Airplane travel in the United States has risen dramatically over the last decade. Although there was a dip in daily flights due to the pandemic in the early 2020s, the United States' daily flights have increased from 23,911 in 2015 to 45,000 in 2024.
The tremendous increase in flights means airports must adjust to continue meeting the needs of the traveling public. Chicago's O'Hare is the second busiest airport in the United States.
The Chicago Department of Aviation (CDA) created O'Hare 21, a decade-long capital improvement program. O'Hare 21 is CDA's "…vision for a modern airport that will be an efficient and accessible international gateway… reaffirming O'Hare's status as a premier international hub while meeting the evolving needs of the traveling public through the 21st century and beyond."
Andy Ryan Photography, courtesy of HNTB photo
A significant part of this modernization program is taking place at O'Hare that airport passengers can't even see. The South Conveyance Project is a major stormwater tunnel project that requires considerable engineering feats.
Location, Location, Location
The purpose of the project is to redirect stormwater runoff intended to be stored within the Central Detention Basin to the South Detention Basin. The Central Detention Basin is in fine working order; however, its location is an issue.
O'Hare 21 envisions using the area where the Central Detention Basin is in a different capacity. Two new satellite terminals, which will include more than 33 new gates, will be installed where the Central Detention Basin is currently. The overall capacity also will be expanded in the area.
The project will convey the stormwater by constructing an 8,000-ft. long and 14.5 ft. in diameter tunnel across the airfield. It will collect water from local, near-surface storm sewers and divert it downstream to the South Detention Basin, which also will be expanded to handle the increase in stormwater. Finally, the central basin is being backfilled.
Credit HNTB photo
The conveyance project is taking place in the heart of the busy airport. The 1.5-mi. tunnel will be approximately 50 ft. below ground and will extend from the airport core to the south side of the airfield, or nearby Terminal One, to the South Detention Basin crossing under two active runways.
"Working adjacent to operating terminals and beneath active runways presents unique challenges that require precision and coordination," said Mark Stephani, principal engineer: geotechnical and tunneling of HNTB.
The firm is overseeing the engineering and serves as the engineer-of-record for the project.
"Thanks to meticulous planning and collaboration, we've been able to maintain airport operations seamlessly while advancing construction," he said.
The team is nearly finished with tunnel construction, with completion expected in the Fall of 2025. The crews are working 24 hours, seven days a week, to meet the schedule and enable future planned projects to proceed.
Underground Work
One of the first steps that HNTB undertook was to implement a comprehensive and thorough subsurface investigation to identify and quantify the subsurface risks as well as specify the proper methodology for tunnel construction.
This enabled the team to understand the ground behavior and subsurface risks and mitigate potential impacts associated with underground construction.
Credit HNTB photo
Stephani noted that while underground construction naturally involves uncertainties, the team's proactive approach to identifying subsurface conditions significantly reduced potential risks — ensuring both safety and constructability throughout the project.
The results of the investigation are interpreted within a Geotechnical Baseline Report.
"The report tells the contractor what to expect during tunnel construction and helps serve as a methods of risk sharing between the contractor and CDA," Stephani said.
The report also helped the team determine the ideal design of the tunnel boring machine (TBM).
German Manufacturer Herrenknecht AG designed the 400-ft.-long TBM, and it was later refurbished in Michigan before mobilization to O'Hare. The TBM, known as an earth pressure balance machine, is designed specifically to handle soil conditions containing water under pressure. The working area inside the TBM is completely sealed against the fluid pressure of the groundwater. This design minimizes settlement and reduces the risk of ground failure.
The team launched the TBM from within the South Detention Basin.
"Throughout construction, we've had to ensure uninterrupted operation of the South Detention Basin, which can hold stormwater depths of up to 45 feet," Stephani explained. "To protect the tunnel and crews from potential flooding, we engineered and installed a 115-foot-long king pile cofferdam within the basin.
"This inventive solution was carefully sized to provide ample working space for launching the tunnel boring machine while keeping workers and equipment safe and dry — all without disrupting the basin's critical stormwater management role."
Delivery Method Provides Benefits
The team is delivering the project using the Construction Manager at Risk (CMAR) method. When using the CMAR method, the construction manager is involved during the design phase, allowing them to provide more input than in other delivery methods.
Andy Ryan Photography, courtesy of HNTB photo
Turner Paschen Aviation Partners is the CMAR on this project. Other team members include Jay Dee Contractors (a specialist in large-diameter tunneling) and K-Five Reyes, a longtime O'Hare contractor.
Stephani noted that CMAR encourages collaboration between engineers and contractors and was crucial on this project.
"Early on, we were thinking about using trenchless technology for the project. However, after consulting with the construction team, we turned to the TBM idea, which minimizes risks and the impacts on the current infrastructure."
CDA has a vision for O'Hare, enabling the airport to continue accommodating the growing crowds. The airport authority is working to create and maintain modern, world-class facilities at the airport. While people are often looking up at the airport, work must first happen underground to make it possible. Hence, the importance of the South Conveyance Project. CEG
