A discovery Monday by construction crews will set back — once again — the completion date for the Garden Avenue bridge project to the end of October.
Construction crews started to remove and replace the collar underneath a loose manhole lid, City Engineer Louis Najar told the Infrastructure Committee at its meeting Monday evening, but they discovered extensive corrosion.
“Typically, you have a manhole and then we build a concrete collar 10 inches thick to match the grade of the road so when you drive over it, it’s smooth,” Najar said Tuesday morning at the site just south of the new bridge.
The crews began to jackhammer the collar to remove it and pour a new one, but as they started, the concrete cone — a structure narrow at the top and widening at the bottom for access to the underground pipes — collapsed. Some of the bricks from the wall under the cone had also collapsed into the sewer.
Hydrogen sulfide — a colorless gas that is produced as a result of the breakdown of organic materials in the absence of oxygen — is the culprit, eating away at the mortar and concrete. The concrete collar was about half its original 10-inch thickness, Najar said.
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The section where the cone and collar collapsed left a gap between the metal manhole cover structure and the street a couple of inches wide.
Considering the truck traffic to Roswell Livestock Auction Sales, 900 N. Garden, Najar said the city is fortunate the problem was found now.
“It’s a bad thing for the project. It’s a good thing for the safety, because this would have eventually collapsed,” Najar said.
The city will excavate the old manhole, which is located in the northbound lane, and essentially build a new one around it, Najar said. The new manhole will be coated with an epoxy to protect it from the hydrogen sulfide.
That will all have to be done without shutting down the sewer line, since it’s an 18-inch collector pipe. Typical neighborhood sewer lines are 8 inches, Najar said.
While inspecting the sewer around the collapsed manhole, crews discovered another manhole several feet to the north that had been paved over and did not appear on any city maps. Two other sewer lines, a 15-inch and a 12-inch, were also found that were also not on any city maps.
Crews will route the sewer lines through the new manhole and also ensure a line from Saddle Barn, 1102 N. Garden Ave., is located and routed to the new manhole.
The second manhole will be decommissioned and filled in, Najar said.
The work will take about another two weeks, depending on how quickly a prefabricated concrete cone can be shipped.
Najar expects both sides of the road around the manhole will be dug up and replaced, extending the street work a bit further, just as work on a waterline extended the project about 300 feet past its original endpoint. The expected end date after that change was Oct. 16. He now expects the street to be open Oct. 31.
“I know it’s going to delay the bridge, but it’s a good thing we didn’t interrupt the traffic and lose a livestock truck or heavy semi truck and somebody get hurt,” Najar said.
City/RISD reporter Juno Ogle can be reached at 575-622-7710, ext. 205, or email@example.com.