Virginia’s Midtown tunnel is 50 years old, and one of the most heavily traveled two-lane roads east of the Mississippi, carrying one million vehicles per month. Since the tunnel 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 main channel in the Elizabeth River, the 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 defense 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 the Midtown Tunnel Corridor Project.

The project comprises several components, among them (see Figure 1):

• 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), led 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 Baltimore 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 to 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.”

The PPP is close to executing a comprehensive agreement, likely before the end of 2011, and expects to reach financial close on the project in late February/March of 2012. Within the first quarter of the new year, minor construction is expected to start with the installation of toll gantries. Major construction for the immersed tube tunnel, such as element fabrication, dredging, support of excavation and pile driving won’t start until late 2012 or early 2013. Immersion 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,’” says Fabian. “The ventilation system in the new tunnel will be provided by jet fans. The existing tunnel has plenum systems for fresh air insertion and stale air extraction.”

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 [12m] 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 minimize any 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. Our point in order to meet naval standards is 58ft.”

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 (16.8m, 8.8m, 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 minimize cracking and avoid water intrusion. A waterproof membrane will be applied to the exterior surface of the element to establish a watertight structure.

“With regards to reinforcement, we’re not that far into the design, but I can tell you there is going to be a lot of reinforcement,” Fabian emphasizes.

“Currently, the designer is calling for 60ksi steel reinforcement in all sections of the elements.”

As mentioned, the tunnel will pass under a federal channel that services the Navy, and is designed to accommodate the Nimitz aircraft carrier. In addition to having a 58ft draft, the top of the structure will have a 2ft (61cm) layer of general backfill, capped with a 3ft 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.

Norfolk sits in the hurricane belt during the summer, Tony Kinn, director, Office of Transportation Public-Private Partnerships, points out. “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 leveling 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 allows 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, creating a vacuum 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 says.

Then ERC will be able to start interior work. A secondary Omega gasket is provided for a second level of protection, 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.

The fill barge will constantly feed this “locking fill” to a center hopper, which, through a conveyor system, will distribute the material to the outrigger hoppers. These are spaced to span the width of the tunnel exterior (between 55ft and 59ft). The outrigger hoppers then place the locking fill underwater by means of tremie chutes some 80ft below the surface. Once the locking fill is placed, it will provide resistance to horizontal movement.

Construction will take three to four years once a final agreement is reached between VDOT and ERC.

The hope is that the new Midtown Tunnel will open to traffic by 2016.


View of the existing Midtown Tunnel from the Norfolk side Figure 1, The layout of the entire extension project A rendering of the new Midtown Tunnel tube element