More than 20,000 precast concrete segments power $267 million tunnel project in Ohio.
By Mason Nichols
Photos provided by CSI Tunnel Systems
From the smallest, most intricately designed pieces to massive components built at scale, precast concrete is the product of choice for nearly any imaginable project. In recent years, use of the versatile building material has become more widespread in the tunneling industry, where owners, engineers and general contractors require a solution that is durable, resilient and capable of being erected with speed and efficiency.
As cities across the United States. seek to reduce overflows from sewer systems into nearby creeks, rivers and other waterways during storm events, officials consistently have turned to precast concrete to produce effective results. Such is the case for the Lower Olentangy Tunnel (LOT) project currently underway in Columbus, Ohio. The work, which began in April 2021, entails the installation of 20,400 precast concrete segments as tunneling contractor Granite Construction builds a combined sewer overflow system in conjunction with local precast manufacturer CSI Tunnel Systems and a slew of additional partners.
LOTs of movement
According to Bob Rautenberg, project executive with Granite Construction, the LOT project calls for 17,000 linear feet of tunnel and requires significant movement of earth to achieve. Specifically, while mining in the urban Columbus area, which includes operations under highways, railways and rivers, the team will encounter both soft ground and weathered rock as they conduct their work.
“The biggest difficulty on this project is the ground itself, where conditions are always changing,” Rautenberg said. “Columbus is also known for its water, which we’ll contend with along with the sands and gravels. Our operators will have to stabilize the ground as we are mining so that we don’t cause any surface settlement or heave.”
He added that Granite Construction will place additives into the ground to assist with the stabilization efforts.
To install the associated precast concrete segments, Granite Construction is deploying a Herrenknecht earth pressure balance tunnel boring machine (TBM) designed and built specifically for the project. The TBM has a bore diameter of 14 feet, 7 inches, which is about 1 foot larger than the outside diameter of each ring that composes the tunnel. Deployment of the TBM is scheduled to take place in October 2022.
Once the TBM is assembled and ready to operate, Rautenberg said his team will begin work at Gowdy Field on the west side of the Olentangy River in Columbus. From this main site, Granite Construction will mine south approximately 5,000 feet – installing the precast rings that make up the tunnel along the way – then retrieve the TBM and perform the same operations to the north, mining another 12,000 feet.
A coordinated effort
Production of the precast concrete segments that make up each tunnel ring kicked off in May 2022. Six segments make up a ring with each ring weighing 26,000 pounds. Overall, the LOT project consists of 3,400 rings totaling more than 88 million pounds. With so much precast needed to be delivered to site, transportation logistics have been an important topic of conversation between CSI Tunnel Systems and Granite Construction.
“We have a very tight site with limited shipping hours available,” Rautenberg said. “A significant portion of our coordination efforts have centered on maximizing the time and space that we have available so that the offloading process is smooth and efficient.”
The plan is for Granite Construction to receive five to eight deliveries per day from CSI Tunnel Systems out of the company’s Macedonia, Ohio, plant, which is located roughly two hours from the project site. Each truck will contain two complete rings with shipments running from 2023 into the first quarter of 2024.
Hammering out these logistics is just one way collaboration has helped catalyze the LOT project. Reece Armitage, vice president of quality control and project relations for CSI Tunnel Systems, said that his team worked closely with the project owners and engineers to develop a tailored concrete mix design capable of meeting the needs of the tunnel.
“We’re running a highly designed concrete mix that uses cement, slag, silica fume, high-strength fiber reinforcement and a water-reducer admixture to maintain a low water/cement ratio,” he said. “The mix design gives us flowability for production along with segments that are reinforced from edge to edge. This results in less chipping and spalling and an extremely high-strength product.”
Leveraging structural fiber reinforcement in lieu of rebar cages is relatively new in the tunnel industry but is becoming more commonplace. When combined with other elements of the mix design, including a higher slag usage, the LOT will boast a life span of 100 years.
Getting it done
When the TBM goes underground at Gowdy Field, the installation process will be relatively straightforward – mine 5 feet of earth, build a precast ring, then repeat. However, the success of this approach is tied directly to the details involved. As Rautenberg said, the segments will be transported on segment cars with three segments on each car. A “quick unloader” on the TBM will pick up the segments and suspend them in the air, allowing the cars to be used for other operations. The segments then are set down, positioned onto a segment transporter and moved into the forward part of the machine to be erected.
The tunnel and precast rings will boast a 12-foot inside diameter. Rautenberg said that there also will be a 6-inch annulus around the bore of the tunnel and the outside edge of the segments. As Granite Construction performs the mining operation, the team will pump grout into that void to stabilize the segments and the adjacent ground.
According to Armitage, the installation process doesn’t only result in a resilient tunnel – it also keeps workers safe.
“Precast tunnel liners are becoming the preferred method because workers are always in a finished tunnel,” he said. “The TBM pushes off the existing tunnel to mine the 5 feet, then the precast pieces are installed, bolted and grouted into place – just like when constructing a building. As a result, the workers are always operating in a controlled environment.”
Armitage added that Granite Construction mandates that the design for projects like this be implemented in a manner that the overhead segments are installed last when building a ring. Workers always start with the bottom pieces first and then work their way up, helping ensure their safety.
As CSI Tunnel Systems manufactures segments for the LOT project through the latter part of 2023, it will take advantage of custom forms designed by its team specifically for this endeavor. And while the team had to make a few modifications to their plant – including the addition of a silo to help supply all the components needed for the mix design – they are well-equipped to handle the job, having completed more than 30 projects of similar size and scope in the past.
The relationship between CSI Tunnel Systems and Granite Construction has been key to the advancement of the project to date. Beyond coordinating logistics and collectively crafting a mix design that will support the CSO tunnel, the two teams also partnered closely to ensure that the precast components would be compatible with the TBM during erection.
Alongside the high-quality precast segments and experienced backgrounds of the major project partners involved, the work also will be spurred along by additional steps the team is taking in anticipation of the needs of the TBM.
“Plenty of maintenance on the machine will be required,” Rautenberg said. “The owner has specified 14 locations along the tunnel route that we’re calling ‘safe havens.’ These are jet-grouted ahead of time so that we can pull the TBM into them as needed and work on it under stabilized ground conditions.”
While much work remains before the LOT project wraps up in December 2026, many steps already have been taken to help set the tunnel work up for success. In addition to strong collaboration between Granite Construction, CSI Tunnel Systems and other partners, the use of precast will go a long way in benefiting the Columbus community and providing a long-lasting environmental impact to the surrounding area.
“This precast tunnel lining approach allows us to make a strong, durable product in a safe and controlled environment,” Armitage said. “Not only does this enable greater precision with manufacturing – it also allows us to deliver a solution that will serve the area well for the next 100 years.”
Mason Nichols is a Grand Rapids, Mich.-based writer and editor who has covered the precast concrete industry for nearly a decade.