More than 115 million vehicles have travelled over the Bridge-Tunnel since it opened in1964. In the 1990s, the existing trestle was twinned to help meet traffic demand and improve safety, increasing capacity from two to four lanes. However, a bottleneck exists with traffic restricted to single southbound and northbound lanes through the existing immersed tube tunnels.

The Chesapeake Bay Bridge and Tunnel District decided to augment capacity by creating a new tunnel under the Thimble Shoal Channel. Mott MacDonald was tasked with designing the new Parallel Thimble Shoal Tunnel that will consist of a new two-lane, 39ft diameter tunnel under the channel constructed between two existing artificial islands.

When complete, the new tunnel will reduce congestion during peak travel season and improve operational safety. The design addresses long-term durability and maintenance, meeting the required design life of 100 years for the structures.

To address these challenges, the design team used Bentley System's PLAXIS geotechnical finite element analysis software to create a 3D model of the immersed tube tunnel to conduct a comprehensive analysis of how the new design impacts the existing infrastructure.

The requirements for the 2,000ft model provided significant challenges, such as determining the construction sequence and consolidation analysis of a large finite element model. In addition, although bathymetry and geology for one of the islands was available, gINT was required to perform manual editing of geometries before PLAXIS could be used for meshing.

PLAXIS analytic capabilities helped demonstrate that the large-diameter tunnel could be constructed with minimal impact on the existing immersed tube tunnel and portal. In addition, the analysis demonstrated that the long-term performance requirements of the tunnel could be achieved despite the presence of soft clays. The unique approach to excavate a tunnel through fill under subaqueous conditions will help to create the largest undersea highway tunnel in the US. It will also see the first use of steel fibre reinforced concrete segments for highway tunnels in the US.

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