The Northdale Sanitary District is taking its first steps to reroute a sanitary sewer main that could be in danger of collapse due to a sinkhole.
The sinkhole opened in the Hideaway Hills neighborhood on April 27, 2020, exposing an abandoned gypsum mine and displacing more than 40 people from 15 homes.
The sewer force main is in danger of collapsing due to the sinkhole, although it isn’t compromised quite yet. There are at least two lawsuits filed in relation to the sinkhole collapse.
During the July meeting, the Northdale Sanitary District board voted unanimously to bore 15 holes about 50-feet deep along the reroute path, which is parallel to Interstate 90 and along the back fence of Hideaway Hills Homes.
Project engineer Leah Berg said the testing is included in the plan submitted to the state board for the Department of Agriculture and Natural Resources $440,000 loan.
The board voted to accept the loan at its June meeting, although no invoices have been submitted yet and no money has been used. Residents also saw an increase of $7.40 per household per connection at the direction of the state to accept the loan. The rate increase would begin Aug. 1 and appear on the Sept. 1 bill.
Hideaway Hills residents who attended the July meeting expressed concerns about the reroute area.
“We don’t know at what point in time, when the houses got put in here, how much ground was down there,” one resident said. “How many times did it fill up with water? How many times did it drain? Was there eight more feet to that ceiling?”
Another resident asked who would be liable or take responsibility if another collapse happened due to the boring or the reroute.
Berg said contractors wouldn’t be responsible for that.
Michael Loos, lawyer for the sanitary district, said there’s risk if the board does nothing and a perceived risk of taking action.
“I’m not sure how to match those up. I’m not sure anybody is,” he said.
Loos said he doesn’t know if the district would be doing its job if they did nothing, though.
The board waited to move forward with boring and engineering on the reroute to hear findings from a geophysical study conducted by Dr. Mohammad Sadeghi, an assistant professor at Montana Technical University.
The study, which studied resistivity levels and boundaries in the north and western areas of the mine, found that there could be an additional sinkhole between Daisy Drive and East Daisy Drive. It also found there could be water beneath homes and backyards on East Daisy Drive heading toward I-90 near the re-route area.
Patrick Ealy, director of research for the Fitzgerald Law Firm of Rapid City that ordered the study, said Sadeghi will return to the area to do a similar study on the south and east area of the mine, although a date has not yet been scheduled.
Berg said the findings showed anomalies but isn’t anything to make a decision on. She said geotechnical testing would have definitive answers about what’s actually underneath.
She said moving the reroute to the west away from the sinkhole would increase the project cost to about $900,000 instead of $440,000.
“Definitely a possibility, but that’s what it takes to go around the block,” she said.
Berg said it also wouldn’t get away from potential void space.
The district meets on the second Tuesday of every month at 7 p.m.
— Contact Siandhara Bonnet at email@example.com —