Pumping and positioning massive structures is key in cofferdam construction.
When it comes to construction projects, it takes a lot to impress Brandon Stidham. Coming from a background in construction and currently a project manager for Johnson Bros. Corporation, he’s been around such work most of his life. But an ongoing job about 30 minutes from Paducah, Ky., certainly has his and his project team’s attention. There, walls for a series of huge cubical structures — “shells” they’re called on the project — are being formed and pumped on a barge, floated into position, lifted, positioned and lowered into place using a specialized gantry barge, then filled with concrete. When complete, the resultant structure will not only serve as a cofferdam for construction of a new lock system, it will also find life as one of the new lock’s main guide walls. But it’s the numbers behind the tasks that are truly worth noting: when lifted for placement, each shell weighs 1.4 million lbs., and more than 65,000 cu. yd. of concrete is being used on the project — 2,400 yards alone on an underwater foundation pour. And this is all being done with the existing lock remaining open to river traffic. How can one not be impressed?
Changing the Locks
Part of the Ohio River Navigation System, the lock at Grand Rivers, Ky., is a 600’ by 110’ structure designed to raise and lower vessels up to 75 feet between Kentucky Lake and the lower part of the Ohio river. According to Stidham, the lock, for decades more than adequate to handle the steady stream of commerce, has become outdated as barge traffic has grown in both physical size and volume.
“The existing lock is very busy, but it is also very inefficient — it might be the most backed up lock on the Ohio River system,” he said. “Since it was built, the size of barge tows has gotten so large that the tugs now have to come in and do a double — or sometimes triple — lockage. That is a time-consuming procedure that inevitably backs up the entire flow of lock traffic; waits of eight to ten hours are not unheard of.”
To alleviate that backup, the existing lock is slated for replacement by a $1.25 billion new structure which will be double in length at 1200’ long by 110’ wide. In advance of that, Johnson Bros. Corporation (a division of Southland Holdings, LLC, Roanoke, Tex.), working as the general contractor to the U.S. Army Corps of Engineers, Nashville District, is creating a cofferdam that will both provide a dry work area from which that new structure can take shape, and tie in to the future permanent lock wall.
Ten Cell Strength
Work on the downstream cofferdam, as the project is known, began in mid-2017. At the center of that effort is the creation of the ten shells that will make up the dam. Together, they will create a 483 foot-long concrete monolithic wall which will serve as one segment of the cofferdam. Getting to that point, however, is taking a mix of creative engineering prowess and solid equipment performance.
“The shells, each of which is 46.5’ wide and vary in length, are being constructed on a barge in the still-active waterway,” said Stidham. “We form it up in an area fairly close to shore to allow access by our concrete subcontractor, Edwards Concrete Construction (Evansville, Ind.). Thus far, they’ve used Schwing S 45 SX, S 46 SX and S 61 SX pumps on this job and, to ensure the best reach with every pump, we constructed some access roads for them. Mind you, we have pumps of our own and we do occasionally perform our own concrete pumping. But purchasing a unit, maintaining it, paying an operator and then have no need for it all at the end of the job doesn’t make a lot of sense for us. It’s much easier and more economical for us to sub out that facet of the job to someone for whom that’s their specialty.
Three of each shell’s four sides are 12-inches thick; the fourth side, which will ultimately serve as the lock-side wall, is 16” thick to accommodate all the steel and rebar within.
In fact, the walls were so dense with steel that getting concrete through it became something of a major challenge. While the first two cells were pumped in three separate lifts to get concrete up around the 35 feet level, each lift took eight hours to pour. According to Randy Edwards, owner of Edwards Concrete Construction, it was then that Johnson Bros. got creative.
“For the third cell, they actually found a way for us do the whole 35 foot pour at one time,” he said. “To avoid segregation of the concrete and keep within ACI specs, we used a tremie with a series of lay-flat hoses and shut-offs on the pump. Using 15 or 16 different hook-ups around the top of the shell, we would attach the tremie to each one of them, make the pour, then they would raise the hose up and cut it off. They had external vibrators working and we did the full 290 yards in about 9 to 11 hours. That really sped up the process, so I suspect we will probably do the same with the remaining seven shells. Having worked with Johnson Bros. before, I’ve learned that they can really come up with some innovative solutions.”
Filling the Gaps
With the walls poured and the frames stripped, a tow boat lines up the 1.3 million lb. structure for pickup by a gantry barge which then moves the concrete shell into position for set-down. On one of them, however, before it could be placed, a gap in the river bed below it needed to be addressed.
“There was a big rock solution feature, essentially a fairly large gap in the river’s floor, that we needed to pump full of concrete in order to level the foundation in advance of placing the shell atop it. Thus far we’ve only encountered one such feature, but it took 2,400 cubic yards of concrete to fill it. Unlike the shells, in which we have to pump much slower to avoid blowing out the form at the bottom, with this pour, we were free to get it done as quickly as we could. We were aiming for rates of 75 to 80 yards per hour (yph) but ended up getting 100 yph.”
With the gap filled, the riverbed was ready for placement of the shell. Because it has to tie in to all the other similar structures as well as to lock work that is still to come, placement was critical — in most cases demanding accuracies to within less than one-inch. “Casting these massive boxes on barges, is quite a feat in itself,” added Edwards. “But then to pick them up, float them in, and place them so accurately is pretty amazing.”
At the Center of it All
Once in place, the center section of each shell must be filled in order for the structure to become whole and stable. That process starts with placement of huge 3’ X 3’ X 6’ sand-filled “super-sacks” around the foundation perimeter. The sacks are used to keep the first concrete pour in place until it gains some strength and it seals the bottom.
“That first pour — about 300 cy of it that we tremied using the 61 m pump — flows against those sand bags and sets,” said Edwards. “We follow that up with a pair of a ten-foot lifts, each one about 800 yards and taking 10 to 12 hours. To date, we’ve completed two shells and it’s gone extremely well.”
When completely filled, the center of each shell will contain approximately 11 million lbs. of concrete.
So Many Mixes
Some of the mix designs at Kentucky Dam are as unique as the project itself, starting with a small rock #8 mix all the way up to a #57/#67 ¾” stone mix. According to Stidham, The Army Corps of Engineers has a preference for mixes with large size aggregate to enhance durability and assist with temperature control while the concrete is curing.
“But, obviously, large aggregate is not good for pumping, so it was sometimes challenging to come up with mixes with smaller aggregate that would meet their demands in all the other areas,” he said. “Because the smaller aggregate takes more cement paste, working with Federal Materials, the concrete supplier, we had to come up with some different admixtures to make sure that the designs didn’t exceed the Corps’ specs for heat allowed. Many of the smaller mixes that we are dealing with took months of trial batching to get right.”
The ¾ stone mix Stidham mentioned was used for pumping the 2,400 yard underwater solution feature. He said the design had to have a great deal of spread so that it could level out — in essence, almost a self-leveling mix. “We definitely wanted to avoid any humps when we were trying to set the shell on it.”
For areas of the job that in which that 1 ½-inch aggregate will be needed, Edwards is ready with a Schwing Model LB 33T4 Loop Belt truck-mounted conveyor which will provide 33 m (107’) of reach. In his quarter century of concrete pumping, Edwards said he’s seen his company undergo a world of changes, growing from just him with a single used 28m pump, to a company with 20-25 employees, 10 pumps, two trailer pumps and a conveyor truck.
“I love what I do and working on different projects like this, for outstanding companies like Johnson Bros., makes it all the better,” said Edwards.”
According to Stidham, the downstream cofferdam should be in place and functional by April 2020 with an estimated open date for the new, extended lock system (per U.S. Army Corps of Engineers) of 2024.
“Once construction is wrapped up, the Army Corps of Engineers will turn the facility over to the Tennessee Valley Authority which, even though they own it, will staff it with Army Corps personnel. Just another way this job is unique.”
Owners: Tennessee Valley Authority, Knoxville, Tenn.
Contracting Agency: U.S. Army Corps of Engineers, Nashville District
General Contractor: Johnson Bros. Corp., div. of Southland Holdings, LLC, Roanoke, Tex.
Pumping Contractor: Edwards Concrete Construction, Evansville, Ind..
Equipment: Schwing S 45 SX, S 46 SX and S 61 SX truck-mounted concrete pumps with placing booms; Schwing LB33 Loop Belt truck-mounted conveyor.