Shelbyville seeks payback for flume collapse costs

The flume topic will be discussed further at next Tuesday's regular study session, scheduled for 6 p.m.

City manager Ed Craig passed out copies of the report from CBC Engineers & Associates to the city council Tuesday and said the report "identifies very well" what the cause of the collapse was -- two sources of water intrusion into the space around the flume, one of which was an open, but unused sanitary sewer line and the other a broken pressure relief valve on the flume itself.

According to the conclusion of the engineer's report, the flume's collapse "was the result of the foundation support below the pipe arch being compromised by the intrusion of water from the ... non-abandoned old sanitary line and damaged pressure relief valve."

The report says that the water that was introduced to the soil below the flume "during the May storm event caused the erosion of the bedding material ... and likely caused softening of the remaining bedding material."

Craig explained the valves are placed down the length of the flume to allow water from outside the structure to flow inside the flume to relieve the outside pressure.

Instead, the valve that had been knocked off allowed water that was inside the flume to go into the region surrounding it.

"Clearly, the sanitary sewer line being open had something to do with Shelbyville Power System and their contractor" when they installed a new sewer line last year near the flume on North Brittain Street," Craig said.

"The old sewer line was supposed to be shut off and the failure to do that allowed the water to get back there."

Craig also said that public works director Mark Clanton has traveled the length of the flume several times taking photos and said that no other valves had been knocked off except the one highlighted in the report.

The flume was installed 47 years ago and the valve had remained in place until the contractors for Shelbyville Power, Mayse Construction Co. of Chattanooga, uncovered the area, Craig said.

"It looks to me like we need to be seeking reimbursement from Shelbyville Power for the cost that we've incurred," Craig said.

Final figures for the repair were tabulated earlier this month, totalling $440,563.86, but whoever ends up paying for the repair will be out $364,455.26.

What it says

The report says that the "significant rainfall event" on May 1-3 had the flume flowing full, but on May 2, city officials observed a ground depression in the grass area between North Main and North Brittain streets. By the next day, the depression had increased and North Main was closed.

It wasn't until a week later when it was safe enough to enter the flume that it was discovered that the invert of the flume had failed by bucking up inside the structure.

The failure of the flume ran for 125 feet toward Duck River, but the report notes that the steel on the rest of the flume "appeared to be in relatively good shape after 40+ years of service" with minor corrosion visible.

The broken valve was located 10 feet downstream of the upstream end of the failed section of the flume, the report reads. Also, the backflow preventing gate for the valve was missing and "backfill material was visible on the outside of the valve."

After the failed part of the flume was removed, engineers returned on June 9 to inspect the area and that's when the open sewer line was discovered.

The report states that during the preparation work for placing in the new flume section, "the new sanitary manhole was plugged in order to bypass sewage around the sanitary lines in the excavation area."

"Shortly after the manhole was plugged, sewage was observed discharging out underneath the flume into the excavation in the same area ...."

The report also says that Shelbyville Power performed a leak test with fluorescent green dye, and the dye was seen discharging from underneath the flume. The old line was also examined with a video camera by the utility and "it was determined that this line has been compromised and was leaking under the flume," the report reads.

The report also says that the valve on the old sewer line had remained open instead of being shut, which allowed water and sewage to continue to flow in the old line, which crossed under the flume.

Also, city personnel reported that during the May 1-3 storm, "several manhole lids in the vicinity of the flume were displaced by the storm runoff."