Beware the Dips: Why Potholes Form Under Viaducts

By March 8, 2021 No Comments

Where water flows to is where pavement is most likely to fail. So in areas where water stands still there may well be potholes lurking under the puddles.

In the balance of life, there are bridges and there are underpasses. On the former a car will go up and over and the latter it’s down and through.

Simple stuff, right? It also explains why, simply, there tend to be potholes under bridges but not on top of them. This is because where water accumulates, so too do asphalt problems – because water is the enemy of intact pavement.

Most but not all bridges are paved with concrete instead of asphalt. The reason has to do with the relative strength of concrete when used in conjunction with the steel structure of a bridge. Asphalt is the preferred and economical material used over land (i.e., 99.9% of the roads traveled), where there is an underlayment bed of gravel, sand, and earth.

An important difference between bridges and underpasses is that water pretty much rolls off the bridge pavement. And where does it go? To the lowest spot it can drain to, of course. Which in the case of a bridge passing over a crossroads means the water drains to the bottommost point under the viaduct. That spells trouble if that underpass has inadequate drainage.

When water accumulates on top of pavement, in all likelihood it is also saturating the sub-layers of the road. Over time, one of two things can and very often does happen: the underlayment will wash out, leading to pothole formation; and in freeze-thaw conditions, the water will expand when frozen, creating larger cavities below the surface. In both cases, potholes and other pavement deterioration occurs.

What’s particularly insidious about viaduct potholes is when they are filled with water – which is often, even days after a rain – the water hides the crevasse. A motorist passing through won’t realize there is a pothole there until the spinning tire abruptly falls into it. The whump! and bump! is usually the driver’s first clue as to what was there.

Potholes are responsible for expenses and injuries. Data collected by the AAA (American Automobile Association) reveal that motorists spend about $3 billion per year to repair their vehicles from pothole incidents. For individual motorists, that averages out to $300 per pothole incident for blown tires, bent wheel rims, and impaired suspension systems.

The solutions are to drive more slowly when passing through accumulated water, or to drive around the water if possible. If the route is a regular part of your commute, pay attention to the quality of the pavement in dry times – and remember the pothole is still there, hiding under the water.

In most municipalities, reporting the pothole through any means provided is more likely to get those road hazards fixed.