Yesterday was World Pot Hole Day and I thought it might be worthwhile discussing a bit of the underlying science behind potholes by re-visiting an old column.
HHH
It would appear Prince George this year is the world's capital for the road hazards. Whether it is Tyner Boulevard, Ospika Boulevard, or Queensway, road hazards abound. We have even experienced a couple sinkholes this past year, resulting in major portions of pavement dropping out of sight.
What is the science behind potholes?
The simple answer is frost heaves and freeze/thaw cycles.
Potholes result from a combination of moisture in the matrix under a roadway and the freezing/thawing cycles experienced during the winter months. Water is the only common substance that occupies more volume in its frozen state than in its liquid state - a consequence of the hydrogen bonding networks that hold water molecules together.
What this means for our roads is that when the water trapped in the soil freezes, it also expands. This tends to lift the overburden - the surface of the road and underlying material - upwards. The asphalt and air above it present the route of least resistance for the expanding water and a bump emerges.
This wouldn't actually be a problem if the whole roadway had the same soil with the same amount of moisture underneath all of it. The whole road would then lift and sink as a single unit. But that is not what happens.
Instead, moisture creeps down through the cracks in the road surface and the moisture in the underlying patch of the soil is higher than elsewhere. Only part of a road is affected as it has a higher moisture content than other parts.
Soil is not really a "solid" but a fine aggregate of grainy material. Typically, a soil might have 50 per cent free volume and half of this could be filled with water. However, the amount of water that a soil can absorb can fluctuate and so it might move up to 35 per cent or 40 per cent under a crack in the road.
More water means more expansion so that the road does not lift in a uniform fashion but bends and heaves.
This results in further physical stress on the road surface both from the ice below and from the pounding that it receives from vehicles as they pass. The result of this stress is more cracks, more water seepage, more moisture in the soil and eventually more damage to the road.
But that is not the end of things with potholes. There is also the thaw part of the cycle. As the ice under the water thaws, it turns back into water and its volume decreases. The road drops down.
Further, the structure of ice is such that it will have pushed some of the underlying soil to the side or outwards as it froze.
The result is that during the thaw cycle, a pocket or hole can start to emerge under the patch of pavement. Each freeze/thaw cycle exacerbates the warping of the pavement.
It is a bit like bending a piece of wire back and forth, to and fro. Eventually it breaks. The same is true of the asphalt. Being pushed up and down, while being pounded by cars and trucks, eventually results in its structure breaking down.
The roadway eventually sinks into the hole created by the freeze/thaw cycle underneath it and we have a pothole - some of which, on our city streets, can be of truly gargantuan proportions.
Of course, filling a pothole is what city crews do during the summer. But because the original asphalt and the patch don't blend or solidify to a single piece, there is still a crack in the road and a place for water to get into the subsoil matrix. That just means that the same hole will be back next year.
Interestingly enough, an increase in potholes may actually be a sign of changing climates.
As E. Ho and W.A. Gough point out in an article in Theoretical and Applied Climatology, "Climate is certainly important to the structure of road pavement, where high temperatures can cause thermal warping, and frequent freezing and thawing results in loss of pavement strength. Furthermore, increased frequencies of freeze/thaw cycles are related to premature deterioration of pavement structures, resulting in increased surface roughness."
Translated, that means that as the climate gets warmer, we get more frequent freeze/thaw cycles during our winter and more damage to the underlying road matrix, giving us more potholes. Our mild winter with its fluctuating temperatures hovering above and below zero have exacerbated the situation.
Ironically, with the damage from this year's potholes, we may see fewer vehicles on the road in the future, which would help to solve the climate issue and decrease the number of potholes.
