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BLOOMINGTON - The city's streets chief says a busy stretch of Washington Street at Prairie Street, closed after an underground cave-in was found there, should reopen to traffic by noon Saturday.

"The repairs should be done to the sewer lines (Friday), but the concrete needs to dry tonight," said Kurt Haas, assistant superintendent of streets and sewers in the Bloomington Public Services Department.

At 6 a.m. Saturday, crews will work to temporarily patch the road over the concrete buffer layered between soil and paved road. The permanent asphalt covering comes in two weeks, when already-scheduled repaving of that block will take place, Haas said.

The hole - which under its initial 6-inch opening proved to be a gaping 8 feet wide, 7 feet long and 10 feet deep - came to light Thursday when city engineering technician Jeff Kohl noticed a dip in the road. The hole was enlarged to give workers more space in which to work safely.

The problem was blamed on heavy rains and damage caused from water gathering around a problem fitting, washing dirt away.

Haas blamed an old clay sewer pipe that had provided service to a home no longer standing. Nowadays, the connections are better fitted to the main line and don't erode as easily. But the mysteries in the ground under a city that's been around more than 150 years aren't always easy to solve.

"We don't know where all these (old clay connections) are," he said.

Some had said Thursday's hole was caused by coal mine tunnels, but Mike Matejka, McLean County Historical Society board president, said a map he's seen of a Bloomington coal mine shows it didn't stretch downtown.

The McLean County Coal Co., which operated in the latter half of the 19th century through the 1930s, was based near the old train station on Lumber Street. The mine tunnels all stretched west.

"Now if this hole had turned up way down on West Washington Street, that might have been a possibility," he said.

Though Bloomington has about 45 holes waiting on repairs, Haas decided to close the spot for emergency repairs because the 200 block of East Washington Street is a high-traffic area.

The city generally handles 25 to 30 pavement holes each year, though most are much smaller than the recent problem, Haas said. He attributes the upsurge to a spring of yo-yo temperatures.

"That cold-warm-cold cycle causes the ground to shift. It heats and thaws more," then refreezes, and causes problems on aging sewer lines, he said.

City crews try to address the list of cave-ins each calendar year. By winter, potholes start to show up and the city temporarily patches those. A few months later, public services start to get reports of the telltale dips in the road. At this time of year, road crews are working to address the holes.

With routine maintenance to repave some city streets beginning July 9, Haas' staff is working to coordinate cave-in repairs to stay one step ahead of that schedule. Otherwise, in certain cases the city would be paving, fixing a pavement hole, then repaving, he said.

Reporter Sharon K. Wolfe contributed to this report.

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