To ensure a high degree of consistency, reliability, and quality, the project team mobilized two concrete batch plants. At the fabrication site, the two portable Lafarge plants produced 72,000 cubic yards of SCC, testing every load for air content, unit weight, slump, and temperature. The specified compressive strength for the concrete on this project was 6,000 psi; however, the high-performance SCC consistently achieved strengths of 9,000 to 10,000 psi at 28 days.
The last concrete pour into the dry dock forms for the first litter was completed in March 2014. The 11 concrete tunnel elements were cast in two cycles, called litters. Litter 1 included the first six elements and litter 2 the final five elements. After each litter was completed, the dry dock was flooded to float the elements so they could be towed the 220 nautical miles down the Chesapeake Bay to the project site.
The first six massive tunnel elements (litter 1) arrived at the Portsmouth Marine Terminal in summer 2014, after being towed one at a time by a tug fleet down the Chesapeake Bay. The remaining five concrete tunnel elements were completed at the Sparrows Point fabrication dry docks and prepared for float-out to the Virginia jobsite. This process included sealing and waterproofing the elements, constructing four temporary, interior ballast tanks in each to provide stability during floating and towing, and installing temporary bulkheads on each end to allow the elements to float.
The blue steel frames, called the main form traveler (on the left in photo) and the egress form traveler (right), were used to form the concrete into the element shape. The Parsons Brinckerhoff rectangular profile design of the second Midtown Tunnel is the first deepwater concrete immersed-tube tunnel in North America and only the second all-concrete immersed tunnel in the U.S.
Flooding of the Sparrows Point dry dock began in June 2014 and the first litter was towed to the Virginia jobsite to be immersed in the Elizabeth River. The last tunnel element was immersed on July 14, 2015, connecting the shorlines of Portsmouth and Norfolk.
The final litter of elements 7 to 11 reached the Portsmouth jobsite in April 2015. They are being placed across the Elizabethe River at the rate of one every five weeks. After each litter was completed, the dry dock was flooded to float the elements so they could be towed the 220 nautical miles down the Chesapeake Bay to the project site.
When completed in 2016, the new 3,800-foot concrete-reinforced tunnel, located adjacent to the existing Midtown Tunnel, will provide a separate road that allows two lanes of traffic in both directions, doubling transportation capacity beneath the Elizabeth River. The new Midtown Tunnel will offer broad and lasting benefits to the Hampton Roads transportation system, its economy, and the lives of those who live and work in the region. Larger Version.