The Pierre City Commission declared an emergency Tuesday at the request of Utilities Director Brad Palmer who says a new sinkhole found Monday in Griffin Park will require hiring outside help quickly to fix the old, faulty sanitary sewer mainline before winter.
The new work could cost $650,000, he told the Capital Journal.
Palmer briefed the Commission, saying that after five weeks of city crews working with local and out-of-state contractors to fix flaws found in the 24-inch clay sewer mainline lying 12 feet under Griffin Park, it seemed the job was done on Friday.
The site of the initial sinkhole which first appeared on a walking path on about July 12 and a site later found by inter-sewer-pipe camera work at a manhole 600 feet “upstream,” at the corner of Missouri Avenue and Ree Street were the scene of much construction ever since.
Smaller-diameter PVC pipe replaced the 60-year old three-foot sections of 24-inch clay pipe. The old clay pipe sections were losing their “integrity” in a year of high levels of ground water pushed higher by high levels in the Missouri River. The increased river flow was caused by above-normal releases from Oahe Dam necessitated by the second-largest runoff in the Upper Missouri Basin in 130 years.
The ground was buttoned up, the hose-and-pumps bypass system was turned off and as of Friday, Aug. 23, the repaired sewer pipe began again taking two-thirds of the city’s sewage for the first time in natural “gravity flow,” since the repairs began July 18, Palmer said.
But Monday morning dawned with a new little sinkhole forming just a few feet from where all the heavy equipment had been parked working on the big sinkhole first seen in the park on about July 12 by passersby.
Palmer said he didn’t know why it happened, exactly.
“We had run cameras through the sewer line . . . and everything looked good,” he told the Commission. His best theory is that the continued high ground water and high river releases have made the soil so unstable it doesn’t support the sewer pipe.
“They call them ‘flowing sands,’” he told the Capital Journal about the nature of the earth below Pierre by the river. When the soils get as saturated as they are this summer, “there’s no structural support” for the sewer pipe, he said. It’s as if lengths of sewer pipe mainline 8 feet beneath the surface of the ground are lying in water, he said. It leads to the old clay pipe sections leaking in groundwater. “The pipe collapses,” he said.
Similar problems developed in 2011 after the historic and devastating flood, he said.
The plan since mid-July was to do a quick fix for now before winter and then go in and do a permanent fix next year, Palmer has said.
But now it’s an emergency requiring a permanent fix this year, he told the Commission, because the bypass system of hoses lying on top of the ground with portable pumps can’t be used in the winter. The emergency declaration will allow the city to use funds that hadn’t been budgeted for sewer work.
The work done the past five weeks on the sewer pipe in Griffin Park using mostly city crews has cost perhaps $100,000 to $150,000, Palmer told the Capital Journal.
It’s been an extreme year, with 160 percent of normal precipitation for Pierre so far this year and in the larger picture, the second-largest runoff into the Upper Missouri River since the 1800s, in the words of the emergency declaration he persuaded the City Commission to approve. He made a late amendment, asking the Commission to add after one of the the “Whereases,” that the needed sewer pipe fix is a matter of “public health and welfare and safety.”
The best answer, Palmer said, is hiring Burns & McDonnell, a “ full-service engineering, architecture, construction, environmental and consulting solutions firm, based in Kansas City,” according to its website.
They are experts who know how to get such a project done, and can find the right contractors who can fit this project into their schedule in time for the city of Pierre to have sewer service this winter, according to Palmer.
The plan now is to replace a 600-foot length of the mainline, from the corner of Missouri and Ree to the site of the original sinkhole in Griffin Park with 24-inch PVC.
He’s already talked with Burns & McDonnell engineers because for two months the firm has been the consultant on a sewer-lining project on smaller pipes uphill across town.
“We are going to work with them and do a complete fix,” he says of the 600-foot area, which handles two-thirds of the city’s sewage on its last run to the wastewater treatment plant to the east along the river.
It will mean redoing some of the work already done under the site of the original sinkhole, where smaller pipe was inserted inside the faulty clay pipe as a temporary fix. But Palmer said the sewer pipe upstream from the repaired manhole area at the intersection of Missouri and Ree has been repaired in previous years and should be OK.
An interesting local connection to the new plan: Working out of the Denver office of Burns & McDonnell as the ramrod on this project will be Brian Knadle, a graduate of Riggs High in Pierre, Palmer told the Capital Journal.
He knows Knadle to be a good engineer, he said.
Burns & McDonnell will be charged with getting all the construction contractors needed to get the job done in time, maybe Octoberish, Palmer said. That will allow city workers to get back to some of the previously scheduled work projects that had to be paused for five weeks, he said.
The Commission voted 5-0 for the emergency declaration.
“We have no choice,” Commissioner Jamie Huizenga said. “We can’t be running that bypass in January when it’s 10 below.”