After the initial sticker shock passed, city officials entered triage mode Tuesday as they try to decide how they’ll move forward on the higher-priced Duck River Dam reservoir project.
Following a full-scale engineering study over the past several months, the utilities board received word last week that the total cost of the project is expected to jump from $70 million to approximately $110 million due to less-than-favorable geology and higher flood standards.
At a Tuesday meeting, the utilities board requested a proposal from URS Corp. — a national engineering firm — to review the current redesigned plan from project engineer of record CH2M Hill. That initiative is expected to take approximately two months, and officials hope a “fresh set of eyes” will turn up some additional cost savings, or at least confirm the new findings.
“Before we commit to anything next, we just want to make sure we’re on the right path,” utilities board chairman Wayne Fuller said.
The dam project is intended to create a 640-acre lake with a 32-million-gallon-per-day capacity in northeast Cullman County, which will work in conjunction with the area’s current sole major water source Lake Catoma.
Officials are eyeing a “hybrid” dam design, combining earthen wings and a roller-compacted concrete center, though the details of the design are still in flux in the wake of the new findings.
CH2M Hill took over the project from the U.S. Army Corps of Engineers once it regained momentum in the late 2000s following some legal challenges, and officials say some details lost in translation during that transition could be at the root of at least some of the problems now plaguing the project.
Engineers with CH2M Hill worked from a set of geological core samples taken by the U.S. Army Corps of Engineers in 1998, which have since been moved several times and might have been compromised in the intervening years.
In addition to those issues, CH2M Hill geotechnical engineer Rick Riker said the Corps’ original notes also included some entries that were left open to interpretation until their own team could get on site and confirm the geology.
“We were looking at the core samples, which were 15 years old and had been moved around, then we looked at the logs that showed notations for ‘loss of core’ without really explaining what that meant,” he said.
To stay on track with the accelerated construction timeline needed to meet the 2016 deadline built into the dam permit, officials opted to combine a round of additional core sampling with the larger excavation aspect of the project — meaning the clay that compromises the site was not fully discovered until excavation work was already well under way.
“We combined drilling into the excavation contract because of the schedule, so at the same time we’re drilling we’re excavating, and at that point we saw a lot of weathered clay materials,” Riker said. “It was at that point we saw a lot of signs that said ‘I don’t know if we can build RCC (roller-compacted concrete).”
Those findings led to the current redesign, which only calls for RCC materials in the more stable, center of the dam.
Though the majority of the cost increases revolve around the geological findings, CH2M Hill also determined the original Probable Maximum Flood (PMF) estimates from the Corps were apparently too low, meaning the design has to be reworked so the dam could withstand a potentially larger flood.
Engineers considered a longer spillway at first, but are now eyeing a “fuse plug” trap door on the top side of the dam, which would allow overflow water to escape to a secondary location in the event of a flood event.
“There was some conflicting [PMF] information in the Corps’ 1998 report, and the Corps couldn’t answer some of the questions we had,” Riker said. “So, we had to ask ourselves if we felt what we had was adequate and defensible to move forward, and we didn’t. So, we reworked it.”
CH2M Hill’s team determined the PMF to be approximately double the Corps.’s original estimate, though Riker said that number could possibly be lowered by 10-15 percent — resulting in some level of savings — with a localized study to determine a site-specific PMF.
“I would recommend some risk-informed decision-making ... and I think we should go through that process and see where we are,” he said. “I know the pain and anguish with this project, and we’re looking at how we can reduce this.”
The utilities board has also reached out to the Corps for assistance in confirming the PMF levels, and project manager Dale Greer said they may use approximately $18,000 in credit the city still has on the books dating back to the project’s genesis.
Trent Moore can be reached by e-mail at firstname.lastname@example.org, or by telephone at 734-2131, ext. 220.