The project involves widening 16km of highway along the congested Interstate 64 corridor to expand it from two to four lanes in each direction. Major activities include construction of two 2.4km-long tunnels between two artificial islands – currently served by immersed tunnels, island expansion, land and marine bridge replacements, and roadway.

The 14m-diameter machine is Herrenknecht’s largest variable density TBM to date. It will operate continuously in slurry mode, using variable-density slurry corresponding to the specific ground conditions.

Soft ground is anticipated throughout the entire drive (mixed-face conditions of sand, silt, and clay), with pressures up to 6 bar.

Depending on soil conditions, the TBM is expected to bore up to 15.25m per day. The deepest point will be about 45.7m below the waterline.

The accompanying slurry treatment plant can treat more than 3,000m3 of excavated material per hour (14,000 gallons per minute).

Together with the nearby Parallel Thimble Shoal Tunnel project, it is the first time a TBM has been used in the challenging soft-soil conditions in Hampton Roads. All 10 of the previous highway tunnels in the area were constructed as immersed tubes.

After completing its first bore from the HRBT South Island to the North Island, the TBM will turn around and bore a parallel tunnel back to the South Island. Tunnel excavation is expected to take about two-and-a-half years.

The project is taking place under live traffic, as Hampton Roads Connector Partners joint venture (HRCP) builds the marine trestles and other roadway structures, and the tunnel runs beneath an active navigation channel that services the US Navy, the Port of Virginia and recreation boats.

The HRBT Expansion Project says constructing a tunnel between artificial islands presents considerable logistical challenges for delivering the necessary supplies and removing excavated material from site. The more than 21,000 precast concrete tunnel segments will be delivered by barge, following casting by Technopref at a plant in Cape Charles, Virginia, 40km across the Chesapeake Bay. The excavated material will also be transported by barge, with trucks available if inclement weather interferes with marine transport.

HRCP, a joint venture consisting of Dragados USA, Vinci Construction, Flatiron Constructors, and Dodin Campenon Bernard, has the design-build contract for the US$3.9bn project. HDR and Mott MacDonald are the lead designers.

The TBM is named Mary, after Mary Jackson, a famous Hampton-born NASA engineer featured in the film Hidden Figures. The slurry treatment plant was manufactured in France by MS and is named Katherine Johnson in honor of the local NASA mathematician also featured in the Hidden Figures film.