Digging Deep into Road Structure

Perceptions by Gerry Warner

Bet you’ve never heard of glaciolacustrine or “GLU” as it’s sometimes known? Maybe it’s time you did hear about it, because living in Cranbrook, you walk on it or drive on it almost every day.

Still guessing? How about “glacial till?” That’s the more common name for the substance sharing a major portion of the blame for the catastrophic collapse last August of the Mount Polley tailings pond dam that dumped millions of cubic metres of water and toxic tailings into the previously pristine waters of Quesnel Lake.

But what has that got to do with Cranbrook? Actually, quite a lot.

The report released by the Mount Polley Review Panel last week said the dam collapsed because of a major engineering design flaw compounded by the fact that the dam was constructed on GLU, or glacial till as most of Cranbrook sits on. And as surely as the experts say the glacial till played a major role in the dam’s collapse that same till is largely responsible for the many drainage problems plaguing Cranbrook that lead to the abysmal condition of our roads and sidewalks not to mention flooded basements, cracked house foundations and collapsed retaining walls.

Glacial till is composed of ground up rock, fine sediments and clay left behind after the glaciers retreat and is almost impenetrable to water, which is what caused many of the problems at Mount Polly and here in Cranbrook. In short, water has great difficulty draining through glacial till so it pools on top of it and over the seasonal freeze and thaw cycle causes the ground above it to heave and subside which causes roads and sidewalks to buckle and dams to collapse as was the case at Mount Polley.

This presents a tremendous construction problem because nothing can be done to stop the natural freeze/thaw cycle which makes building on soils underlain by glacial till a mostly futile proposition because anything built on it is bound to fail in a fairly short time as the ground heaves and subsides underneath it. It took 20 years for engineers to find this out at Mount Polley and Lord only knows we’ve seen this for the last century or so in Cranbrook.

However, roads, sidewalks and house foundations built on glacial till aren’t nearly as problematic as huge tailings pond dams because they’re relatively small in comparison. In the case of municipal infrastructure, the standard construction technique for dealing with the problem is relatively simple – remove the glacial till and replace it with a standard subgrade such as gravel that water can drain through and stop the ground from heaving.

I have seen this done first-hand when the City redid the street in front of my house two summers ago and did it the right way, namely going eight feet down and removing all the glacial till, which you can see in the accompanying pictures and replacing it with a gravel subgrade and then laying the pavement down on top of that. “Good for 30 years,” the construction contractor told me and I don’t doubt it although I don’t think I’ll live long enough to see it. So why have successive City Councils done this so seldom in the past? You know the answer – cost. In the case of 2A St. S, more than $600,000 for three short blocks! That eats an awfully big hole out of the City’s annual roads budget of $3 million or so.

gravel  subgrade,ready to be paved

going deep to do it right

soil profile showing glacial till

The reason I bring this up is the new Council is doing its budget deliberations for the 2015 – 2016 fiscal year and there’s been lots of talk about potholes and the sub-standard condition of our roads. Much of the debate has centred on “mill and fill” where the worn-out pavement is ground up and replaced by new pavement laid down on top of the old road bed without removing and replacing the glacial till underneath. That’s the cheap way to go, but it’s throwing good money after bad.  Therefore, I’m, hoping the new Council doesn’t go this way because the result is obvious – yet another repaved Cranbrook street crumbling in five or 10 years. Or even less.

The political choice is simple – take the short view or the long. From a taxpayers’ point of view, I think the best choice is obvious