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Contractors World International Vol 7 No 5


Wor ldwide Panorama

A new sewer line was installed in the finished tunnel

– after half a century in use the old pipe was simply worn


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 ma-

chine with exceptional safety features. A Herrenknecht

Combined Shield offered the necessary adaptability.

Additionally, the front area of the machine had to be

accessible at all times during the challenge off Miami 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 bulk-

head 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

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 a novative ground

treatment from the TBM.

“Dorsey” began the drive near the treatment plant

on Virginia Key in April 2015. Right from the very first

meter the project was characterized by its special re-

quirements: to save space, with a diameter of twelve

meters the launch shaft was rather small. At the begin-

ning there was no room for the HSC machine’s back-ups

and they were only able to be used one by one after 70

meters 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


At a depth of up to 21 meters below sea level, the

3.13 meter diameter “Dorsey” dug its way forward in

the months that followed. After 227 working days came

the breakthrough on Fisher Island on February 16, 2016

– top performances of up to 24 meters per day and about

300 meters per month confirmed the optimum config-

uration of the TBM. 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.

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The launch shaft on Virginia Key with a diameter of only

twelve meters saved space. As a result, the HCS ma-

chine had to begin the drive in pipe jacking mode using

a jacking frame adapter.