Miami: Tunnel breakthrough in the Sunshine State
The tunnel boring machine (TBM) "Harriet" successfully completed the second tube of the "Port of Miami Tunnel" at the beginning of May 2013. The TBM with a diameter of 12.86 meters mastered complex geological and hydrogeological challenges when crossing under the harbor twice. A machine concept was employed that Herrenknecht developed in collaboration with the client specifically for this ambitious project. The road tunnel will significantly alleviate the traffic congestion in the world's largest cruise port and the city center of Miami (Florida).
"With this second breakout, we have successfully completed one of the most technically challenging tunnelling projects undertaken in the world to date", said project manager Louis Brais from the executing construction company Bouygues on the occasion of the breakthrough celebrations in Miami. From 2014 onwards, the twin-tubed tunnel with two lanes for each direction will connect the largest cruise ship port in the world with four million passengers a year with Interstate 395 and Interstate 95, thus relieving downtown traffic considerably. At the present time, around 16,000 vehicles make their way through the narrow streets of the city center day after day. In addition to the cruise traffic, Miami is an important freight hub with around seven million metric tons annually.
The solution, developed specifically by Herrenknecht for Miami together with the client was to extend the application range of the EPB Shield machine type and adapt it to the unique geological conditions of the project. The aim was to safely control both the soft but stable grounds at the tunnel entrance and exit as well as the porous limestone containing corals subject to expected high water pressures beneath the middle of the fairway. The jointly developed system that ensured that the water pressure could be coped with and at the same time ensured transport of the excavated construction ground was referred to by the engineers as "Water Control Process" (WCP). The water-soil mixture is transported away via the screw conveyor with an attached Slurryfier box and an integrated stone crusher via the closed slurry circuit, rather than via the open belt conveyor commonly used in EPB Shields. "The TBM has done well", said the Herrenknecht project manager Georg Schleer. "The customer used the EPB mode on two thirds of the route and the WCP mode on the middle section where pressures were higher than three bar."
After the start of the construction work in November 2011, the TBM needed nine months to build the first tunnel tube. Following breakthrough of the first tunnel in late July 2012, the 2,900 tonne machine was turned on Dodge Island and began its return journey for the excavation of the second, western tube in October.
