The US Megaproject You've (Probably) Never Heard Of

TL;DR

The Hampton Roads Bridge-Tunnel in Virginia — the world's first bridge-tunnel — is undergoing a $4BN expansion using the world's largest variable-density TBM.

Key Points

• 1.The Hampton Roads Bridge-Tunnel was the world's first bridge-tunnel, built in 1957. It was designed to cross Norfolk Harbour without blocking Navy ships, using two artificial islands and immersed tube tunnel segments — a method where pre-built sections are floated out and sunk onto the harbour bed.
• 2.Virginia's Hampton Roads region holds a world record in immersed tube tunnel technology. The area has 10 immersed tube tunnels — the largest collection anywhere on Earth — and VDOT is considered a global leader in this construction method.
• 3.The current $4BN expansion adds two bored tunnels and widens the trestle bridges to bring total capacity to eight lanes. The project uses a 46-foot diameter, 430-foot long TBM named Mary after NASA mathematician Mary Jackson, the largest variable-density TBM ever fabricated.
• 4.Mary the TBM completed a rare 180-degree turnaround between the two tunnel bores using nitrogen skate technology. The manoeuvre used 16 nitrogen-infused hydraulic jacks to move the 2,500-tonne machine — a technique previously used only once before, on London's Silvertown Tunnel.
• 5.The project is on budget at just under $4BN — a rare feat for major US infrastructure. Funding comes from a local construction finance fund backed by regional sales and fuel taxes, and the cost is less than Canada's Gordie Howe Bridge despite including a tunnel.
• 6.The expansion is running roughly 18 months late, with completion pushed from late 2025 to early 2027. VDOT cited the design-builder's failure to meet projected production rates; however, the project is considered the largest infrastructure undertaking in Virginia's history, combining tunnelling, bridge building, marine construction, and road widening simultaneously.