Precast on Tap

By Jerry Soverinsky

Opening a faucet is a lot like magic: Lift a handle, turn a knob or push a button and you have instant access to clean drinking water. It’s a benefit that’s often taken for granted, and, for many people, what makes it possible is of little consequence. But the engineering that allows water to move from source to drinking glass is anything but ordinary.

For more than a decade, residents of Gary, Ind., and surrounding communities have relied on a water intake crib located approximately 1 mile off the shore of Lake Michigan for potable water. Operated by Indiana-American Water from its Borman Park Treatment Plant, the crib – which is attached to the lakebed – helps deliver nearly 40 million gallons of water each day to area residents.

About 50 feet below the surface of the lake, water enters an 8-foot-diameter vertical shaft that extends down another 150 feet before being passed through a 15,500-foot-long horizontal tunnel. It then travels up to ground level, where it is processed by Indiana-American Water’s water treatment facility and distributed to nearly 70,000 customers.

This long journey would not be possible without the crib, which functions as a filter by diverting lake bed sand and sediment away from the intake and preventing larger debris from entering the tunnel. However, the original wooden crib had somehow been damaged, resulting in half the structure being sheared off. As a result, Indiana-American Water commissioned a design-build project in conjunction with McMillen Jacobs Associates and Ballard Marine Construction to provide a new, more resilient crib.

Indiana-American Water tasked the team with finding a solution to the many challenges that face a structure submerged in water. The answer quickly became apparent: precast concrete.

Many considerations, one ideal building material

Although the design-build team initially considered rehabilitating the structure with wood, Grant Horeczy, P.E., senior structural engineer for McMillen Jacobs Associates, explained it wasn’t a viable solution for several reasons.

“An important part of this process was the decision to use the existing foundation,” he said. “Precast concrete allowed us to use the inherent self-weight of large concrete members to resist wave and impact loads.”

Horeczy added precast was also preferable over wood because it doesn’t require the use of ballast stone, which is crucial to counteract wood’s buoyancy. It’s also far easier to remove the lake’s ubiquitous zebra mussels – an ongoing nuisance in the area – from precast concrete.

“The zebra mussels in Lake Michigan tend to grow on just about anything,” said Trent Nedens, general manager of Midwest operations for Ballard Marine Construction. “But they love things that are porous most of all. So they attach to wood very, very quickly.”

Nedens stressed the use of precast would also enable the structure to counteract frazil ice formation, which can occur when water at very low temperatures moves at a high velocity. If frazil ice is allowed to form on the crib, it can choke off the intake, limiting or preventing water from entering the treatment plant in the winter.

“We needed a material that has some of the insulative properties of wood, but with the negative buoyancy of steel, and that’s really what led us to precast concrete,” he said.

After the design-build team decided on precast concrete, they engineered the new water crib. Horeczy said the final design included a perimeter ring and additional components that create “table legs” to support a cover over the intake shaft. The team partnered with Wieser Concrete Products of Portage, Wis., to manufacture the 17 precast pieces for the project.

“Precast concrete is easy to install, and if anything ever hits it and damages the structure, the pieces can be easily duplicated since they were made from steel molds,” said Mark Wieser, vice president of Wieser Concrete Products.

Beneath the surface

The design-build team specified that divers assemble the structure underwater – rather than pre-assembling it and floating it out – due to concrete’s physical properties. Thanks to a dry run assembly at Wieser Concrete’s plant, the team was well-prepared for the installation, which took place in July. Setting the precast required that divers work 50 feet underwater, placing each piece precisely on top of the existing foundation. To accomplish that, BMC used a self-centering steel jib.

Using surface-supplied equipment, BMC’s certified commercial divers performed the installation by working in pairs. Each pair worked 2.5-hour shifts. The divers, who had communication and video equipment attached to their helmets, worked closely with the crane operator and dive supervisor on the surface to ensure all of the individual components were placed correctly. Once the precast units were in proper position, divers employed a series of specialized underwater tools to pull them together. They then secured the water crib to the existing foundation.

Finally, the crew installed PVC piping around the perimeter of the roof, which disperses chlorine into the crib area.

“That helps to deter zebra mussels from growing on the crib as well,” Nedens said. “So not only do we have the ability to clean them off the crib very easily, we also have the ability to keep them from growing in the first place by dispersing chlorine.”

A clear-cut winner

Engineering, designing and installing the water crib may not have been as simple as turning on a faucet, but thanks to the use of precast concrete, residents in northwest Indiana can continue to enjoy the benefits of clean drinking water for years to come.

“Precast concrete was a clear-cut winner among the alternatives explored,” Horeczy said. “Simply put, it allowed us to easily make the shapes we desired with a quality that could be verified prior to installation.”

Jerry Soverinsky is a freelance writer and journalist who covers more than a dozen industries.