Thing: Sinking substructures

>> Not all potholes are created equal

by PHILIP PREVILLE

 Need proof that the earth moves? Just have a look at the road beneath your feet. Over the course of the winter, water seeped beneath the roadways and expanded as it froze, pushing asphalt in all different directions; now that the ice has melted, the road surfaces are cracking and collapsing.

 And that's just the surface. Underneath, more nefarious movement may also be underway.

 Some potholes are worse than others. The pothole pictured above doesn't look that bad. It doesn't even look like a pothole; it's more of a wide, extended divot in the road. But the dip from the surface measures at least six inches, which is pretty severe. And what makes this divot so bad is the presence of the man-hole in the centre. Normal, everyday potholes are just cracks in the surface, but in this case an underground tunnel is shifting--and the problem can reach many feet underground.

 Cars and trucks take a serious toll. Every time a vehicle passes over top of a sunken man-hole, it reverberates through the underground structure. "Down in the tunnels, you can feel the vibrations even though you're 12 feet underground," says public works spokesperson Pierre Bonin. "And that's in tunnels that don't have structural problems." Between now and May 12, trucks are supposed to reduce their loads by 20 per cent to ease up on the toll they take.

 For repair crews, sunken man-holes take priority over potholes. In a worst-case scenario, an entire segment of the tunnel system could collapse. Usually the problem isn't that bad; they just need to compact and reinforce the ground around the man-hole, raise the man-hole entrance a couple of inches and patch up the surface asphalt. But Bonin says city crews keep an inventory of all sunken man-holes, fixing up the worst ones immediately while taking care of the less severe cases over the course of the summer. "It's the kind of problem that people should report to the city when they see it."

 Final factoid: The worst-case scenario actually happened in the spring of 1993, when a series of underground tunnels under the Decarie service lanes collapsed beneath the weight of a loaded truck. The rig descended into the hole as though it were on an elevator. "The driver got the scare of his life when he realized he was being sucked into the earth," says Bonin. :

 

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