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Tunneling the Norris Cut

Herrenknecht Combined Shield Navigates Difficult Geology

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The Herrenknecht Combined Shield (HCS) for the Norris Cut Project was produced at the Herrenknecht plant in Schwanau, Germany.

In February 2016, construction company Bessac completed work the Norris Cut tunnel, a project in extremely difficult geology in South Florida. In just 10 months, crews used a customized Herrenknecht Combined Shield (HCS), with a diameter of 3.13 m (10 ft), to complete the 1,613-m (5,200-ft) tunnel while adhering to the highest safety standards. A new sewer line was installed in the finished tunnel to allow replacement of a 50-year-old forcemain that was in need of replacement.

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, but the complex Fort Thompson formation to be tunneled through was full of sand-filled cavities. The tunnel face was therefore prone to instability.

For this reason the construction of tunnel under Biscayne Bay between Virginia Key and Fisher Island required a special and highly flexible machine with exceptional safety features. The HCS offered the necessary adaptability: available in slurry mode as well as in EPB mode depending on the requirements, the HCS machine is optimally prepared for changeable ground conditions.

The launch shaft on Virginia Key had a narrow working shaft to save space. As a result, the HCS had to begin the drive in pipe jacking mode using a jacking frame adapter.

The launch shaft on Virginia Key had a narrow working shaft to save space. As a result, the HCS had to begin the drive in pipe jacking mode using a jacking frame adapter.

Additionally, the front area of the machine had to be accessible at all times during the drive 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.

RELATED: TBM Breaks Through on Norris Cut Project

In the end, the safety measures were not needed. Neither the sophisticated lock system nor the EPB mode of the HCS machine were used. The ground was highly permeable as expected, however, the proper design of the cutterhead and the appropriate disc cutters enabled the customer to perform only one maintenance stop, performed under compressed air after an innovative ground treatment from the TBM.

The HCS, named “Dorsey,” began the drive near a treatment plant on Virginia Key in April 2015. Right from the start, the project was characterized by its special requirements. The launch shaft, for instance, was built at 40-ft diameter in order to save space. The small shaft size meant that there was no room for the HSC machine’s back-ups and they were only able to be installed one by one after 225 ft (70 m) of tunneling. The first section of the TBM was therefore pushed forward in pipe jacking mode using a jacking frame adapter developed by the contractor. Once the HCS was complete, the rest of the tunnel was built with concrete segments.

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In February 2016, “Dorsey” reached the target shaft on Fisher Island.

At a depth of about 70 ft 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 Feb. 16, 2016. During the drive, the crew achieved a top performance of 24 meters per day and about 300 meters per month, confirming the optimum configuration of the TBM. By the end of 2016, the new 60-in. discharge pipeline is due to be installed within the finished tunnel and put into operation.

The successful drive on the Norris Cut project has pushed the boundaries of what is possible in Florida’s tunneling 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,” said Bernard Theron, President of Bessac.

Nicholson Construction was the general contractor for the $72 million design-build project for the Miami-Dade Water and Sewer Department. Arup was the tunnel design firm, while AECOM served as the consultant for the owner. Woolpert provided pipe design services. Hobas was selected for the carrier pipe.

With the construction of the Port Miami Tunnel previously, machine technology from Herrenknecht has 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 13 m (42 ft), in 2013 the EPB Shield S-600 reached its target reliably thanks to an additionally installed slurry circuit. According to internationally renowned accounting and consulting firm KPMG, in 2012 the Miami Port Tunnel was one of the 10 most innovative transport projects in the world.

RELATED: AECOM Awarded $91 Million Miami-Dade Sewer Repair Contract

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