In the US, TBM assembly is under way on the Hampton Roads-Bridge Tunnel Expansion Project (HRBT) – the largest construction project ever awarded in the state of Virginia.
Tunneling with the 46ft-diameter (14m) Herrenknecht variable density machine is expected to get underway this Spring. The work involves constructing twin parallel tunnels between two artificial islands.
The HRBT project involves widening 10 miles (16km) of highway along the congested Interstate 64 (I-64) corridor to expand it from two to four lanes in each direction.
The project is being developed by the Virginia Department of Transportation (VDOT).
Major activities on the HRBT projects include construction of two 1.5 mile-long (2.4km) tunnels between two artificial islands. The islands are currently connected by a series of links and measures – immersed tube tunnels )very common for the prior transport connections in the area), island expansion, land and marine bridge replacements, and roadway.
Soft ground is anticipated throughout the entire drive (mixed-face conditions of sand, silt, and clay), with pressures up to 6 bar. The deepest point of the tunnel alignment will be about 150ft (45.7m) below the waterline.
The Herrenknecht machine is the manufacturer’s largest variable density TBM to date. It is to operate continuously in slurry mode, using variable-density slurry corresponding to the specific ground conditions.
Depending on the soil conditions, the TBM is expected to achieve progress rates of boring up to 50ft (15.25m) per day.
The slurry treatment plant can treat more than 3000m3 (3924yd3) of excavated material per hour.
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 tube tunnels.
After completing its first bore from the HRBT South Island to the North Island, the variable density TBM will be turned around to bore a parallel tunnel back to the South Island. In total, tunnel excavation for the parallel tubes 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 JV) 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.
More than 21,000 precast concrete tunnel segments are to be delivered to the project site by barge. The segments are to be cast by Technopref at a plant in Cape Charles, Virginia, approximately 25 miles (40km) away, across the Chesapeake Bay.
Excavated material is also to be transported by barge, with trucks available if inclement weather interferes with marine transport.
HRCP is a joint venture consisting of Dragados USA, Vinci Construction, Flatiron Constructors, and Dodin Campenon Bernard. The JV has the designbuild 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, manufactured in France by MS, is also named – after Katherine Johnson, the local NASA mathematician who also featured in the Hidden Figures film.
