer expects the Rideau cavern to be completed in a total of 18 months, while Lyon and Parliament each took 20 months from start to finish.
What are the different techniques for hard-rock and soft-ground excavation?
Much of the tunnel excavation must go through limestone, but a 110-metre stretch of soft clay and sand near the future Rideau station requires a different technique.
In rock, crews use a 135-tonne roadheader, a 20-metre-long machine with a spiked fist at the front that punches away at the rock, to hollow out the upper sides of the cavern, followed by the middle section, then the bottom. The upper portions, which are called drifts, take longer because workers also have to install an umbrella, a temporary, cone-shaped support structure comprised of steel pipes that hold the tunnel up. It’s about 18 to 20 metres wide from side to side.
In clay and sand, crews use a smaller excavator with an arm that rotates, so the bucket can scoop like a traditional excavator but also twist to dig out the sides of the cavern. They complete one side of the cavern, reinforce it temporarily with a layer of concrete, then move to the other and finish by clearing out the middle and bottom.
Openings in soft-ground excavation are about one-fifth the size of openings in rock because the excavator takes smaller bites and more reinforcements are required in soft ground, while the roadheader just smashes the rock. This helps explain why progress is somewhat slower in soft-ground than it is in bedrock.
Long before putting the LRT project up for tender, the city did its homework, Craig said. It paid for geotechnical work to understand the challenges posed by the formation known as the Rideau Valley and provided that information in advance to interested bidders.
“We wanted to understand what we were potentially facing but, more importantly, we wanted the proponents to understand, so that when they were pricing it out, they had good information, they didn’t have make assumptions about what the conditions might be,” Craig said.
The soil encountered so far is “pretty much what we expected,” Ferrer said, adding OLRT conducted further geotechnical work once it was awarded the contract.
What else is happening underground?
In Lyon station, crews are busy pouring concrete for the walls and floor of the cavern, upon which the tracks will eventually be laid. In the tunnel between Lyon and Parliament stations, work on the final concrete lining of the tunnel arch is ongoing.
In Parliament station, crews are nearly finished digging out the station cavern, while excavation to the east has reached as far as the National War Memorial and will soon hit the Rideau Canal.
At Rideau station, the focus is on digging out the middle, which is called the central pillar.
Work at either ends of the tunnel (from an entrance near LeBreton Flats to Lyon station in the west and from an entrance at the University of Ottawa to the future home of Rideau station in the east) is finished.
What happens when the tunnel goes under the canal?
Surprisingly little, actually.
That’s because the tunnel will be nowhere near the Rideau Canal, a UNESCO world heritage site. There will be 20 metres of rock between the bottom of the canal and the top of the tunnel when a roadheader starts rumbling below the waterway in April.
Ferrer expects it will take seven to 10 days to pass under the width of the canal as workers dig toward Rideau station.
What’s the timeline?
Ferrer and Craig said crews will hopefully finish pouring concrete in the sections of tunnel between stations and on the lower half of stations by the end of this year, so that track installation and work on the mechanical systems can begin over the winter and into next spring.
Station construction at Lyon will begin in July, followed by Rideau in September and Parliament in October.
With files from Jon Willing
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That plan doesn't seem to jive with the rendering, it shows the track partially covered
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