Early in the morning on June 9, 2014, workers began to flood a dry dock in Maryland, signifying the beginning of the interstate journey for the 11 elements that will create a new immersed tunnel. The first of six fabricated elements left for its 220-mile tow down the Chesapeake Bay to Virginia.
Less than a year later that first litter, elements one through six, had been successfully immersed at the bottom of the Elizabeth River, marking the halfway point of construction for the new Midtown Tunnel.
The two-lane road tunnel is part of the largest design build project in the Hampton Roads region’s history. Hampton Roads refers to the area in the middle of the Eastern seaboard where the James, Nansemond and Elizabeth rivers pour into the mouth of the Chesapeake Bay. Some 1.6 million people live in the region and rely on Virginia’s existing Midtown Tunnel, which carries more than one million vehicles per month.
Crossing the main channel in the Elizabeth River, the 50-year-old Midtown Tunnel connects Portsmouth with Norfolk. Back when it opened, it accommodated 8,400 vehicles a day. Today that volume has increased to 38,000 vehicles.
Building the new Midtown Tunnel is only one component of the USD 2.1bn Elizabeth Rivers Tunnel project, a P3 between the Virginia Department of Transportation (VDOT) and Elizabeth River Crossings (ERC), a sole-purpose company created to finance, deliver, operate and maintain the project, led by Skanska Infrastructure Development and Macquarie Infrastructure and Real Assets. The project also comprises rehabilitation work on the existing Midtown and Downtown tunnels among other construction.
ERC’s design-build team is a joint venture composed of Skanska USA Civil Southeast, Kiewit Construction Company and Weeks Marine (SKW Constructors).
Parsons Brinckerhoff led the design team on behalf of SKW that includes Volkert & Associates and COWI.
The new tunnel is being built parallel to the existing Midtown Tunnel, which will be converted from two-way traffic to one-way once the new one is completed. The second tunnel measures 4,198ft (1,280m) from portal to portal, and the total length of the tunnel is 5,441ft (1,658m) from end to end.
The elements are being towed from SKW Constructors’s fabrication facility in Sparrows Point, Maryland, to the Portsmouth Marine Terminal in Portsmouth, Virginia. ERC members’ Skanska and Kiewit fabricated the tunnel elements for the Fort McHenry Tunnel at this same facility.
Each element is 350ft (106m) long, and weighs 16,000 tons. The shape varies depending on road’s curvature, and is on average approximately 30ft (9m) tall by 54ft (16.5m) wide.
"The most challenging part of constructing the tunnel elements was the tremendous planning effort that had to take place to assure overall success," says ERC’s construction director, Dallas Marlow. "The second biggest challenge was getting the concrete mix design right; starting in the laboratory then assuring a workable design allowing ease of placement in the forms."
Parsons Brinckerhoff recommended an all-concrete option for the two-lane tunnel over a more conventional design using a steel tube encased in concrete.
"It offered substantial economic savings resulting from changing the immersed-tube sections to a shallower rectangular configuration, thus reducing the scale of the steel fabrication," says Fred Parkinson, design manager for Parsons Brinckerhoff. "The use of reinforced concrete in lieu of fabricated steel also significantly reduced the schedule risk for SKW, since tubes could be constructed without relying on specialized steel fabrication labor."
The elements are reinforced with plain black rebar and the concrete is a dense specialty mix designed to achieve a 120-year design life, Marlow explains.
The first concrete element was cast in January of 2013, and the second litter (Elements 7-11) started in July of 2014. The second litter launched on March 24th and the seventh element is scheduled for immersion as Tunnels & Tunnelling goes to print.
Tunnel elements are launched from the dry dock at Sparrows Point. The first element of the litter leaves immediately for Hampton Roads and the others are tied up dockside until the tugs can return for the next one.
Once an element is on-site, it is outfitted to be lowered into a prepared (excavated and gravel bedded) trench across the Elizabeth River. "Once an element is ready it is placed inside the lay barge, which is used to move the element into position in preparation for immersion," Marlow says. "During the immersion process, water is pumped into the internal ballast tanks adding additional weight. Computers on the lay barge, GPS positioning from the survey tower, and scuba divers all play a role in positioning the element and connecting it to the preceding element."
After the element is placed on the river bottom, hydraulic jacks are used to pull it toward the preceding element to make initial contact between the end frame and the Gina Gasket, supplied by Trelleborg. After a good seal is formed, water is pumped out of the space between the two element bulkheads compressing the Gina gasket and forming a permanent seal
"Once the surveyors and divers verify that the element is placed correctly the immersion joint is dewatered," Marlow explains. The newly-placed tunnel element can then be accessed from the preceding tunnel element through submarine doors in the temporary bulkheads. After the tunnel elements are securely connected, the trench is backfilled with a locking backfill to hold the elements in place then regular fill and a band of armor stone are placed over the elements to protect them from ship traffic and anchors.
Tunnel elements one through three were placed into the slot from land using cranes and elements four through six were placed in the river using the lay barge. "Placement of Element 6 marked the midway point for us and is the element that rests at the deepest point," Marlow says, "approximately 90ft [27.4m] below the surface."
By mid-June elements seven and eight should be placed. Fine grading of element nine should be well under way with placement schedule in early July.
