ABSTRACT
The Parallel Thimble Shoal Tunnel (PTST) consists in the duplication of the existing Thimble Tunnel part of the Chesapeake Bay Bridge Tunnel (CBBT) district. The 1.9 km new tunnel starts at the southernmost Portal Island, PI#1, and will emerge at northern PI#2, using a 13.25 m diameter Earth Pressure Balance Tunnel Boring Machine (EPB-TBM), named Chessie. TBM Chessie was successfully launched in February 2023, making good progress under the artificial island and man-made scour protection Berm on PI#1, until an obstruction was intercepted three months later. The obstruction was identified as an abandoned ship anchor weighing approximately 8 to 9 Tn, based on the parts extracted with the screw conveyor, along with cutting tools sheared off from the cutterhead of the TBM. Operational parameters were drastically impacted, and wear detection sensors were also reporting damages to the cutterhead.
Two hyperbaric interventions were attempted with limited actual working time due to excessive air losses. The second intervention identified a large section of the anchor located right in front of the central portion of the cutterhead. Using a temporary trestle, part of the marine enabling works, a working platform was installed to accommodate a jet grout operation in front of the TBM. Columns with different jet grout techniques and diameters would be installed to create a safe haven, allowing safe retrieval of the remaining parts of the anchor and facilitating the necessary cutterhead inspection and repair, under the sea.
This paper covers the operations since the first hyperbaric intervention post-obstruction until the full repair on the cutter head in atmospheric conditions.
