A customized Herrenknecht Combined Shield (HCS) has successfully negotiated the challenging karstic and permeable geology of the Fort Thompson Formation to deliver a 1.6km-long subsea sewer tunnel in South Florida, USA.
With a tunnel face expected to be prone to instability and flooding as a result of sand-filled cavities, the construction of the Norris Cut protection tunnel for a new sewer line between Viginia Key and Fisher Island required selection by the contractor, Bessac, of a highly flexible machine, fitted with extra safety features.
With the capability of operating in both slurry and EPB mode, depending upon the requirements, the HCS machine was 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. The overpressure that this innovation enabled, meant that muck and water could not penetrate the machine at the tunnel face so that should high water pressures lead to flooding, the dive pit would allow for safe locking into the flooded area.
In the event the specially designed safety features were not needed, and neither the sophisticated lock system nor the EPB mode of the HCS machine were used. The ground was highly permeable as expected, but only one maintenance stop – performed under compressed air following novative ground treatment from the TBM – was required.
The 3.13m diameter TBM began the drive near the treatment plant on Virginia Key in April last year (2015), from a tight launch shaft measuring just 12m in diameter. Back up sections of the machine could only be fitted, one section at a time, following an initial advance of 70m. The TBM was therefore pushed forward in pipe jacking mode using a jacking frame adapter developed by Bessac; the rest of the tunnel was then lined with concrete segments.
At a depth of up to 21m below sea level, breakthrough on Fisher Island was achieved in just 227 days, on February 16 (2016). Top performances of up to 24m/day and 300m/month confirmed the optimum configuration of the TBM. By the end of the year the new 60in (150cm) discharge pipeline is due to be installed in the finished protection tunnel.
The successful drive on the Norris Cut Project has pushed still further 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 has demonstrated solutions for deep sewer lines in the porous Fort Thompson Formation," said Bernard Theron, President of Bessac.