Monday, March 06, 2006

Potholes and Roads

It's that time of year when the potholes appear. This year in Montreal they appeared early due to warmer temperatures.

I have been really surprised at the state of roads. The potholes here are huge and plentiful. And they pop up very quickly. I have seen potholes grow to a foot or two (30 - 60 cm)wide by four inches (10 cm) depth in as short as a week. The freeze/thaw cycles just rip the roads apart. On my daily commute there are a handful of locations where I've given up tryin to find a 'line' to take to avoid a pothole riddled section of road. Even after repairs are done, it's like driving over rumble strips. Sorry for the lack of pictures with this post.

An obvious response to the pothole problem would be to build the road out of concrete. Though concrete is more durable, I suspect it is cost inhibitive. Sidewalks are made of concrete, but there is more leeway as far as unevenness and roads would require more concrete (thickness and width) more often. So it seems asphalt is the answer as long as it remains less expensive.

The snow plows give the roads quite a beating. In addition to the interaction with the asphalt, I have noticed how all the painted lines are noticably fainter every spring. My guess is that they must be repainted every three or four years if not sooner.

And it is not just the potholes. The roads are noticably bumpy in general. Drawing on my knowledge of what we deal with in the building industry, here are a few educated guesses as to what else may take their toll on the roads. If there are any road construction or maintenance experts out there, it would be great to be enlightened whether these are also factors. Maybe the subgrade can be engineered to withstand these forces.

In addition to the freeze/thaw cycles a road goes through there is also some other forces at work from below. One is ice lensing where water coming down from the road surface or up from the ground below encounters a freezing layer. The water freezes and causes water following behind to stop and freeze also. Thereby creating a lense of ice inside the earth. This pushes up on the surface above and when warmer temps arrive the water melts and the surface above settles back into it's original position.

Frost heave is similar, but involves the moisture present in the earth. In this case the earth expands together due to how water expands when it becomes ice. This can be damaging if different areas rise at different rates due to different moisture contents. I'm pretty sure that this is why if you go to a park soon after the thaw you find that the earth is nice and soft. I'm not positive this could have a large affect on the surface, but it could.

Lastly there is the soil makeup. Near where I work there are expansive clays. What this means is that the clay expands with a higher moisture content and contracts under low moisture content. We have many building nearby which had settled severely (1 to 2 feet from one end to another) when there was a drought a few years back. There is a section of sidewalk that does dips like a roller coaster in the areas adjacent to trees. Probably because the trees were drawing out all the moisture during the drought. So I think if the presense of expansive clays was not taken into account during the construction of the road, it would have an adverse affect on the road surface.

So in short, the forces of nature take a great toll on the roads. I really think that the cold temps instead of lack of maintenance is the reason for the difference from other places I've been.


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