Why did the sinkhole happen in downtown Pittsburgh? Could it happen again?

It's the sinkhole seen all over the nation. On October 29, a Port Authority of Allegheny County bus was swallowed by a sinkhole on 10th Street and Penn Avenue in Downtown Pittsburgh. READ MORE: https://bit.ly/2CXJ2w8 Subscribe to WPXI: https://on.wpxi.com/YouTube Connect with WPXI online: https://www.wpxi.com/ Download our FREE apps: https://wpxiapps.com/

After the incident, Channel 11 started asking questions about why it happened and if it could happen again. Investigative reporter Rick Earle asked the city what is under the roads of Pittsburgh.  When city officials didn't answer, he went to someone who probably knows the most about Pittsburgh streets, former city CEO and longtime Public Works Director Guy Costa.

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Earle met Costa at the sinkhole site where repairs are ongoing.

"The mystery is what caused it.  Obviously, it was water," Costa told Earle. "But where did the water come from?  Did it come from the storm sewer?  Was there a leak?"

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Costa said this isn't the first time there have been problems under Pittsburgh roads.  He predicts it won't be the last.  He told Earle about a similar situation on Liberty Avenue and 6th Street.  He said workers started noticing the road buckling and the subway dropping about a decade ago.  He added the road had dropped nearly a foot in some places but never collapsed.

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"When we excavated, we found the fill washed away.  It could be the when the fill was installed when the subway was installed, it may not have been properly compacted or the right materials weren't used," Costa said.

The road was dug up in the early 1980s for construction of the subway, but Costa said it took nearly three decades to see and feel the impact.  Crews replaced the fill and concrete slabs one section at a time, and that seems to have worked.

Four years ago, Costa said the city noticed parts of Reedsdale Street near Sandusky Street on the Northside cracking and dropping.  He said when crews dug up sections of that road, they made a shocking discovery.

"Someone compacted it with washers, dryers, stoves and hot water tanks," Costa said.  "As we dug it up we just found more and more appliances under there.  And what happened over time, the appliances rotted away and dropped and it brought the base down and brought the roadway down."

Costa suspected the old appliances were left behind when crews tore down the homes in that area years ago to make way for the roads.

Earle asked Costa if we should expect more sinkholes to open.

"Yeah, you should be concerned," Costa told him.  "It's very hard to tell or know when it's going to happen again."

Costa added the key is to keep a close eye on all Pittsburgh streets, and as soon as one buckles or dips to get in there and start shoring it up right away.