Winter may be over, but spring isn’t the only thing starting: get ready for sinkhole season.

Sinkholes have already proved irksome for some commuters. Last week a sinkhole on Eglinton Ave. W. caused almost a week’s worth of repairs. And on Freeland St., near the Star’s offices on the waterfront, a small sinkhole forced a patch of road to be closed down for reconstruction.

Pothole vs. Sinkhole

A lot of people confuse potholes with sinkholes, says Hector Moreno, manager of road operations for the city.

Potholes are holes caused by damage to the surface of the road. They form when cracks in the asphalt fill with water, pushing up the asphalt around the crack, Moreno said. Freezing temperatures or heavy rains increase this stress to the crack, as does heavy traffic.

“It’s usually just the top layer of the asphalt,” he said.

Sinkholes, however, are something different. They are caused when leaking pipes or water mains wash away soil or rock deep beneath the surface, Moreno said.

Two kinds of sinkholes

Not all sinkholes are the same, says Steve Worthington, an expert on natural sinkholes from Dundas, Ont.

In Toronto, most sinkholes are caused by man-made failures, like leaky pipes. Naturally occurring sinkholes (otherwise known as karst sinkholes) are formed when soluble rocks between the surface are gradually washed away, Worthington said. In Florida, heavy rains, which are slightly acidic, wear at the limestone beneath the roads and have caused massive sinkholes up to 600 metres across.

How the weather affects it all

Potholes are especially common during the winter, as temperatures fluctuate above and below the freezing point, causing the water to expand and contract, expand and contract. This freeze-thaw cycle puts extra stress on the pavement and causes greater damage.

Although most people bemoaned this February’s icy temperatures, Moreno said it was actually beneficial for the roads because sustained frozen ground is stronger than ground that constantly cycles through freeze-thaws.

“It basically keeps the entire structure as a whole,” he said.

There have been 61,000 potholes so far this year, Moreno said, and they’ve cost the city $1.3 million in repairs.

A year ago, when temperatures fluctuated more widely, there were 104,000 potholes in the same time period, costing the city a total of $1.8 million in repairs.

When it comes to sinkholes, however, the danger seasons are spring and summer. Water is the enemy, and the spring thaw means that frozen water underneath the earth’s surface will start to flow again.

“The most is happening just at the end of the snow melt,” Worthington said.

This year’s harsh winter also brought frozen pipes, Moreno said, leading to a lot of leakage beneath the surface. The frozen ground can also serve to “bridge” holes below, temporarily hiding the subterranean caverns.

The scope of the damage

Moreno cautions that it can take a long time for sinkholes to emerge, sometimes months and sometimes years. While the spring thaw may begin to wash away the earth beneath the surface, it takes time and the constant strain of traffic to actually cause the pavement to collapse.

“You’re going to start seeing a lot more low settlements, not necessarily sinkholes,” he said. “You’re hardly ever going to see a complete road collapse into a sinkhole — boom!”

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In the next four weeks, Moreno said the city will be on the lookout for small depressions, which may signal that a larger hole is eroding underneath. Crews look for water leaks and then determine how far water travelled, possibly causing erosion all along the way.

“Unfortunately we’re not going to be able to see (the damage) until for many cases, years from now,” he said.

While potholes cost as little as $25 per hole to repair, sinkholes are basically money pits, said Moreno. In 2006, a 10-metre-wide sinkhole on Sheppard Ave. W. cost the city close to $1 million.

Still, Moreno deals with far fewer sinkholes than potholes — he estimated about 20 to 50 sinkholes a year in the city.