The big blue bridge you use to cross the Peace River in Taylor, BC when you travel the Alaska Highway is not the original, it's actually the replacement.
My Dad operated heavy equipment during the building of the Alaska Highway in 1942, long before I was born.
I remember driving north with him and when we crossed over the long Peace River Bridge he told me that the original had collapsed in 1957. Dad said a landslide caused it.
I did a bit of digging and found a great article by the Association of Professional Engineers and Geoscientists of BC.
The original wooden suspension type Peace Bridge was built in 1942 as an essential piece of the Alaska Highway and was considered one of Canada's great bridges at the time of it's opening. It was the longest on the whole highway.
In 1957, a truck driver noticed unusual settlement in the road so the army (responsible) immediately closed it. Over the course of the next day, the bridge structures continued to shift and just after noon on October 16, 1957, the bridge collapsed into the Peace River.
They determined that the bridge failed due to a landslide in the shale bedrock that it was built on. Shale is simply mud that has hardened over millions of years... from the time when dinosaurs walked the earth.
The problem with shale is that once it's exposed again to water, it will eventually turns back to mud.
It's believed that the bridge foundation likely began to deteriorate soon after it was constructed and continued until the time of the collapse.
A portable Bailey bridge was erected for immediate use while the Train Bridge was planked for traffic until the new bridge was built and completed in 1960.
Now, when you cross the big blue bridge, you will notice a strange feel and sound. Don't worry, it's the steel deck. It's a unique sensation and no matter how many times you drive across it, you will notice it. We all do.