So critical is the timing, the countdown continues to the completion of the Gold Coast Desalination plant, a vital piece of infrastructure that will provide 125 megalitres of fresh drinking water per day to the South East Queensland region on Australia’s east coast.
South East Queensland is Australia’s fastest growing region, attracting 55,000 new residents annually. Regional growth projections, the extended drought and supply considerations have highlighted the need for new initiatives to guarantee water supplies in the longer term.
This region is currently experiencing the worst drought in recorded history and water storage levels are at record lows. This, combined with unprecedented population growth, has necessitated the implementation of a number of new water initiatives.
Queensland has joined a growing number of countries throughout the world that have opted for water desalination as a proven solution to meet future community water needs.
The US$1.15bn Gold Coast Desalination project is currently under construction and located on a six hectare site at Tugun on Queensland’s Gold Coast, adjacent to the western side of the Gold Coast Airport. Construction of the plant commenced in November 2006. The plant is due to begin producing water at the end of November 2008, with operation at full capacity by January 2009.
The desalination plant will form a vital part of the Queensland Government’s new US$8.6bn SEQ Water Grid and the Gold Coast City Council’s Waterfutures Strategy.
This water grid will link all water sources across SEQ together. This will enable water to be easily transferred from population to population across the region. The desalination plant at Tugun will be integrated with the other sources of water through the development of the water grid. Investigations into desalination as an emergency water source are consistent with the implementation of this strategy.
Desalination was chosen as the community’s most favoured fresh water option after extensive community consultation in 2005. This survey asked the community to select an option that did not rely on rainfall or other surface water storage methods.
Tunnelling
Tunnel construction is on target as two 150-tonne TBMs powered their way through bedrock 50m beneath the sea to complete the Gold Coast Desalination Project’s two ever so crucial marine tunnels, which will link the largest desalination plant on Australia’s Eastern Seaboard to the ocean.
Following the arrival of the two US$9.6M purpose built slurry TBMs, manufactured in Germany by Herrenknecht, they have been working 24 hours a day, seven days a week to construct the 3.04m o.d. 2.2km long intake and 2km long outlet tunnel, which were both completed in February 2008 in preparation for the November 2008 deadline. The intake tunnel will collect the seawater and the outlet tunnel will disperse the brine.
International interest
The nature of the project and the strict time frame for its construction attracted the employment of leaders in this field nationally and globally.
The Gold Coast Desalination Project is being constructed for SureSmartWater (a 50/50 joint initiative between Queensland State Government and Gold Coast City Council) by the GCD Alliance, comprising John Holland Group, Veolia Water Australia, Sinclair Knight Merz and Cardno. The Alliance will also operate the project for 10 years.
At the helm of the project’s tunnelling and marine operations is Tony Bermingham, who is working for John Holland Group. Bermingham has worked on a number of iconic tunnels throughout the world, including the Channel Tunnel linking the UK to France, Athens Metro, and the CTRL Thames Tunnels to name a few.
Bermingham, who is assisted by tunnelling manager, Matt Lennon, and marine manager, John Holmes, for John Holland Group, identified that the tunnel design and construction provided numerous challenges, which his expert team has met eagerly. These challenges included:
• Designing and constructing the tunnels and having the necessary equipment delivered within the tight time frames
• Assembling an efficient tunnelling crew from scratch
• Working 40m below the seabed
• Working under 3.5 bar of compressed air, when water ingress into the TIM cutter-head was 4000l/minute, required numerous interventions
• Training the tunnelling work force to work in compressed air conditions and how to operate the air-lock
Bermingham said that the slurry tunnelling method was chosen over other mixed-ground methods, such as EPBM, due to the sub-aqueous conditions and in response to the soft ground conditions and settlement control. “We chose two purpose-built slurry TBMs because they have minimal impact on both the environment and the community. They are also safer and more robust, particularly when going under the seabed with limited geotechnical information available,” said Bermingham.
The slurry TIM and supporting equipment was adaptable to handling the geological conditions using slurry pipelines in the tunnel. A significant solids separation treatment plant (STP) was installed at the portal to support the tunnel excavation sequence.
“We also needed to ensure the tunnel lining was durable enough to cope with the constant flow of saltwater, 70m underground (40m below the sea bed) for a minimum design life of 100 years,” said Bermingham.
“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 that will be able to help provide drinking water for the region well into the future,” Bermingham said.
The TBMs consist of three shield sections, which were lowered down the two 70m deep access shafts by a 300 tonne crane and then assembled in the tunnel launch chambers. The remaining six backup gantries were lowered to form the complete 71m long TBMs.
Each TIM cutterhead has 24 disk cutters. The excavated rock is crushed and mixed with bentonite to form a slurry, which is then pumped to the STP where the clay is separated from the crushed rock. This is only the second time slurry tunnelling has been used in Australia. The first was the Herrenknecht system used on the Sydney Airport Tunnel project back in 1998 (T&TI, October 2000).
The TBMs have excavated some 52,000m3 (approximately 125,000 tonnes) of high quality metamorphic siltstone (argillite), which is being used as engineering fill at the neighbouring Gold Coast Airport area. The siltstone is very fractured and relatively soft (up to 120MPa), and there is the potential for direct connections through the 70m overburden of the tunnels to the maximum 76m depth of the sea above. This provided potential for a maximum 7.5 bar water pressure on the tunnelling system. There was also the risk of sudden, large volume inflows into the excavation chamber.
Having started in June 2007, excavation of the tunnels was completed by February 2008. During the eight months of constant 24h/seven days/week, tunnel excavation averaged an impressive 75m per week.
A lining first for Australia
As the TBMs advanced, a pre-cast lining ring, consisting of six steel fibre reinforced concrete (SFRC) trapezoidal segments, were placed using a vacuum erector and held in place with 12 shove rams. This is the first time trapezoidal SFRC segments have been used in Australia. The system was selected to negotiate tight radius curves on the alignments and to meet high long-term durability criteria.
The tunnel lining specifications included:
• A compressive strength of 50MPa
• A first crack flexural strength of 4.6MPa
• Quality benchmarked to a Quality System AS/NZS ISO 9001:2000
To ensure durability and strength, and to minimise 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 of .35. Finally, to increase corrosion resistance, ductility and durability, 35kg/m3 of steel fibre was added to the mix.
Each segment is connected to the preceding ring using plastic dowels with steel bolts on the radial joints. These steel bolts will be removed to avoid corrosion and spalling of the concrete. The longitudinal joints of the nominal 1200mm long trapezoidal rings are offset 10° to the axis of the tunnel and the circumferential joints taper from 1193mm to 1207mm allowing the tunnel to navigate through the minimum 400m radius curves.
Forty-eight segment moulds were designed and fabricated by Precast Concrete Products (in Wacol, Brisbane). A programme of 96 segments/day was required by the casting yard to meet the 75m/day programmed advance rate of each TIM. Trapezoidal vertical joints were incorporated in the design to ensure the structural integrity of each tunnel and to allow rapid installation. On average each ring build took approximately 11 minutes.
The journey of each TIM followed a precise route. A survey grid was coordinated by GPS, and this information programmed into the VMT guidance systems on each TIM. The machines were then guided by a laser beam to each target (e.g. from the plant site to the marine riser locations). Once commissioned, each TIM excavated approximately 75m/week of the 3.4m o.d. (2.8m i.d.) tunnels.
Intervention
During construction of the two tunnels, planned cutterhead interventions were required at least every second day. On occasions, these TIM interventions required the need to work in compressed air conditions, due to the rapid inflow of groundwater (approximately 4000 l/min) into the face of the TIM.
Increasing the air pressure at the front of the TIM via compressed air pumped through a pipeline, the inflow of water into the work area was reduced allowing workers to enter into the cutterhead to carry out maintenance works. This meant that team members would spend up to two hours in a hyperbaric chamber after each shift intervention to safely decompress to normal atmospheric conditions.
About 10 specialised tunnellers, including TIM operators and tunnel constructors, were required to work in each TIM. Approximately 100 tunnellers were required to work on both the intake and outlet tunnels (above and below the ground) in two 12-hour shifts. Some of the other tunnelling roles include:
• Project and shift engineers
• Tunnelling superintendent & shift bosses
• Air lock operators (compressed air conditions)
• Safety coordinator
Access to the tunnel is via an Alimak lift in the 70m deep shafts. Five, six-tonne diesel Plymouth locomotives are used in each tunnel to transport equipment, materials and crews from the base of the shaft to the TBMs. Ventilation was an installed 600mm diameter ventilation duct installed in the crown of the tunnel. The fans were multi-stage axial flow.
Once the work of the TBMs was complete, their trailing gantries were recovered. The shield sections and cutterheads were left behind, buried deep under the seabed, as there was limited time available to allow for their complete removal.
Marine
Off-shore, a self-elevating platform (SEP) barge is being used to construct the two marine risers, positioned approximately 1.5km off the coast of Tugun. The marine risers, when in place, will connect via a cross-cut, to the intake and outlet tunnels at about 40m beneath the seafloor.
Construction of the cross-cut involved using traditional hand mining methods through exposed ground. Steel sets and timber lagging were used to construct the 2m diameter x 4m long cross-cuts.
The SEP barge carried a 600 tonne crane and 120 tonne piling hammer to create these marine risers by driving to refusal a 3m diameter caisson. Once the required depth was reached a 1.5m diameter fibreglass reinforced marine riser pipe was lowered into the caisson alongside the tunnel. The space between the two was then filled with high strength concrete to form a reinforced concrete riser pipe.
During plant operation, the intake tunnel will be gravity fed through the low velocity riser at approximately 0.5m/sec, less than ambient sea currents, so the capture of sea organisms in the intake tunnel is low.
The marine riser for the outlet tunnel is fitted with a high-efficiency diffuser dispersing the brine in a relatively small mixing zone of about 120m by 225m. At the edge of this mixing zone, the salinity of the water is modelled to be almost at background salinity levels.
Operating at 100% capacity, approximately 334 megalitres of seawater each day will be fed into the plant to produce 125 megalitres of fresh drinking water. This plant is one of only two in the world to separate backwash material before returning salt water to the ocean.
All tunnelling and marine programme works are on track and are scheduled for completion by mid-2008.
When complete, the Gold Coast Desalination Project will provide 125 megalitres of fresh drinking water per day to South East Queensland, which is approximately 20% of South East Queensland’s water needs.
An aerial view of Australia’s Gold Coast desalination plant Australia’s Gold Coast desalination plant A view of the sections of TIM on the surface waiting to be lowered down the 70m shafts Sections of TIM on the surface A view of the lined outlet tunnel, 70m underground A view of the lined outlet tunnel Fig 1 – Longitudinal section of the tunnels and risers Fig 1
