A Dam Site Better
With continued stability and performance in mind, Ameren Missouri’s Bagnell Dam gets a major overhaul.
Hydroelectric power has been providing energy to Americans since the late 19th century. It currently provides for nearly 7% of our overall energy needs, and represents roughly 35% of this country’s total renewable energy. While that longevity underscores its value as a proven, effective technology, it is also a reason many of today’s larger dams — some of which were built in the 1930s and ‘40s — are showing signs of aging and are in need of upgrade. One such structure, Ameren Missouri’s Bagnell Dam, recently underwent a $52 million overhaul designed to improve both strength and stability. To make that happen, ACI Concrete Placement dealt with a host of challenges enroute to pumping and placing more than 17,000 cu. yds. of concrete. The net result of their effort, and that of the other trades involved in the project, is confidence that the dam will provide residents in the “Show Me State” and beyond with electricity well into the next century.
Better Safe Than Sorry
Situated on Lake of the Ozarks — in fact, the reason for Lake of the Ozarks — Bagnell Dam is a half-mile in length and rises to a height of 148 feet. Completed in 1931, the 176 megawatt dam provides power to some 52,000 homes, and impounds the Osage River to create the iconic lake mentioned above. While still structurally sound, the decision to make the upgrades was strictly a preventive measure based on engineering reports and an eye toward future use. According to Mike Hartwig, project manager for MC Industrial (St. Louis, Mo.), the general contractor at Bagnell, ACI’s concrete pumping role included two main efforts.
“Bagnell Dam consists of a west retaining section, a spillway in the center with the power house, and an east retaining section,” he said. “On the west and east ends, we had ACI pour 13’-6” thick segments of concrete between the existing bridge piers which added mass to hold the dam down and prevent it from shifting. Also on the west and east sides of the structure, they poured an overlay of the downstream face which didn’t really have a structural element to it, but was instead done to remove old worn concrete and replace it with new for a more durable skin. Those pours ranged from 9” to 12” in thickness.”
A Perfect Fit
With offices in Columbia, Springfield and Kansas City, Mo., ACI Concrete Placement is one of the state’s pre-eminent suppliers of concrete pumping services. Formed in 2005, the company leans heavily toward commercial work but has also maintained a hold on residential accounts over the years. According to Kurt Wagner, the company’s Columbia office manager, the Bagnell Dam project, while not one of the biggest they’ve tackled, was still a decent size and a good fit for their fleet of pumps.
“Company-wide, we have some 20 Schwing concrete pumps, with five here in our Columbia office: two Schwing S 46 SX units, a pair of S 34 X pumps and an S 31 HT. At Bagnell Dam, we primarily used the 46 meter and 34 meter pumps, with the 34s doing the bulk of the work and the 46s taking over for the higher parts of the job.”
Wagner added that the 46 meter pumps were actually units they had converted from 47 meter pumps through Schwing’s Concrete Pump Repair (CPR) program. “Doing so allows us to be able to run 5-inch system all the way through, without having to taper down to 4 ½ inches at the end. We sacrifice three feet of reach off the boom, but to us that’s not a bad tradeoff.”
Pours Were Staggering
For work involving resurfacing the face of the dam, MC Industrial contracted with a hydrodemolition firm which used a robotic device with an ultra-high-pressure water jet to scale back three inches of the dam’s face. With that removed, MC-Industrial set scaffolding and forms for placement of 9-inches of new concrete.
“On the infill section of the project, it was decided that a Peri formwork system that the company developed while working on the Panama Canal, would also be a good fit for us,” said Hartwig. “It sits on a single ‘doorknob’ at the bottom that supports all the framing and braces for both the formwork and the scaffolding on the back side. Project specs stated that, for those infill pours, we were limited to six-foot lifts. So we set the formwork up at 12 foot heights so that we could pour a six-foot lift, break our forms, relieve the pressure, set them back, and pour again, then jump after we poured that second 6-foot lift.”
As mentioned, the sole purpose of the infill pours was to increase the weight of the structure, the science being that the additional 66 million pounds of new concrete throughout, coupled with the force of 67 newly installed anchors, will provide about 200 million pounds of strength pulling the dam down to the bedrock below.
Cool by Design
The mix design selected by MC Industrial was a 4,000 psi 60/40 slag-based mix for the infill pours which, because of its workability was also used on the overlay work. According to Hartwig, that decision was based on a need to keep down the heat hydration.
“By cutting out 60% of the cementitious material and replacing it with slag we accomplished that,” he said. “Doing so also helped prevent thermal cracking. Using that mix, we knew we could gain the strength the concrete would need to work with the forms and keep our production schedule.”
ACI’s Wagner added that some coarse aggregate in the design made pumping a challenge, albeit one they knew they could handle. On the overlay portion of the project, with the custom-built forms placed at an angle, however, his team noticed that trapped air rising to the surface was creating pock marks on the concrete.
“When we first stripped the forms we saw that bubbles had worked their way to the surface, leaving marks that were definitely not part of the plan,” said Wagner. “After inspection, MCI‘s engineers opted to use a form liner called Duoguard Formtex which holds the concrete in, but allows water and air to escape through the liner. After they added that, the surface was smooth and uniform moving forward.”
Just Out of Reach
Though the 46-meter pump was able to reach almost all the upper areas of the dam’s face, ACI did need to improvise to access the final corner of the structure. In most cases, they would have simply contacted either their Springfield or Kansas City offices for a larger boom. At Bagnell Dam that was not an option.
“We had a number of things working against us, foremost of which was the limited available work area,” said Wagner. “The ‘apron’ at the base of the dam was roughly 30-feet wide, just barely enough available area for the outriggers of the 46-meter pump — that took using a larger pump out of the discussion. And while access from Bagnell Boulevard that runs across the top of the dam was a possibility, that would have meant closing down the road which was also deemed not do-able. We had no recourse but to put about 30-foot of system onto the 46 meter pump, lay it out across some scaffolding and get the access we needed. But it actually worked out well and got the job done.”
“Show Me” the Coastline
Concrete for the project was provided by Ozark Ready Mix which had a batch plant set up about ten miles from the dam. That location was extremely convenient, given the unique geography of the area — Lake of the Ozarks covers 55,000 acres and reportedly has more coastline than the length of the state of California.
“We probably never needed more than seven or eight trucks working this job because the pours were slow and steady,” said Wagner. “In fact, our biggest pours were just over 200 yards — small compared to most projects. However, because the Bagnell jobsite is located on a river, there were some times when the yardage might not have been huge, but the days were long. It was not uncommon to do some pumping on one side of the river at the base of the dam, then have to clean out, move over to the other side of the base, set back up, and pump a segment there.”
In light of the work space constraints they faced on a regular basis, it is noteworthy that the Bagnell Dam project, slated for 18 months, actually wrapped up two months early. “At times it was very crowded out there,” said Wagner. “Between the demolition of the dam face going on, the anchors being drilled, ready-mix trucks coming in and out, and us setting pumps set up, tearing them down and moving them elsewhere on site, it must have looked like chaos. But everyone knew their roles, the pumps’ performance was impressive and we got it all done. It was a great job to be a part of.”
Owners: Ameren Missouri St. Louis, Mo.
General Contractor: MC Industrial, St. Louis, Mo.
Pumping Contractor: ACI Concrete Placement, Spring Hill, Kan.
Equipment: Schwing S 46 SX and S 34 X truck-mounted concrete pumps with placing booms.