Parallel Thimble Shoal Tunnel
Project Overview
Opportunity
Completed in 1964, the Chesapeake Bay Bridge-Tunnel crosses the mouth of Chesapeake Bay, an environmentally and economically significant watershed. The four-lane crossing consists of a series of low-level trestles interrupted by two tunnels, each about one mile (1.6 kilometers) long, beneath the Thimble Shoal Channel and Chesapeake Channel.
To provide an easier crossing for an increasing number of motorists, the Chesapeake Bay Bridge and Tunnel Commission decided to create a new bored tunnel between two artificial portal islands in Chesapeake Bay. Maintaining strict state and federal environmental standards was a priority.
Solution
As part of a design-build joint venture created by Dragados USA and the Schiavone Construction Company, Mott MacDonald was retained as the lead designer for the Parallel Thimble Shoal Tunnel, which will run alongside one of the two existing tunnels. We are responsible for obtaining necessary permits and documenting compliance with NEPA.
A tunnel boring machine (TBM) will be assembled inside a large pit and will excavate the tunnel from Island 1 to Island 2, a distance of about one mile. The TBM will install precast concrete tunnel segments as it proceeds, and will be disassembled and removed after it reaches Island 2. Because the environmental challenges of large-diameter tunneling are new to local authorities, Mott MacDonald has helped describe the technology in a clear and timely fashion.
Outcome
The tunnel boring machine, named Chessie, began her initial excavation at the tunnel’s headwall in February 2023. As of fall 2023, Chessie had installed more than 100 rings and advanced more than 700 feet from the launch pit.
Clear communication has helped keep the project on schedule. Mott MacDonald is exploring ways to reduce the impact of pile driving on seals and other marine mammals, and on endangered species such as sturgeon.
The new parallel tunnel will reduce the number and total hours of lane closures caused by accidents, disabled vehicles, or other interruptions. It will provide operational redundancy and improved levels of service, giving new life to a landmark of engineering.