In February 2016 construction company Bessac completed work on the Norris Cut protection tunnel in extremely difficult geology in South Florida. Norris Cut is a shallow inlet between Fisher Island and Virginia Key, just south of the Main Channel to the Port of Miami. In just ten months and using a customized machine designed especially for the mission, a “Herrenknecht Combined Shield” (HCS, diameter 3.13 m), 1613 m were tunnelled adhering to the highest safety standards. A new sewer line was installed in the finished tunnel – after half a century in use the old pipe was simply worn out.
The Miami-Dade Water and Sewer Department had awarded Nicholson Construction the 72 million dollar design-build contract in January of 2014. Nicholson served as the General Contractor for the project, working closely with sister company and tunnel construction manager Bessac and tunnel design firm, Arup.
Safeguard Measures against Flooding
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 is 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 the flexibility and special safety features of the HCS machine. Available in slurry mode as well as in EPB mode depending on the requirements, the HCS offered the necessary adaptability to changeable ground conditions.
Additionally, the front area of the machine had to be accessible at all times during the excavation to allow for tool changes, for instance. For maximum safety a bulkhead with a dive pit was developed especially for the project. Thanks to the bulkhead between the front two machine parts and the overpressure thus enabled, muck and water cannot penetrate into the machine at the tunnel face. Should high water pressures nevertheless lead to flooding, the dive pit allows safe locking into the flooded area.
In the end the safety reserves were not needed. Neither the sophisticated lock system nor the EPB mode were used. The ground was highly permeable as expected, however the proper design of the cutterhead and the appropriate disc cutters necessitated only one maintenance stop, performed under compressed air after an innovative ground treatment from the TBM.
Construction started in Pipe Jacking Mode
The TBM “Dorsey” began the drive near the treatment plant on Virginia Key in April 2015. Right from the start the project was characterized by its special requirements: to save space, with a diameter of 12 m the launch shaft was rather small. At the beginning there was no room for the HSC machine‘s back-ups and they were only able to be used one by one after 70 m of tunnelling. For the first section the TBM was therefore pushed forward in pipe jacking mode using a jacking frame adapter developed by the customer, the rest of the tunnel was then lined with concrete segments.
At a depth of up to 21 m below sea level, the TBM dug its way forward in the months that followed. After 227 working days came the breakthrough on Fisher Island on February 16, 2016. By the end of the year the new 60“ discharge pipeline is due to be installed in the finished protection tunnel and put into operation. “The project has set standards for work in Florida‘s underground and showed solutions for deep sewer lines in the porous Fort Thompson Formation,” said Bernard Theron, President of Bessac.