More specifically, Herrenknecht MTBMs are being used, which are highly sophisticated machines that can be steered to achieve accurate tunnelling. These MTBMs are capable of excavating through a wide variety of ground conditions but are best suited for tunnelling through rock.
Two MTBMs are being used on the project; one that the SRWP Alliance has purchased, the other the Alliance is renting – both of the MTBMs have been imported from Herrenknecht AG, in Schwanau, Germany, and both have 1,800 mm diameter heads.
During the tunnelling operation reinforced concrete jacking pipes are pushed into the ground behind the MTBMs using large hydraulic cylinders mounted onto jacking frames. The jacking frame has an 850 tonne capacity.
The wall thickness of the concrete jacking pipes is 150 mm and each pipe is 3m long. Each pipe weighs approximately 5tonnes.
The inside diameter of the tunnels (after the jacking pipes have been installed) is 1,500 mm diameter. The sizing of the machines has been based on a combination of the size of the water pipeline that needs to be installed within the tunnels and cathodic protection isolation that needs to be provided.
The MTBMs utilised are the smallest diameter machines available which have an air lock facility. This allows for the cutters on the head of the machine to be changed under compressed air conditions if required. Under normal conditions the machines work under atmospheric pressure. The Wesley Centre for Hyperbaric Medicine is working closely with the tunnel teams in preparation for the river crossing works.
Material excavated by the MTBM is transported to the surface via steel pipelines that are installed along the length of the tunnel and up the walls of the shaft. When the slurry reaches the surface it is pumped through a series of vibrating screens, hydro cyclones and ultimately a centrifuge, which all assist in removing the solid material from the slurry. The separation equipment described has been imported by SRWP from Pigott Shaft Drilling, based in the UK. All water separated in the process is re-circulated and is pumped back to the head of the machine through a second steel pipeline down the walls of the shaft and along the full length of the tunnel.
The majority of the tunnelling teams are direct employees of the SRWP Alliance. One man from each shift is employed by the manufacturers of the MTBM as an expert operator. Most of the tunnelling crews have a background in tunnelling and or mining, bringing with them a range of relevant skills, such as blasting, which means teams can self-perform rather than subcontracting out.
The MTBM operates between two shafts (a drive shaft and a receival shaft). The deepest of these shafts on this project is 30m. The longest drive length on this project is 530 m under the Coomera River.
Progress of the tunnelling differs according to ground conditions, and the MTBM will tunnel under the Bremer River, the Logan River, the Albert River, the Coomera River, the Smith Street motorway, Binstead Way, the Pacific Motorway, Stapylton Hill and Belivah Hill.
Approximately 10 km of access roads have been built to provide crews with all weather access to drive and receival shafts.
The team is very aware of the ground water discharge criteria for dewatering the shaft and tunnel sites when working adjacent to waterways. Water treatment plants have been mobilised which can correct pH levels, treat the water for turbidity and contain any oils and grease that might be picked up through work processes.
Wherever possible biodegradable oils and grease are used in the process.