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.

The Waterview Connection’s Tunnel Boring Machine

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.

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1 Tunnel face2 Cutting wheel3 Excavation chamber4 Pressure bulkhead5 Thrust cylinder 6 Screw conveyor7 Concrete lining segments8 Tail skin