An important part of the Adelaide Desalination Project is a pipeline that will deliver desalinated water from Port Stanvac to the Happy Valley water treatment storage facility, a distance of approximately 11.5 kilometres. The pipeline is nominally 1.5 metre diameter MSCL (Mild Steel Concrete Lined). Winslow Infrastructure is currently completing six tunnel bores along this pipeline. Winslow has been engaged to undertake this tunnelling work by the pipeline contractor, McDonnell Dowell Built Environs Joint Venture.
The bores, ranging from 30 metres to 160 metres each in length, are excavated using a MTS-2000 TBM (tunnel boring machine). This equipment was selected to ensure accuracy to within 20 mm and minimal settlement under roads and rail lines. Minimal cover and hence decreased depth to these assets could then be achieved. A mixed ground cutting disk was selected to excavate through the varied ground conditions including sands to stiff clays and weathered rock.
The MTS unit enables cutters to be changed from within the tunnel during the drive, though this is unexpected in the soft ground conditions experienced. The excavated material is separated from slurry at the surface by a Schauenburg MAB 300 separation plant. The slurry is water-based which assists in disposal, and mimimises environmental issues as compared to a bentonite-based system.
Steel sleeves of 1,800 mm ID are used as the enveloper pipe for the drives requiring a 1,870 mm cutterhead OD. This ensured a smaller diameter TBM could be used whilst maintaining a minimum 100 mm annulus between the product and enveloper pipes. Sleeves were supplied in three metre lengths, lowered into the shaft, then welded together externally during the pipe jacking operation. Winslow has been able to achieve advance rates of up to 12 metres per shift with this pipe jacking technique.
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Shafts at various depths of between 5 and 15 metres, in close proximity to existing roads and services, have dictated the use of a variety of shaft construction techniques. These techniques include:
(i) specifically fabricated shoring boxes;
(ii) sheet piles and whalers;
(iii) shotcrete and soil nail walls; and
(iv) in situ concrete caissons.
In addition to shaft construction and tunnelling operations, Winslow is contracted to install, weld and grout all product pipe within the tunnels; install necessary bends, risers and thrust blocks; and reinstate each location for later connection to the open cut pipeline.
The microtunnelling option was chosen to ensure minimal disruption to the community throughout the project. Extensive community liaison has ensured all works could proceed within close proximity to major roads, schools and residents without complaint.
Winslow, through the use of its highly technical and specialist plant and experienced crews, is tracking well to program, and all works are expected to be complete in October.
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