Stephen Rees's blog

Thoughts about the relationships between transport and the urban area it serves

Will shaky soils kill the bridge?

with one comment

Massey-Bridge-rendering

I am not an engineer or a geologist. But I do know that soil liquefaction is a huge problem for structures in earthquake prone areas, like the one we live in. When the shaking starts what seemed to be solid ground is actually waterlogged sands and similar material – the result of millennia of silt being deposited by the Fraser River as it slows on its way to the sea – starts to move. The damage to buildings in San Francisco in its famous quake was due to similar soil conditions. They still cause issues there: a high rise called Millennium has piles that do not reach bedrock and it is both sinking and leaning.

When the Massey crossing was first contemplated it was these soil conditions that caused the engineers to reject the idea of a bridge and chose a tunnel instead. Those conditions have not changed since. The Geological Survey of Canada in 1995 reported that bedrock is around 1,970 to 2,300 feet below where the new bridge is proposed. More recently  B.C. Ministry of Transportation and Infrastructure had two holes bored to 1,099 feet “without tagging bedrock” – not really a surprise since there was another 1,000 feet to go.

We know that Greater Vancouver is going to experience a major earthquake since there has not been a major shift in the tectonic plates since European settlement started, but there was apparently a “big one” which was recorded as a tsunami that hit Japan. These events are hard to predict with any accuracy but many seismologists think it is “overdue”. No-one has ever built a cable stayed bridge of this size in these kind of conditions. Indeed it is very hard to think of why anyone would propose taking such a risk – anyone who has the imagination to envisage what happens to two massive towers unsecured to bedrock but linked by cables and a bridge deck when the soil beneath them liquefies and shakes.

“I think people tend to focus on the Big One. If you’re looking at the statistics there’s a one in 10 chance that it will happen within the next 50 years. I think of those as fairly high odds. If we had a lottery with that kind of probability you’d probably buy a ticket,” she said.

The “she” quoted is Earthquake Canada seismologist Alison Bird

Ask yourself, as Premier Christy Clark wants you to buy a bridge, do you feel lucky?

Written by Stephen Rees

December 1, 2016 at 11:25 am

One Response

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  1. Wow. Thank you. Shared on facebook. Funny, I’ve heard the same thing about the north slope of Site C.

    Kim Poirier

    December 1, 2016 at 7:09 pm


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