Lake City, in Clayton County, Ga., is part of the Atlanta metro core and parts are very densely populated. More to the point, and like most urban areas, density has increased in most areas, and new construction has overgrown some existing underground assets. This makes rehabilitation of existing infrastructure a challenging proposition.
This was certainly the case when the Forest Parkway culvert failed. The 66-inch corrugated metal pipe was 20 feet deep, 220 feet long, and failing so dramatically, that parking lots and commercial buildings overhead were subsiding noticeably and had to be monitored throughout subsequent rehabilitation. Inspection revealed completely corroded inverts, sidewalls that were curling up into the pipe interior, and some collapsed pipe.
After discovery, the failure accelerated. “From quote to award, the culvert had collapsed even further,” says Anita Clyne, president of Utility Asset Management Inc. of Reynolds, Ga., a licensed utility contractor specializing in trenchless technologies for manhole and sewer construction and rehabilitation. “That’s when we realized typical procedures wouldn’t be enough for this project.”
“It wasn’t a surprise,” agrees Terry Moy, PE, program management and engineering manager for the Clayton County Water Authority (CCWA) and an active member of the American Society of Civil Engineers’ Pipeline Division. “We’d identified it for rehabilitation a year before and were working through access and business impacts with property owners, but increased storm runoff accelerated the migration of the backfill around the pipe. This eliminated the possibility of a cured-in-place solution.”
Surprise or not, it was almost a worst-case scenario:
- The pipe was no longer reliably supporting overhead soil, so just being in it was unsafe.
- There were commercial buildings directly over the culvert, so trenching was out of the question.
- Capacity couldn’t be reduced significantly. Sliplining and other minimally invasive procedures were ruled out.
Tedious but effective
Eventually, Utility Asset Management developed a two-phase procedure that was slow and difficult, but effective.
Phase one: Fifty-four-inch tunnel liner plates were used to stabilize the culvert structurally. These are four-part steel rings, 18 inches wide, which were assembled within the pipe.
“First, we had to cut out curled-in pieces of the old culvert by hand,” Clyne says. “Then we assembled the liner plates inside the culvert section by section. Basically, we were building a new pipe 18 inches at a time. Meanwhile, cars and trucks were driving by in the parking lot above us.”
The liner plates’ four arched sections were bolted together manually. Each completed section created a new 18-inch safe zone for workers, who then repeated the process 146 times.
After the new, bolted-together “tunnel” was completed, the void between the liner plates and what was left of the existing pipe was filled with injected grout—and yes, that was expensive. It was tedious work in a cramped space but it was safe and effective. The phase was completed without injury and no further subsidence of overhead buildings. The culvert was at least stable.