bishop at sinkhole

Bill Bishop noticed this sinkhole appearing over the weekend on the walking path in Griffin Park in Pierre next to the Missouri River, seen in the background. Bishop said he regularly walks here near his home and had noticed a depression on the path a week ago. (Stephen Lee/Capital Journal)

A 10-foot deep sinkhole that gaped open the past weekend on a walking path along the Missouri River in Griffin Park, not far from the tennis courts and the outdoor swimming pool in Pierre, is attracting attention of passersby and city leaders.

At Tuesday’s City Commission meeting, Commissioner Jim Mehlhaff asked other city leaders about it, saying he’d heard about it and wanted to know what the plan was.

“We are going to fix it in the next couple of weeks,” City Administrator Kristi Honeywell said.

“So that part of the park is safe?” Mehlhaff asked

“That is ringed off from the public,” Mayor Steve Harding said.

Bill Bishop, a retired railroad man, walks this way often and on Wednesday morning stopped with his two tiny dogs to check out the sinkhole ringed with orange plastic fencing.

He’s noticed a depression, or settling, on the ground along the path that seemed to get more apparent this spring, Bishop told the Capital Journal..

“Then the hole was there, Sunday, or maybe it was Saturday,” he said. “The first time I saw it there weren’t any signs around it or anything.”

Looking at it Wednesday, Bishop said the sides of the hole appeared to have fallen in more since the weekend.

“Ever since the flood in 2011, there are a lot of voids — I call them — along Missouri Avenue here, there’s one at my place,” he said, describing what he suspects are underground spaces he blames on effects of the disastrous flood eight years ago.

Lynn Patton, the city’s construction and operations manager, said the hole goes down about 10 feet to an old sewer trunk line.

He thinks. There’s still some research to be done before the fix can be done.

“There was a small dip on the edge of the path more than a week ago that we were made aware of,” Patton said. “We were in the process of starting to come up with a plan, and then it just collapsed and has opened up even bigger yet.

If it is an old sewer trunk line, the pipe itself dates back decades to the first moves by the city to switch the sewer disposal plan from simply piping it down to and into the river, to collecting it from those old ways and piping it to the first wastewater treatment plant, Patton said.

The trunk line was lined back in the 1990s with a plastic inner surface to extend its life.

Now his crews have to find out; just what happened so they can fix it, he said.

“We’re going to try to videotape it during a low flow, and suck some flow out to haul cameras in the line to see if we can determine (the cause.) That’s an old pipe that has been lined. The outside pipe is clay and that’s probably where the material and inflow is doing into. We’re hoping the it’s just between the liner and the old pipe. But until we get a camera in there, we won’t know. This is a trunk line, so it flows quite full.”

If relying on a low flow period doesn’t work, the crews will have to set up pumps to get the flow to where a video camera can be inserted, he said.

A sort of city colonoscopy.

“We don’t know at this time if any sewage leaked out. If ground water is getting in between the pipes and the liner is in good shape, than the sewage is staying within the pipe.”

No one has lost any utility service yet from the sinkhole.

But if the exam finds some repairs are needed to underground pipes, “there will be some bypassing,” to keep everyone with water and sewer service and any other utility affected, he said.

This area of 100 feet or more from the river used to be mud flats from regular floods before Oahe Dam was built in the 1950s and completed in 1962, bringing a new measure of flood control, Patton said.

Back in the day, the river and mud flats were used as dump sites and some of those things still are found when digging goes on, he said.

“There are a lot of things underground we’re not aware of.”

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