The Chesapeake Bay Bridge-Tunnel carries US 13, the main north-south thoroughfare on Virginia’s eastern shore, across the Chesapeake Bay to Virginia Beach. At almost 23 miles, it is considered the world’s largest bridge-tunnel complex, with 12 miles of low-level trestle, two one-mile tunnels, four artificial islands, two bridges, and five-and-a-half miles of approach roads.
Shortly after opening in 1964, the Bridge-Tunnel was selected as one of the Seven Engineering Wonders of the Modern World. More than 115 million vehicles have traveled over the Chesapeake Bay Bridge-Tunnel in its first 50 years. In the 1990s, the existing trestle was twinned to help meet traffic demand and improve safety, increasing capacity from two lanes 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, 39-foot-in-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 PLAXIS 3D 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 2,000-foot 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 United States. It will also see the first use of steel fiber reinforced concrete segments for highway tunnels in the United States.
One of the Seven Engineering Wonders of the Modern World continues to advance infrastructure, and augment capacity by creating a new tunnel under the Thimble Shoal Channel. Image courtesy of Mott MacDonald
The largest undersea highway tunnel of this kind in the U.S., used PLAXIS to model and analyze the unique approach to excavate tunnel through fill under subaqueous conditions. Image courtesy of Mott MacDonald