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Posted Feb 4, 2010, 6:44 PM
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Join Date: Jul 2001
Location: Vancouver
Posts: 41,814
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The Canada Line columns have spread footings with multiple pilings in an array that is intended to survive the Big One.
Here's an old post:
Quote:
From the Richmond News:
Quake won't topple Canada Line
By Nelson Bennett
Imagine you are riding the Canada Line rapid transit system when the earthquake hits.
It's a bad one - the same magnitude as the one that collapsed causeways and toppled buildings in Kobe, Japan in 1995.
You are on the Richmond segment, which is elevated five metres off the ground - ground that is prone to liquefaction during earthquakes.
You feel the train shake and sway, and you wonder if it will come off the tracks and plunge onto the cars below you on No. 3 Road.
The drivers of the cars below you wonder the same thing.
Roger Woodhead, engineer in charge of ensuring the Canada Line's design, says the swaying back and forth should be the worst thing that would happen during an earthquake.
"If a car did derail, it wouldn't go very far," said Woodhead.
The rail system is designed with concrete curbs to prevent cars from bouncing off the track, in the event they derail.
That's just one of the engineering methods used to mitigate the impact of a major tumbler, like the Kobe quake, which killed more than 5,000 people.
The earthquake damaged more than half of the city's bridges, collapsed elevated tracks for the famed Shinkansen fast train, and knocked over a huge span of the Hanshin Expressway - an elevated causeway built on concrete columns.
"I think the Kobe earthquake was a wakeup call," Woodhead said.
When they were given the task of designing the Canada Line, mitigating earthquake damage on the Richmond portion was one of the top things on the minds of engineers.
"It was one of the first things we looked at - what are we going to do as far as pilings in Richmond," Woodhead said. "There were 10 or 12 technical issues we had to deal with, and that was one of them."
Another challenge is boring a two-kilometre long tunnel underneath False Creek.
Richmond is an island built up over the millennia by silt deposited by the Fraser River. It has a subsurface three to 10 metres thick made up of water-saturated granular soils, says John Claque, a geologist with the Department of Earth Science at Simon Fraser University. On top of that is a layer of clay-like soil.
It's the subsurface of granular soil that liquefies during an earthquake.
"It's not like everything is going to slump into the sea," Claque said. "Not everything beneath Richmond is going to liquefy."
Liquefaction happens rapidly, but stops as soon as the ground stops shaking.
"It doesn't take very long - a minute or so," Claque said.
During liquefaction, the subsurface soils begin to flow, which can cause structural damage.
Much of the damage caused in Kobe was due to liquefaction.
Fortunately, the Kobe quake taught engineers some valuable lessons on mitigating the impact of earthquakes.
Roughly half of the Canada Line's 19 kilometres of track will be below ground, including a two-kilometre stretch running 35 metres deep beneath False Creek.
While the prospect of being trapped 35 metres down in a train during an earthquake might make the bravest person claustrophobic, it is probably one of the safer places to be, Canada Line officials say.
"Tunnels are very stable in an earthquake," Woodhead said. "You tend not to get very high forces underground."
Then again, the architects of the Shinkansen train in Japan were shocked to find parts of the train's tunnel collapsed.
One of the biggest challenges for Canada Line engineers was designing the North Arm bridge that will bring trains across the Fraser River from Vancouver to Richmond. The Middle Arm is a shorter span, so designing the bridge taking trains to the airport was not as challenging.
Below the silty soil that makes up Richmond is a sloping mountain of glacial till, which is relatively solid - solid enough to anchor the bridge with steel and concrete pilings.
On the Richmond side, engineers must go 45 metres down to anchor the pilings in till.
Building a bridge in sandy soil wasn't the only challenge. Because ships use the North Arm, engineers had to design the North Arm Bridge with only two piers, set 180 metres apart. Typically, they would have used more piers set closer together to give the bridge added strength.
"The challenge is to be able to build a bridge there and not interfere with the shipping channel," said InTransit public affairs vice-president Steve Crombie.
Having only two piers means they each must bear a heavier load. Moreover, because of the bridge's proximity to the airport, engineers were restricted in how high they could go with cable stay towers. These towers, like the ones on the Alex Fraser bridge, rise high above the bridge and strengthen it by applying upward pressure with steel cables.
They added strength to the bridge by running braided steel cables along its length.
"It holds it tight, basically, and takes the load," Woodhead said.
Half of the Canada Line will be elevated, coming out of the ground around 64th Avenue in Vancouver, and continuing above-ground for the entire Richmond segment, and for a portion of the airport segment.
There will be 250 concrete guideway columns, each one of which will have four to eight concrete and steel pilings. The pilings will be driven 12 metres down into what Woodhead describes as "fairly compact sand."
The tendency during an earthquake would be for the elevated causeway to bend in one direction, as the ground slips underneath. That slipping would pull pilings on one side upwards, and push the ones on the other side deeper into the ground.
To prevent that, Canada Line engineers came up with pilings that essentially have roots, like a tree. These "expanded base pilings" have a concrete bulb at the end.
This bulb - acting like a deeply buried root - would prevent the pilings on one side from being pulled up, and the ones on the other side from being pushed deeper.
The worst thing that could happen is that one of the concrete guideways could actually snap.
"I guess the column could break," Woodhead said, "but they're very heavily reinforced (with steel)."
As for the pilings themselves, there are doubly reinforced steel casings inside. They are also narrower than usual. Smaller diameters make it easier for liquefying soil to flow around the pilings, which decreases pressure on them.
This was discovered when computer modeling was done to test the design.
"Instead of this material pushing the piles over, it flows around them," Woodhead said.
Asked what the chances are that a train will be knocked off the elevated guideway during an earthquake, Woodhead said: "zero."
published on 12/15/2006
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. I think this in itself is partly contributing to suburban sprawl.
At that price, we would have been able to paint the trains properly, use the more expensive exterior sliding doors instead of pocket doors, nicer seat covers, a LIVE train map, LCD screens, more bars and handles, a faster and more powerful train motor, a better heater and air con....and you could probably add a lot more extras.
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