Virginia’s Midtown tunnel is 50 years old, and one the most heavily travelled two-lane roads east of the Mississippi, carrying one million vehicles per month. Since the tunnel has opened, population in the region has increased 69 per cent and tunnel use by a whopping 600 per cent.

The Port of Virginia drives much of this traffic. It has deepwater channels that allow for large ships and will see a noticeable increase when the Panama Canal expansion project completes in 2014.

Crossing the Elizabeth River, Midtown Tunnel connects Portsmouth with Norfolk. The U.S. Navy’s largest installation is located in Norfolk and increasingly relies on the tunnel.

"If there’s a natural disaster, if there is an emergency where the nation was placed on a higher level of defence alert you’re going to have all those federal workers trying to use that single tunnel at virtually the same moment and it would be nothing but gigantic gridlock," explains Craig Quigley, executive director, Hampton Roads Military and Federal Facilities Alliance.

With all of these factors in play, the Virginia Department of Transportation (VDOT) is undertaking a massive infrastructure program to improve transportation in this region: The Downtown Tunnel/Midtown Tunnel/ Martin Luther King Freeway Extension (MTT/MLK), formerly known as the Midtown Tunnel Corridor Project.

The project comprises several components, among them:

  • A new two-lane tunnel under the Elizabeth River parallel to the existing Midtown Tunnel
  • Maintenance and safety improvements to the existing Midtown Tunnel
  • Maintenance and safety improvements to the existing Downtown Tunnel
  • Extending the MLK Freeway

To finance the project, VDOT has partnered with Elizabeth River Crossings (ERC), lead by Skanska Infrastructure Development and Macquarie Financial Holding Limited, with construction joint venture members Skanska USA Civil, Kiewit and Weeks Marine. Parsons Brinckerhoff is leading the design team that includes Volkert & Associates and Cowi. ERC has collectively built nearly 52,000ft (16km) of immersed tube tunnels in the United States. Skanska and Kiewit constructed, and Parsons Brinckerhoff designed the Fort McHenry Tunnel in the Balitmore harbor, which is the world’s widest underwater highway tunnel. Parsons Brinckerhoff also designed the first Midtown Tunnel, which was completed in 1962.

In looking at the design and construction of the new immersed tube Midtown Tunnel, Frank Fabian, senior project manager for the MTT/MLK, says he considers it to be almost like a sister project the Limerick Tunnel in Ireland.

"There are some subtle differences between the two because of existing site conditions," he says. "But there are also a lot of similarities in length and size, the type of construction and all the ancillary support systems that go along with a tunnel, for example the ventilation systems."

VDOT and ERC reached financial close in April, releasesing all the funding needed to build the USD 2.1bn project. Major construction for the immersed tube tunnel, such as element fabrication, dredging, support of excavation and pile driving won’t start until later this year early 2013. Immersion of tube elements won’t start until the second or third quarter of 2014.

Aligning with the old

The advancements in engineering since the original Midtown Tunnel was constructed in the middle of the 20th century have improved installation techniques and materials alike. Other than both being immersed tube, the new and old tunnel will have very few similarities.

"From a structural perspective, the existing tunnel is a steel pipe encased in concrete. The new tunnel will be very similar to a heavily reinforced ‘box culvert,’" Fabian explains. "The ventilation system in the new tunnel will be provided by jet fans, whereas the existing tunnel has plenum systems for fresh air insertion and stale air extraction.

"Another major difference will be in the fire life safety arena. For example, the new tunnel will have an enclosed pressurised escape corridor to provide safe egress for motorists, should a tunnel fire occur and evacuation is required."

The new tunnel’s approach on either side of the alignment are close to the existing tunnel. ERC will use sheet pile walls to protect the tunnel at the approaches, and to keep existing surrounding soils in place. As the alignment moves away from the approaches, the new tunnel will bow out into the river.

"We want to stay away from the existing tunnel as much as possible," Fabian says. "The problem is that when we start dredging, if we dredge deep enough to insert a 40ft [12.2m] high tunnel the side floats are going to be quite long. We really bow out from the existing tunnel so our side slopes are still a safe distance from the existing tunnel to minimise and movement or undue settlement that will be caused."

The tunnel project will also need to maintain a navigational draft of approximately 58ft [17.7m], he explains. "We have a navigational channel that we undercut that is serviced by the U.S. Navy, and there are some very large ships that use this channel and they carry with them a large draft."

Specifically, 58ft will accommodate the Nimitz-class aircraft carrier, the largest vessel that the U.S. Navy uses in this region.

The Elizabeth River has a very slow current and shouldn’t cause any issues with flow. However, siltation may be a problem. ERC isn’t expecting a lot of siltation to go downstream, but it will be installing silt curtains for the dredging operations.

Elements

To connect the two cities there will be 11 tube elements to immerse, starting on the Portsmouth side and working northeast to the Norfolk side. From outside to outside, the dimensions of an individual element are approximately 55ft wide, 29ft high and just over 300ft long — the same as a football field [16.8m by 8.8m by 91.4m].

The elements will be fabricated at an existing dry dock facility in Sparrows Point, Maryland. ERC members’ Skanska and Kiewit fabricated the tunnel elements for the Fort McHenry Tunnel at this same facility. As the elements are constructed and pass final inspection they’ll be floated out of the dock and taken by tugboat, attached by cable, down the Chesapeake Bay — a journey of approximately 63 hours. There will be an area to moor the elements at the project site while final preparations are made and they are ready for immersion.

This facility is capable of fabricating one element at a time and current plans will leave minimal lay time between when the tugboat takes one down and returns for the next. The boats are capable of floating two units down the bay at one time, but at this point in the conceptual planning, at least to start, they will be taken one-by-one.

The concrete will be a 6,000psi mix with a major emphasis on thermal control during placement to minimise cracking and avoid water intrusion. A waterproof membrane will be applied to the exterior surface of the element for watertightness.

"We’re not that far into the design, but I can tell you there is going to be a lot of reinforcement," Fabian emphasises. "Currently, the designer is calling for 60ksi steel reinforcement in all sections of the elements."

The top of the structure will have a 2ft [0.6m] layer of general backfill, capped with a 3ft [0.9m] layer of armor stone. There will also be an anchor release band designed to deflect a dragging anchor. "We also have that capstone on there to distribute the weight over the tunnel should a ship sink and rest upon the tunnel," explains Fabian.

Tony Kinn, director of the Office of Transportation Public- Private Partnerships, points out that Norfolk sits in the hurricane belt during the summer. "You have to be prepared for all of these contingencies."

After dredging, the bottom of the excavation will receive a varying depth of fill material consisting of either stone or sand and capped by a 2ft layer of bedding material, a fine course aggregate, explains Fabian. All of this material will be brought in and placed by a screed barge. "This material is placed by clam shell and then leveled into final place by use of a moveable screed," he says. "Using GPS technology, the submerged screed runs back and forth along the trench by means of a cable system with a levelling blade to place the material within acceptable elevation tolerances."

Immersion

There are going to be three major types of backfill: bedding, side fill and the top and cap fill described above. The bedding material is a very fine, granular material. It could almost even be considered sand, says Fabian. This material will also be installed by a screed barge.

A lay barge will bring the elements into place and immersion will be controlled by ballast tanks constructed inside each element. GPS technology will direct the vertical and horizontal alignment of each one.

Fabian describes the method of aligning the elements. "Each tube section has a blockout precasted near the ends, which allow a cable or system of cables to be inserted through them. One end of the cable becomes fixed on the section being immersed, while the other cable end is attached to a hydraulic jack set in the blockout on the previously set section. Jacking then brings the section being immersed into the previously set section and starts the seal compression of the Gina gasket."

Trapped water between the end bulkheads on each element will be pumped out and will creating a in the space. "Hydro-static pressure acting on the water side bulkhead of the section being placed, provides an additional push to further compress the gasket and place the sections into their final location," he explains.

Then ERC will be able to start interior work. A secondary Omega gasket is provided for a second level of protect, at all joints. There will be a team of divers assisting the immersion by monitoring the connections as best they can.

Once the bedding is prepared and the element lowered into place, a fill barge will place material along the sides of it. "This particular material is more granular in nature, and it’s gradated from smaller size stones to larger size stones," he explains. Construction will take three to four years once a final agreement is reached between VDOT and ERC.