DETROIT -- A sinkhole that damaged homes in a neighborhood north of Detroit and cost $75 million to fix was caused by human error that allowed the quick release of waste and water into a sewer line, according to a top county official.
An engineering assessment determined the surge fractured the pipe in Fraser which drew in sand and created a void in the surrounding soil, Macomb Public Works Commissioner Candice Miller told reporters Wednesday.
The pipe collapsed and caused the Dec. 24, 2016, sinkhole which grew to 100 feet (30.4 meters) wide and 250 feet (76.2 meters) long. Nearly two dozen homes were evacuated and three houses later were condemned. The collapse had threatened to dump raw sewage into thousands of basements in the county through the broken pipe.
But the problem started in 2014 after a gate was closed to hold back sewage as crews performed pipe maintenance. The gate was supposed to be gradually raised to allow a slower release of the water and waste. Miller said that wasn't done properly on a number of occasions, which she attributed to "human error."
"When the workers leave the pipe, the operator then has to raise the gates," she said. "The protocols for raising these gates call for the flow to be released maybe over several hours."
On at least eight different occasions the water was released faster.
"One time in particular, they released the flow in about seven minutes," Miller said. "When that happened ... you had a tsunami of water — a huge water hammer — that traveled down the pipe" and hit a wall.
It "was like a bomb went off," she added.
Miller said it caused "hairline fractures" in the pipe.
"Silt begins to come into the pipe," she said. "The ground just keeps sinking, sinking. Pretty soon you got a sinkhole. That's what happened here."
The 11-foot (3.3-meter) diameter pipe is 60 feet (18.2 meters) below ground and serves nearly 500,000 county residents. Officials say it has since been repaired with 4,000 linear feet (1,219 linear meters) of new pipe.
The collapse and repair work forced a nearly yearlong closure of a major roadway.
A $70 million bond was sold by the Macomb Interceptor Drain Drainage District to help pay for the sewer line repairs. About $5 million from the state also was used for the work.
An insurance claim will be filed to recover some costs, Miller said.
"I think it was a mistake that was made, and that's why you have insurance," she said.