Premier Brumby said “The project is unique because it delivers clean energy without the need for new dams or new water by reusing water already used by the McKay Creek Power Station.
“The design has ensured that this project has been great for the economy and great for the environment, producing zero emissions energy while conserving our valuable water resources.”
The fourth and final link in the scheme has added another 140 MW to the existing 241 MW generating capacity of the Kiewa Valley Hydroelectric Scheme.
This is the storage reservoir for the water that powers the entire hydro scheme, currently made up of a sequence of three existing power stations that descend from Falls Creek and down the Kiewa Valley, in the Bogong region of Victoria’s Alpine National Park.
Owned by AGL Southern Hydro, the scheme was built over a period of more than 20 years, commencing in the late 1930s and, after construction works were interrupted by World War II, completed in 1961. The project, awarded to McConnell Dowell Constructors (Aust) by AGL Southern Hydro in September 2006 and commissioned in October 2009, is the largest hydro power project constructed in Australia for 25 years.
Mr Brumby commented on the significant engineering feat. “The seven km water tunnel between McKay Creek Hydro Power Station and the Bogong Hydro Power Station is one of the largest hard-rock tunnels constructed in the Southern Hemisphere in the last 40 years using tunnel boring equipment.”
The Bogong project harnesses the water supply already used by the existing three power stations to produce additional power for periods of high demand. The additional power is created by diverting flow from the McKay Creek Power Station at peak times via a seven km tunnel running under the mountains of the National Park, and connecting to the newly constructed Bogong Power Station, positioned adjacent to Junction Dam.
After first conceiving the project in 2000, AGL Southern Hydro awarded the $A160 million contract to McConnell Dowell for the design and construction of a new tunnel connecting the McKay Creek Power Station with the proposed Bogong Power Station, along with the associated infrastructure.
McConnell Dowell Project Manager Paul Thomas explained why MacDow was the successful tenderer for the job.
“Basically it came down to the most cost-effective solution. MacDow has a very good history with TBMs. The company was formed in 1960, and has not only worked in tunnels but also pipelines, civil and electrical works, so it’s a multi-discipline contractor. In terms of TBM, they’ve done tunnelling work in Singapore, New Zealand, and also some major projects here in Australia, so they’ve got a very good track history of difficult and challenging tunnelling jobs.”
MacDow established a reputation as a leader in tunnelling during the mid-1990s, setting three world records for TBM tunnelling rates during drilling operations at the Blue Mountains Sewerage Transfer Scheme. Completed 17 months ahead of schedule, the project drilling records included 70.5 m in a single eight-hour shift, 172.4 m in a single day and 703.3 m in a single week.
“Certainly from a hydro point of view, time is of the essence. There’s always going to be time constraints, always pushing harder to get it finished earlier,” said Paul.
The company proved its capacity to work within tight time frames when it successfully completed the Eastern Distributor Tunnels for the opening of the Sydney 2000 Olympic Games. The very tight construction program meant that the workforce was organised into a 24 hour day roster, working 7 days a week. The tunnels were completed one month early.
Value engineering
Design optimisation is also a strength of the company. By undertaking hydrojacking tests and onsite coring tests, the length of the High Pressure Head Race Tunnel steel liner was reduced from 950 m to 700 m. In addition, the diameter was reduced from 3.4 m to 3 m, without compromising any hydraulic requirements. This resulted in significant cost savings for AGL Southern Hydro.
“Originally it was all steel liner, and that’s to do with some of the pressures that build up when the water is going into the tunnel, to stop it exploding, literally – that’s why the steel lining was used. But when we checked the stresses in the rock, we found we didn’t need such a robust steel lining section. We could replace that with concrete. So there was a bit of a cost saving scheme, about 400 m of concrete, and then 700 m of steel liner.”
Other design optimisations included increasing the alignment on the main tunnel from two per cent to four per cent, which resulted in a shorter depth of the head pond drop shaft.
“In hindsight, this is quite steep because we did have some problems with our locomotive capacity. We’ve got locomotives which are about 15 tonne capacity, but the steepness of the gradient meant that they weren’t efficient and sometimes couldn’t get up the tunnel.”
While this challenge wasn’t anticipated, it was quickly solved by lessening loads, using larger capacity locomotives, and introducing more rigorous maintenance on the rail tracks.
For MacDow, winning the contract was an important step in getting a foothold in the hydro market, given the growing importance of environmentally sustainable operations, and the increase in demand for renewable energy sources.
“For McConnell Dowell to have this almost as a signature project, so to speak, it’s very important for us as a company,” he said.
Challenges
One of the main challenges encountered during the project was the variable geological conditions, ranging from ‘massive’ rock with Q values in excess of 100 to highly fractured rock with Q values of less than 0.1.
While extensive geological and geotechnical testing was conducted prior to the project’s commencement, during drilling operations the rock was found to be harder than originally anticipated.
“We had advanced the tunnel, and the rock itself was harder than originally anticipated. The average strength is in excess of 180 MPa, which makes it five or six times stronger than the concrete in that structure,” explained Paul.
“It’s very similar to granite, in composition, and it’s very massive, which means there are very few joints, and therefore it’s very difficult to mine on occasions.”
At one point during excavation of the main headrace tunnel, operations had to stop because the rock was self-mining, meaning the TBM couldn’t be advanced at all. While this resulted in significant delays and required additional ground support, the project was completed on schedule.
The Aurora Australis TBM
A 5 m diameter Robbins TBM was chosen to excavate the 800 m headrace access tunnel and 5,700 m main headrace tunnel. Refurbished by Herrenknecht in Germany, the machine had completed two previous projects in Europe, the last being a water tunnel in Lugano, Switzerland.
The TBM is a single gripper, main beam hard rock machine, allowing it to grip against the side walls of the excavated tunnel and propel itself forward using four no. hydraulic propulsion cylinders connected to the main beam.
The combined force of these cylinders is approximately 1,000 tonnes. The rotating cutter head is connected to the main beam and a series of six no. frequency controlled electric motors producing 1,800 kW. The cutter head is made up of 33 x 19 inch cutters, which are front loading and require special maintenance procedures.
The maximum cutter head thrust is 10,395 kN and maximum head rotation is 10.9 rpm. One stroke of the TBM is
1,800 mm, and in anticipated conditions this takes approximately 40 minutes. The machine is also equipped with a probe drill with a range of 360 degrees and two no. roof bolting hydraulic rock drills for the installation of rock bolts.
The TBM’s best single week was in November 2008, when it excavated
220.62 m, while the best single shift was in May 2008, excavating 30 m.
Asked about MacDow’s plans for future hydro projects, Paul said there are potential projects nearby in Victoria and elsewhere in Australia. MacDow is currently working on a hydro project in the Philippines and is strongly focused on the Asia-Pacific, Sub-continent and Middle East for future opportunities. Paul is now in Singapore to work with MacDow and the Singapore Government’s Land Transport Authority on an underground station and tunnelling project.