Dropping in on City Hall

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What lies beneath the asphalt of city streets

There's a rupture in the space-time continuum down at the corner of Jeanne-Mance and Ste-Catherine these days. Step into it and you are automatically transported 100 years back in time, when horses roamed the streets and roads were the domain of stonemasons. The asphalt has been worn away to reveal the old cobblestone street beneath it.

Normally, asphalt road surfaces are built on top of a bed of concrete, which is itself built atop a bed of gravel. These modern structures--built with state-of-the-art technology and designed for the wear and tear of cars, buses, trucks and other industrial vehicles--usually last about 20 or 30 years at best. So it's amazing to think that the old cobblestones still provide a solid foundation for the modern roadway. Here's how they have stood the test of time.

Built to last: Engineering was still a nascent science back then. Builders tended to err on the side of caution and build things that, beneath the gaze of modern science, were far stronger than required.

They were also exquisitely crafted. Today's engineers have a tough time replicating the old roadbuilders' feat. "We're re-paving Le Royer street in old Montreal with stones to preserve the area's cachet," says Public Works Manager Gilles Robillard. "Making sure the road is even, grading it so that the water runs off it properly... it's a tricky job. And it costs four times more than simply laying down a new asphalt road."

Built with solid rock: The principle behind concrete is as follows: take some rocks, crush them, and fuse them into one big man-made rock-slab. It seemed like a good idea at the time. Alas, as it turns out, nature's rocks have greater structural integrity than man's concrete.

Not surprisingly, as the cobblestones continue to last, it's the asphalt that eventually gives way. Normally, says Robillard, crews cap all roadways with at least four inches of asphalt; down at this pothole, it's been worn down to less than two--and, obviously, has started to peel off altogether.

Built on solid ground: Modern roads have a maze of water and sewer pipes running beneath them. They shift and leak over time, and when they move, the road surface moves with them. But back in the old days the underground pipes were installed beneath the sidewalks, in part because the streetcar rails ran down the middle of the street. The logic was different: since you couldn't ask a streetcar to take a detour around a repair site, you built your underground infrastructure off to the side. In many cases, the old rail lines are still lying beneath the asphalt as well.

Final factoid: No one knows exactly which streets still have cobblestones beneath the asphalt. "When a road needs repairs, we send a survey team down and they're the ones who usually make the discovery," says Robillard. "We can make an educated guess based on when the street was first built, but we never really know what we're going to find."

--Philip Preville