Heavy Lifting: Precast Shells Require World’s Largest Equipment

U.S. Army Corps of Engineers lifts and transports heavy precast concrete shells for one-of-a-kind Ohio River lock and dam project with the largest gantry crane, catamaran barge and strandjacks in the world.

By William Atkinson
All photos courtesy of The U.S. Army Corps of Engineers, Louisville District.

You have to see this dam to believe it: the view of a new, one-of-a-kind navigation project being built on the Ohio River near Olmsted, Ill., is, to say the least, spectacular. Large precast concrete shells are built on the shoreline at a specially created concrete batching plant, precast concrete facility and storage yard. Precast shells are lifted and carried by a  heavy-duty gantry crane to a ramp that transports them down into the river, where a catamaran barge lifts the shells and floats them to their final location in the river. Once in place, the space under the shells is filled with tremie concrete. Then, more concrete and steel structures are built atop the underwater precast shells. Impressive? What is even more impressive is the size and scope of the project, including 4,700-ton (4,260-tonnes) precast shells and the lifting capacity of the equipment, some of which is the largest in the world.

Maximizing American’s inland shipping
Built during the 1920s, a number of crumbling locks and dams on the Ohio River between Pittsburgh, Pa., and the Ohio’s convergence with the Mississippi at Cairo, Ill., have been replaced. Just upriver from Cairo are Locks and Dam 52 and 53, built in 1929 and currently being replaced by the new Olmsted Dam. This $2.1 billion project consists of two 110-ft (34-m) x 1,200-ft (365-m) locks; a dam comprised of five tainter gates1; 140 boat-operated wickets2 that total 1,400 ft (427 m) in length; and a fixed weir. The total structure will be 2,700-ft (823-m) long.

The U.S. Army Corps of Engineers (Corps), the project owner, awarded a $564 million building contract to a joint venture comprised of the Washington Group International and Alberici Constructors (known as WGA).

This strategic section of the Ohio River is the confluence of the Ohio, Tennessee, Cumberland and Mississippi rivers. More water-borne tonnage passes this point than at any other place in America’s inland navigation system. Once Olmsted Dam is complete in 2015 and operational in 2016, it will allow tows to pass through this busy stretch of river in one hour, rather than the current five hours. The Corps estimates this project will produce economic benefits to the nation of more than $440 million annually.

In-the-wet construction: uninterrupted shipping
Olmsted Dam features a new method of construction that will not obstruct the flow of the river during construction. After studying several dam-building methods, the Corps decided to take advantage of in-the-wet (underwater) construction rather than the more traditional dry-cofferdam method. With this underwater approach, precast shell sections are transported while submerged in the river.

About 3,300 pipe piles, 24 in. (610 mm) in diameter, will eventually be driven for the entire dam foundation. Prior to final placement, the precast shell foundation is further prepared by grading the riverbed to fit the shell’s geometry, using an aqua digger (a barge-mounted Komatsu PC3000). Once a shell’s foundation work is complete, the final underwater installation begins. A catamaran barge, described in subsequent text, moves the submerged precast shell into position for installation.

Heavy precast shells moved with largest equipment
Large precast concrete shells are built on site at a specially created concrete batching plant, precast facility and storage yard. The largest shells measure 100 ft (30 m) x 125 ft (38 m) and are 3-ft (0.9-m) thick. The smallest are 75 ft (23 m) x 100 ft and 2.5-ft (0.8-m) thick. Shells, including the lift frame, weigh anywhere between 3,000 tons (2,720 tonnes) to almost 5,000 tons (4,535 tonnes).

Before lifting a shell from its casting bed, a lifting frame is attached to the shell; the frame distributes the line loads from the lift points over the entire shell. Without the frames, the relatively thin shells (about 2.4-ft [0.76-m] thick) could not be hoisted without damage. The lifting frame also serves as a work platform once the shell is installed in the river. A typical lift frame is 75-ft (23-m) long, 100-ft (30-m) wide and 45-ft (13.7 m) high.

After the lifting frame is attached to the shell, the shell is moved to the top of a marine skidway by crane; this  $9.5-million Super Gantry Crane, the largest of its kind in North America, can lift up to 5,304 tons (4,810 tonnes). The crane is wheelmounted, travels on steel rails and is capable of moving the lifting frames perpendicular to the rails. A self-contained piece of machinery, the crane operates off a generator. The top of the crane supports twelve strandjacks3 that are the primary lifting mechanisms for the precast shells; each strandjack costs about $240,000. The crane’s uppermost beams, known as strandjack beams, can be hydraulically adjusted to various configurations.

The crane sets the shell on a wedge-shaped steel structure with wheels, used to transport the shell down the skidway to the catamaran barge at the river. The wedge shape of the $4-million Cradle Transport Equipment System (CTE) keeps the shell horizontal while slowing movement down the 8% slope. When transporting the loaded cradle on the upper part of the skidway (in-the-dry), the CTE is attached directly to the cradle. When the cradle is to be lowered into the river, it is locked in position on the skidway (at a point just prior to being fully submerged) and disconnected from the CTE. The CTE is then moved up the skidway so that 500-ft (152-m) long pendant extensions can be installed between the CTE and the cradle. The pendant extensions ensure that no hydraulic or electrical components of the CTE become submerged while the cradle is being spotted on the skidway for loading or unloading by the catamaran barge.

Catamaran barge largest of its kind
When the cradle reaches the end of the skidway, a $19-million catamaran barge, with a maximum capacity of 4,500 tons (4,082 tonnes), moves into position. The lifting frame is attached to the barge and raised off the cradle from the barge. A towboat then pushes the catamaran barge so that the precast shell may set down in its permanent position in the river.

The Super Gantry Crane and the catamaran barge both utilize strandjacks to raise and lower loads. Loads vary from the lighter individual lifting frames (up to 900 tons [816 tonnes]) to complete precast concrete shells and their associated lift frames mounted on top (up to 4,900 tons [4,444 tonnes]). The gantry crane utilizes 10 strandjacks, each with a working capacity of 1,100 tons (998 tonnes).

As the catamaran barge moves the precast shell out into the river, the shell remains submerged in about 15 ft (4.6 m) of water. In places, the shells are lowered as deep as 30 ft (9.1 m) below  the water’s surface. Once a shell is in place on the riverbed, the void between the prepared pile foundation and the underside of the shell is filled using a tremie process, wherein fresh concrete is dropped through pipes.

Stilling basin and sill shells (large, flat shells) are set first, and then the lower pier shells are placed on top of the sill shells to support the tainter gates. Finally, the upper pier is cast in place. Large steel gates are installed to finish the assembly. This sequence is repeated until the Olmsted Dam Project spans the river using a total of 32 precast concrete shells. While the concrete batch plant and precast facility operate year-round casting the shells, actual river placement  typically takes place during the warmer months the when river is at optimal level for construction.

Project’s huge scale is the biggest challenge
According to William J. Gilmour III, P.E., the resident engineer for the Olmsted Dam at the Louisville District of the Corps, plans call for six seasons of setting precast shells, beginning in 2010 to project completion in 2015. “We are planning to set six shells this season,” says Gilmour.“ Three have already been completed and three more are under construction.”

”The biggest challenge is the size of the project. Everything is so big,” explains Gilmour. “For example, the specially designed Super Gantry Crane that picks up the shells is the largest in world. The catamaran barge is also the largest in the world. The strand-jacks that sit on top of the super gantry and pick the load are the largest ones ever built.”

People who visit the site to see the project in action are in awe of the project’s huge scale. “When visitors see that the world’s largest crane is going to pick up a piece of concrete 125 ft × 100 ft, and 2-to-3-ft (0.6-to-0.9-m) thick, they realize this isn’t your normal precast job,” says Gilmour. Despite the great size and weight of the dam’s structures and equipment, the project’s safety record is outstanding, Gilmour proudly reports. “We have had a few injuries, but no one has been seriously hurt to date.”

Precast yard manager explains shell production
When the Corps issued the bid specifications, it envisioned two separate concrete batch plants: one on the river and one on shore. WGA, however, proposed one batching plant that could supply both the precast operation and the fresh concrete needed for the tremie process.

“The precast production and storage yard were constructed in 2006,” says Robert Wheeler, precast yard operations manager for URS (URS Corp. purchased Washington Group after the contract was let). Wheeler explains that the precast facility is not NPCA certified, because, technically, it is not a typical precast operation. “We don’t do typical pretensioned beams, prestressed panels and so on. Rather, we build 3,000 ton to 5,000 ton (2,700 to 4,500 tonnes) blocks. We have 1,000 to 1,500 yd3 (765 to 1,147 m3) of concrete in each of the shells,” said Wheeler.

Intricate shell structure
Wheeler explains that the structure of the precast shells is very intricate and complex:

• “The shells have deep beams; all of the beams have soffits and very complex forming, because the Corps wants the tremie underneath to fill in a certain way.”
• Drill collars are embedded into the shells. “There is a rod that screws into the collar and it is bolted to the shell in order to lift it.”
• “We have a lot of holes in the shell where we put pipes for the tremie concrete once the shell is set in the river. The shell is filled from the bottom of the river up to the bottom of the shell.”
• “There is an incredible amount of rebar. In fact, the rebar is so incredibly dense that WGA had to model the shells in 3-D to resolve the interferences. Once we modeled the rebar, we found more than 2,000 interferences per shell that we had to resolve before we could begin construction.” (See article on 3-D modeling in the precast industry in Precast Solutions Spring 2010 issue).

2-foot-thick precast shells: strength, hydration and curing
In terms of mix design, the precast shells are not particularly super high strength. “We use a fairly typical 5,000 psi (35 MPa) concrete,” states Wheeler. Controlling hydration temperatures, which is a critical concern in mass-concrete construction projects, is no problem for Wheeler. “Since the shells are only about 2-ft (0.6-m) thick, we’re not talking about a deep piece of concrete, so we don’t have to put in thermistors to measure temperature.”

Spray-cure is not used. “We water-cure everything with burlap, the old-fashioned way,” says Wheeler. “We are placing concrete in a range of 56 F to 75 F (13 C to 24 C) and we use ice chips from the batch plant’s ice house to maintain proper curing temperature. Fresh tremie concrete has to go in the river at no more than 65 F (18 C), so we have to keep everything cool. We try to keep our curing temperatures under 100 F (38 C).”

Wheeler admits that managing a one-of-a-kind project is a challenge: “A typical day? We do work long hours. A dam like this has never been built before. Every day, we invent something new and on a scale that has never been done before. On the other hand, we are never bored and the days fly by!”

Bill Atkinson, Carterville, Ill., is a freelance writer who covers business and safety issues.

1 A tainter gate is a spillway gate whose face is a section of a cylinder and rotates about a horizontal axis on the
downstream end of the gate and can be closed under its own weight; also known as radial gate.

2 A wicket is a smaller gate or door that is part of a larger gate or dam assembly.

3 Strandjacks are jacks used to lift very heavy loads for construction and engineering purposes. Capable of lifting loads of well over 22,000 tons (20,000 tonnes), strandjacks were invented in Europe in the 1970s as a development of  precast concrete post-tensioning-cable anchoring systems.

Check out installation videos at www.youtube.com/louisvilleUSACE