The Waterview
Connection’s Tunnel
Boring Machine
On March 5, the Well-Connected Alliance
officially took ownership of the Tunnel
Boring Machine (TBM) that will be used
to build the Waterview Connection
project’s twin 2.5km tunnels. The TBM
had been custom built by German
company for Herrenknecht over the
previous year at its manufacturing facility
in China, to the exact specifications
demanded by the project’s scope and the
region’s geological conditions. Following
this final inspection and handover, it
has been dismantled to be shipped
New Zealand, arriving in July to be
reassembled in time to start tunnelling
in late October.
The Waterview Connection’s TBM is an
Earth Pressure Balance Machine (EPBM).
The primary advantage of an EPBM is that
it maintains the pressure in the shield’s
cutting head to that of the soil around it,
thereby stopping the ground above from
subsiding as the machine drills through it.
This allows the machine to pass shallowly
beneath the surface. For the Waterview
Connection this is particularly significant
as it will enable the last part of the tunnel
in Waterview to be bored underneath
Great North Rd, rather than trenched
across it as had originally been envisaged.
Not only will this limit disruption to
commuters, it will reduce the project’s
construction footprint.
How does it work?
At over 14 metres in diameter (about the
height of a four-storey building), and almost
100 metres in total length, this TBM will be
the 10th biggest ever built worldwide – and
the largest built for use in the southern
hemisphere. It will house a crew of at least
15 people, headed by the TBM pilot in the
machine’s operating cabin.
At the head of the TBM is a 12 metre
long shield, behind which sit three
back-up cars, or gantries, that house
all the equipment needed to run it.
The shield has two functions: its steel
cutting head drills through the ground to
create the space in which prefabricated
concrete panels are then placed to create
the tunnel lining.
During the cutting process the ground
is conditioned using a polymer which is
injected, along with water and compressed
air, into the cutting area through pipes in
the rotary cutting head. These pipes are
connected to injection nozzles at the front
of the head which spray the conditioning
mixture on to the cutting face. The ground
is conditioned so that it does not clump
and stick to the steel on the inside of the
cutting head and takes on a toothpaste
consistency for ease of removal from the
cutting chamber.
The TBM will have an absolute top
speed of 80mm a minute… or 0.005
km/h. This will allow it to cover the total
4.8km journey to Waterview and back in
two years. It will remove approximately
800,000 cubic metres from both tunnels
over the two-year dig. To provide a sense
of scale, this equates to filling 320
Olympic-sized swimming pools.
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Tunnel face
Cutting wheel
Excavation chamber
Pressure bulkhead
Thrust cylinder
Screw conveyor
Concrete lining segments
Tail skin
With its rotating cutting wheel, the TBM removes the material at the tunnel
face (1). The soil enters the excavation chamber [3] through the cutting wheel
[2] openings. It then mixes with the already available plastic soil mush. The
thrust cylinder force is transferred to the soil mush via the pressure bulkhead
[4], balancing the earth pressure. Excavated material is removed for disposal
via the screw conveyor [6].
The propulsion system (5), which consists of cylinders around the
circumference in the shield, pushes against the concrete lining to project the
machine forward. There are ten concrete lining segments per ‘ring’, each two
metres long that are installed by an erector, which picks up these segments
that enter the chamber by way of a gantry, and installs them one at a time. So
as the machine moves forward, it leaves lined tunnel in its wake. Grouting is
continuously forced into the remaining gap between the segments’ outer side
and the soil to ensure that there are no gaps between the excavated earth and
the tunnel lining rings installed, thereby ensuring stability.