Overview
The Chesapeake Bay Bridge Tunnel is a 17.6-mile bridge and tunnel complex that spans the lower Chesapeake Bay, connecting Virginia’s Eastern Shore with the mainland in Virginia Beach. The Parallel Thimble Shoal Tunnel project involves constructing a two-lane bored tunnel from the CBBT’s southernmost island (#1 Island), under Thimble Shoal Channel, to #2 Island. Once completed, the new tunnel will carry two lanes of southbound traffic, while the existing tunnel will accommodate two lanes of northbound traffic. The new Parallel Thimble Shoal Tunnel will be the first transportation tunnel built in the Mid-Atlantic region using a tunnel boring machine (TBM).

Scope of Work
The TBM, Chessie, was manufactured by Herrenknecht of Germany and is an Earth Pressure Balance TBM. It measures 308 feet in length and has a cutter head size of 43.5 feet in diameter. In February 2023, Chessie began the excavation of the new tunnel. However, after advancing approximately 700 feet from the launch pit, Chessie encountered an unknown obstacle beneath the Bay bottom. Pieces of steel, identified as parts of an old anchor manufactured by W. L. Byers & Co., were found on the TBM’s conveyor belt, halting tunneling progress. While it is not uncommon to encounter obstructions during tunneling, this discovery of a large anchor suspended tunneling. From June 2023 to early December 2023, Chesapeake Tunnel Joint Venture (CTJV) developed a plan to construct a safe haven into which the TBM could be driven for a maintenance intervention. This plan included performing ground improvements by injecting foam grout to seal the area around the cutterhead and between the cutterhead and the safe haven, preventing potential cave-ins or blow-outs. Grouting the ground in front of the TBM pressurized the excavation chamber, preventing water and sand from flowing into it.

Innovative Solutions
With air-lock chambers, bulkheads at the front of the TBM allowed Ballard compressed air workers to access the cutterhead for inspection, remove any remaining anchor debris, and replace any damaged tools. All of this work was completed underground, under the seabed, in front of the TBM. After removing the anchor pieces, CTJV conducted a full inspection of the cutterhead, determining that 389 of the 442 tools on the cutterhead needed replacement. Each tool weighs approximately 150 pounds. Additionally, 48 wear plates were replaced. A comprehensive inspection was also performed on the mechanical and drive systems to ensure that no further damage was done to the tunnel boring machine.

Results
The Ballard team has completed 6 rounds of Interventions and 460 compressed air dives and will continue to support the project throughout its lifecycle.