New geotechnical information gathered during reconstruction of Interstate 40 through the Pigeon River Gorge has provided engineers a more complete picture of conditions beneath the roadway, resulting in revised cost and schedule estimates for the largest reconstruction project in N.C. Department of Transportation history.
Based on the information available today, NCDOT’s current estimated cost to reconstruct the corridor is approximately $2.8 billion, compared with an approximately $2 billion estimate earlier this year. The Department currently estimates restoring four lanes of traffic by late 2029 and continues to evaluate opportunities to accelerate that timeline.
Cost and timeline estimates reflect the project team’s current understanding of the work required and will continue to be refined as design and construction advance and additional information becomes available.
Drivers of cost and schedule
As site access improves, crews have been able to test from the causeway every six feet instead of the interstate level to determine depth from the surface to bedrock.
In some places along the 5-mile-long worksite, the bedrock is exposed. In others, it’s more than 50 feet below the surface. The detailed information received from the samples taken at river elevation show the depth to bedrock is more than samples previously taken at the interstate level. Reaching the deeper bedrock requires longer steel piles, which has increased the estimated cost and timeline to complete these repairs.
“This is the most complex reconstruction NCDOT has ever conducted, and we remain focused on delivering the safest, most durable solution while carefully managing taxpayer dollars,” N.C. Transportation Secretary Daniel Johnson said. “We are rebuilding while we continue to investigate and design, which means our understanding of the work becomes more precise as construction advances. We will continue to be transparent as those estimates evolve, while looking for every opportunity to reduce costs and restore four lanes of traffic as quickly as possible without compromising the long-term resilience of this critical interstate.”
Rebuilding for Long-Term Resilience
Hurricane Helene destroyed or severely damaged approximately five miles of I-40 in North Carolina through the Pigeon River Gorge, where the interstate is constrained by steep mountainsides and the Pigeon River.
As planned, the reconstruction is extremely robust. The combination of roller-compacted concrete walls, interlocking pipe pile walls and associated elements are designed to withstand forces that were exerted by the Pigeon River during Hurricane Helene. It is intended to make the interstate more resilient to the hydraulic forces associated with future extreme events.
“History shows us that I-40 in the Gorge is vulnerable to flooding and failure, and not addressing the vulnerabilities now would result in the same damages at some point in the future,” said Division 14 Engineer Wesley Grindstaff, who oversees highway operations in the 10 westernmost counties. “The cost and consequence of slope failure is very high along this critical transportation corridor.”
NCDOT and its project team evaluated other construction approaches, including tunnelling, a viaduct, new alignment, rip-rap, pile-panel walls and secant walls. Each failed to meet the needs of either constructability, schedule, federal reimbursement criteria or the ability to maintain traffic during construction.
“Our team continues to examine opportunities to expedite schedules and minimize costs within the design, construction methods, materials and sequencing without compromising the long-term resilience of the interstate,” Grindstaff said. “We are reconstructing it to withstand future extreme weather and reduce the risk of another catastrophic failure.”
Building While Designing
NCDOT is using the Construction Manager/General Contractor, or CMGC, delivery method for the reconstruction, allowing design, engineering and construction activities to advance concurrently.
That approach has been particularly important following Hurricane Helene, as it allowed NCDOT to restore interstate access quickly while rebuilding it.
Early cost and schedule estimates were based on the best information available while data collection and design were in the early stages. The contract process allowed behind-the-scenes work to advance while some construction operations were underway. Those operations — from soil-nail wall support, causeway creation, loose material excavation and others — provided new opportunities to obtain new and more precise information to inform design plans.
Experts from every discipline of engineering have been involved, working collaboratively instead of one after another like most projects. Each discipline overlaps with others — leadership, hydraulics, geotechnical, geomorphological, environmental, structural — creating an incredible team of engineers.
Under the CMGC process, NCDOT selected Ames Construction following a competitive procurement. Ames participates in the design process and provides real-time input on constructability, pricing, and scheduling.
The early input from Ames also helps manage risk, lower cost, and accelerates construction timelines compared to traditional bid-build contracts.
“Our responsibility is to make the best decisions we can with the information available and adjust as better information becomes available,” Grindstaff said. “We will continue looking for opportunities to save time and money, but our primary commitment is to build this interstate to withstand the same type of event in the future.”