The grouNd coNditioNs and project circumstances of the mission at Norris cut were anything but standard. Not only did the karstified, permeable geology pose the risk of flooding the machine. the complex Fort thompson Formation to be tunnelled through was also full of sand-filled cavities. the tunnel face was therefore prone to instability.

For this reason the construction of the protection tunnel for a new sewer line between Virginia Key and Fisher island required a special and highly flexible machine with exceptional safety features. A herrenknecht combined shield was chosen for the bore: available in slurry mode as well as in ePB mode depending on the requirements, the hcs machine is optimally prepared for changeable ground conditions.

Additionally, the front area of the machine had to be accessible at all times to allow for tool changes. For maximum safety a bulkhead with a dive pit was developed especially for the project. Because of the bulkhead between the front two machine parts, and the overpressure thus enabled, muck and water could not penetrate into the machine at the tunnel face. should high water pressures lead to flooding regardless, the dive pit allows safe locking into the flooded area. In the end however these safeguards were not needed. Neither the lock system nor the EPB mode of the HCS machine were used. The ground was highly permeable as expected, however the contractor was only required to perform one maintenance stop, which was carried out under compressed air after ground treatment from the TBM.

The machine began the drive near the treatment plant on Virginia Key in April 2015. Right from the very first, the project was characterised by the requirement to save space. With a diameter of twelve metres, the launch shaft was rather small. At the beginning there was no room for the HCS machine’s back-ups and they were only able to be used one by one after 70m of tunnelling. For the first section the TBM needed to be pushed forward in pipe jacking mode using a jacking frame adapter developed by the contractor, the rest of the tunnel was then lined with concrete segments.

In the months that followed the 3.13m diameter machine, nicknamed Dosey, dug its way forwards at a depth that reached 21m below sea level. After 227 working days, on 16 February, the machine broke through on Fisher Island. Top advance rates of up to 24m per day were achieved with about 300m per month.

By the end of the year the new 60″ (1.5m) discharge pipeline is due to be installed in the finished protection tunnel and put into operation.

The successful drive on the Norris Cut project has pushed the boundaries of what is possible in Florida’s tunnelling industry and contributed to its further development: “The project has set standards for work in Florida’s underground and showed solutions for deep sewer lines in the porous Fort Thompson Formation,” confirms Bernard Theron, president of Bessac.

A Herrenknecht spokesman added: “With the construction of the Port of Miami Tunnel already, machine technology from Herrenknecht had demonstrated that even the most complex ground conditions such as the Fort Thompson Formation can be safely mastered with optimally adapted technology.

“Despite its huge diameter of nearly 13m, in 2013 the EPB Shield S-600 reached its target reliably thanks to an additionally installed slurry circuit. According to accounting and consulting firm KPMG, in 2012 the Miami Port Tunnel was one of the ten most innovative projects in the world.