Miami Tunnel Builders Dig Deep for Cutting-Edge Project
Tricky geology and a dense urban environment marked the journey of Harriet, a lone tunnel boring machine that excavated dual 1.26-kilometer-long, two-lane highway tunnels under the Government Cut shipping channel in Biscayne Bay, to connect Dodge and Watson islands. The tunnels, plus approximately 4 km of roadway and a bridge widening, create a link to the PortMiami from Interstates 395 and 95, taking some 16,000 daily trucks off local streets.
Completed this summer, the $710-million PortMiami Tunnel project features the first large-diameter tunnels—with internal diameters of 11.3 meters—to be constructed through Florida's challenging carbonate sedimentary strata, requiring one of the state's most comprehensive ground investigations. Until this project, only shallow, small-diameter tunnels had ever been built in the state.
Bouygues Civil Works Florida self-performed approximately 80% of construction as the design-build contractor and 10% equity partner. The team had to contend with multiple layers of varying geologic features. The coralline limestone—found in the seventh layer, approximately 80 ft to 110 ft deep—is extremely porous and unstable. Before beginning the eastbound launch from Watson Island, the design-build team embarked, in 2010, on an extensive supplementary geotechnical investigation that lasted 22 months. Contractors ultimately utilized a unique grouting system that involved more than 30,000 meters of drilling and 50,000 cu meters of grout.
Additionally, the project team successfully used ground freezing for the first time in Florida. Two of five cross passages could not receive cutter soil mixing (CSM) panel reinforcements due to their locations in the water, and therefore required artificial ground freezing. For each passage, crews installed 44 pipes to inject the earth with freezing brine. Steel support frames protected the two main tunnels' segmental lining from potential deformation due to the pressure of the frozen ground in the cross passage area. It took about a month to freeze the ground enough to begin mining the cross passage. After about two months of excavation, the ground was thawed out over several weeks.
A temporary lining of steel arch ribs and shotcrete supported the passage during construction, then was replaced by permanent cast-in-place concrete. A steel door was installed so that, if something went wrong, workers could seal off the cross passage and keep water from entering the main tunnels.
The specially developed grout stabilized the dissolvable limestone with the aid of more than 1,000 holes drilled both onshore and in the water. The team also utilized CSM-reinforced walls to support the excavation. A 1.5-m-thick concrete bottom seal built underwater facilitated dewatering of the excavation.
It took two months of preparation to turn Harriet after she finished eastbound excavation. Crews scrambled to complete a 32-ft-dia, 5-ft-high, 120-ton Teflon disc to carry the 2,000-ton shield, which was detached from its 361 ft of trailing gear, on its 9-day, 180° spin. Then, crews reattached her gear for westbound excavation, which took another two months.
Much attention was paid to aesthetic detail and context-sensitive design. The 80-ft-tall portals at each end of the tunnel feature smooth exposed concrete surfaces formed with conjugations of the Latin word for "navigate," and translucent metal mesh sections glow at night like beacons. The five cross passages also sport graphics of a human swimming with sea creatures.
The administrative and operations buildings are designed with ribbon-like orange shade canopies. A variety of native plants enhance the landscape of the approach roads.
In 2011, Bouygues Travaux Publics in France launched a Zero Tolerance biennial global safety initiative for its work force. In 2013 the event occurred in 10 countries around the world, on 15 projects. In Miami, the Bouygues team held the event annually. It consisted of a mandatory three-hour safety training program that included not only workplace safety but home and driving safety.
The project team worked more than 2 million man-hours without a lost-time incident before an extremely minor injury occurred. After that, the team recorded 4 million man-hours with no lost-time incidents.
Southeast Infrastructure Project of the Year, Best Highways/Bridge Project - PortMiami Tunnel Project
Contractor/Engineer Bouygues Civil Works Florida, Miami
Owner Florida Dept. of Transportation, Miami
Lead Engineer Jacobs Engineering, Doral, Fla.
Geotechnical Engineer Langan Engineering, Miami
Architect Arquitectonica, Miami