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Concrete In Australia : March 2008
NATIONAL PRECASTER NUMBER 47 • FEBRUARY 2008 … story continued from front page Forty-eight machine moulds were designed and fabricated by the precaster and were required to form the segments. Trapezoidal vertical joints were incorporated in the design to ensure the structural integrity of each tunnel and to allow rapid installation. – bentonite clay – to form a slurry. It is then pumped, via pipelines inside the tunnel, to the surface at the plant site. After the TBMs cut through 1.2 metres of rock, they stop and the lining is erected from the back of the machines. Six segments are linked together to form a ring, which is grouted into place. Using hydraulic rams, the TBMs then thrust off the completed ring and begin the cutting process once again. These rams are capable of pushing about 1200 tonnes on the edge of the segments – a critical issue that also needed to be considered in the design and fabrication of the segments. Failure to do so would have resulted in local pressure points which would cause cracks and possible failure of the segments. The segment geometry is also complex, as all of the longitudinal joints are offset 10 degrees to the axis of the tunnel, resulting in trapezoidal shaped segments. In the circumferential joints the nominal 1200mm long ring tapers from 1193mm to 1207mm allowing the tunnel to navigate around large radius bends. To ensure durability and strength and to minimise any porosity, the concrete mix included silica fume and fly ash. In addition, a high-range water reducer was used to provide a low water/cement ratio to enhance durability, and steel fibres (35 kilograms per cubic metre of concrete) were included to increase corrosion resistance and ductility of the segmental lining. “QA is always at the top of our minds on this project, so all elements and aspects of the segment fabrication process were required to be tested, including the raw materials from the quarry,” Mr Bermingham said. “Our tunnel design and construction is the result of considerable research to ensure the best possible product for the South-East Queensland community – tunnels that will be able to help provide drinking water for the region for at least 100 years.” Dimensional, material and concrete tests were conducted on each segment fabrication shift. The moulds were also required to undergo significant testing in order to meet tight dimensional tolerances for approximately 500 casts. Colin Ginger, of Precast Concrete Products, said the amount of testing undertaken for this project was the highest recorded since the company was established 40 years ago. In order to meet the strict time frames required for the project, the company operated two shifts throughout the manufacturing process, with 48 segments cast twice a day (early morning and evening), six-days-a-week. This allowed for approximately ten truck loads of segments to be delivered daily to the project site at Tugun. Once delivered, the segments were carefully lowered into the tunnel shaft, 70 metres below ground, and taken – by locomotive – to the TBMs for installation. Each TBM has a round, rotating cutter head which is covered in disk cutters (like giant teeth). As the head spins, the teeth cut into the rock. The rock is crushed and mixed with a naturally occurring lubricant Desalination plant “We used 3D computer software to model the mould components in a language that could be directly downloaded to the machine shops that milled the parts, and then assembled all of the fabricated components in our mould workshop,” Mr Ginger said. “A special curing regime was also used with variable amounts of steam piped to each mould. The temperature of the concrete and rate of temperature rise were tightly controlled to allow for stripping of the moulds only six hours after casting. This enabled us to undertake the daily double castings. “Concave and convex vacuum lifters were custom made for us in Melbourne. These were used to lift the segments out of the mould and move them to storage. In addition, a customised rotating device was built and attached to a forklift, allowing the segments to be rotated 180 degrees prior to delivery to Tugun.” The Gold Coast Desalination Project is being constructed by the GCD Alliance, comprising John Holland Constructions, Veolia Water Australia, Sinclair Knight Merz and Cardno. The Alliance will also operate the project for 10 years. Shaft Tunnel in precast segmental construction Note:Image is not to scale and is an artists impression of the plant and tunnel design 26 Concrete in Australia Marine riser