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News

Digging Deep for Bogong Hydro Power Station

16/09/2008
Digging Deep for Bogong Hydro Power StationThe Kiewa hydroelectric scheme is the largest scheme in Victoria, Australia. It was built over more than 20 years from 1938 until 1961, with construction interrupted when World War II broke out. Located in the Australian Alps in northeastern Victoria, about 200 km from Melbourne, the scheme has been developed solely for power generation. It has three power stations (Kiewa 1, 3 and 4) with a total capacity of 241 MW and an average annual electricity output of 340 GWh. The scheme diverts and harnesses the Rocky valley and Pretty valley branches of the East Kiewa river and the West Kiewa river. The three power stations in the scheme are McKay Creek (150 MW), Clover (29 MW) and West Kiewa (62 MW). The flow of water through these stations at successively lower levels enables the same water to be used repeatedly to generate electricity before being discharged into the river system. Water for the McKay Creek power station flows from the Rocky Valley dam reservoir and via a diversion dam from the Pretty valley. It flows by tunnels nearly 5.5 kilometres under the mountains, then by 1,250 metres of surface pipeline and finally by steel-lined pressure tunnel. The station is located 80 m underground on the slopes of Mount McKay. Water discharged from the McKay Creek power station then flows down the Pretty valley branch to Bogong village. Just below the junction is Lake Guy, which forms the head storage of the Clover power station. Water is conveyed to the Clover power station through a rock and steel-lined pressure tunnel. After use, it is discharged into a pondage formed by the Clover dam. This forms the head storage for the last power station in the chain, West Kiewa, built 140 metres underground.A tunnel from the Clover dam storage is joined by another which carries water from a diversion dam on the West Kiewa river. The combined flow passes through an unlined rock pressure tunnel to feed the West Kiewa underground power station. Water from this station is then discharged to a regulating pondage at Mount Beauty for release into the Kiewa river.Now Kiewa 2, called Bogong, will add the fourth and final power station to the Kiewa hydro grid. The project will add 140 MW of power to the Victorian grid. It is Australia's largest hydroelectric power plant in 25 years. Australian Gas Light (AGL) Southern Hydro is building the 140 MW hydroelectric power station at Bogong to provide additional capacity and increase the amount of zero emission electricity produced by AGL's power generation assets. The AUD230 million project will divert water from the McKay Creek power station's tailrace tunnel normally discharged into the Pretty valley creek. The water will run down a 6.5 km-long, 5 m-diameter headrace tunnel to a new power station on the shores of Lake Guy. The project will minimise the current environmental impacts of the operation of McKay Creek power station, by returning the water flow within the Pretty valley creek to near natural conditions. The Bogong hydroelectric scheme also addresses Victoria's need for peaking and renewable electricity generation without having to construct any new dams. Visit www.agl.com.auAlong with the main tunnel, the scope of work also consists of a 1.1 km steel-lined high pressure headrace tunnel, two vertical shafts, a power station to house twin 70 MW generators and a tailrace outfall into neighbouring Lake Guy. McConnell Dowell Constructors, the principal civil contractor responsible for construction of the tunnel, have been awarded a design and construction contract for the project. The designers are Halcrow Pacific. Click au/27. Visit www.macdow.com.au and www.halcrow.comThe start of tunnelling works was officially given in November 2007. Bogong is scheduled to be fully commissioned by October 2009 and will produce enough energy to power more than 14,000 homes each year.Main headrace tunnelThe main headrace tunnel is the major part of the project, featuring a 5 m bored diameter and a 6.5 km length. It is bored through granitic rock by a hard rock TBM built in the USA by Robbins and refurbished in Europe by Herrenknecht after being used in Lugano, Switzerland. Prior to the TBM starting its work, 134 metres of the main headrace access tunnel has been excavated through softer material before solid rock is reached. Eighty metres have been dug by road header and the remainder (54 m) by drill and blast in solid rock to make the launch tunnel ready for the 200-tonne hard rock TBM, named "Aurora Australis". The machine is 30 m long, but with all the back-up gantries including control cabin, transformers, staff amenities and conveyor belt it is 140 m long. The TBM requires around 11,000 volts. Visit www.therobbinscompany.com and www.herrenknecht.comThe temporary hydraulic rams are slowly pushing the TBM forward. Once at the face the cutting head supports itself with the help of gill type grippers just behind the main bearing. Behind these are four solid rams that are used to push the cutting head forward and behind these rams are the grippers that expand against the rock walls of the tunnel to secure the whole TBM while the rams drive the cutting head forward. At the full extension of the rams (about 2 m), the gills come out and lock into the walls. The grippers contract and the rams retract, dragging the rest of the TBM up to it. The grippers lock in again, the gills contract and the rams push the cutting head forward again into the solid rock with a pressure of 2,200 tonnes. The rock the TBM is working through has a very high strength of 180-240 MPa.Staff works in two ten-hour shifts per day. The remaining four hours is downtime when the boring tools are replaced, the power cable and the conveyor belt extended. Construction commenced in September 2007 and as of 1st August, the main tunnel of the Bogong hydro power station had been drilled some 4 km through the mountain in Victoria's Alpine National Park.The ground is reinforced with 2.4 m fully grouted CT bolts, various support classes from spot bolts to full pattern and steel sets, and synthetic fibre-reinforced (SFR) shotcrete with BASF admixtures typically. A hydramatic ARO equipment is mounted on the TBM and a rail bound Jacon spraying equipment. An example of the hardness of the rock is that solid concrete must have a hardness of 18 to 20 MPa. Visit www.ct-bolt.com, www.basf-admixtures.com or www.basf-cc.com.au and www.jacon.com.auThe rock from the tunnel is carried on continuous H+E conveyor from the cutting head over the top of all the carriages on gantries and then by a transfer conveyor out of the tunnel to a designated stockpile where it is loaded into trucks for loading into trucks and transport to designated storage points for future road rehabilitation. Much of the 300,000 cubic metres of rock that will come out of the 6.5 km main headrace tunnel will be used to form and seal 37 km of the Bogong High Plains road from Falls Creek to Shannonvale on the Omeo highway. Visit www.helogistik.de High pressure headrace tunnelThe smaller of the two tunnels is the high pressure headrace tunnel, which is three metres in diameter and will leave the power station site. It is drilled and blasted in a southwesterly direction or 1.2 km to meet up with the 90 m drop shaft from the main headrace tunnel. This high pressure headrace tunnel will then be completely lined with steel, grouted in with concrete.The high pressure headrace tunnel is 1.1 km and has a 4 m x 4 m excavated diameter, with concrete lining on 400 metres, 3.6 m in diameter, and 700 metres of steel lining, 3 m in diameter. It has been drilled and blasted from a single headling by an Atlas Copco drill rig 252, 8HR Häggloader and Haagcars. For ground reinforcement, the same techniques as for the main hearace tunnel are used. Excavation is completed and lining is underway. Visit www.gia.seDrop shafts and Bogong stationA 100 m-long 4.6 m excavated diameter, 4 m concrete lined drop shaft and a 140 m-long 5 m-diameter unlined high pressure drop shaft are raise bored with Robbins LM97 raise borer from Atlas Copco. One shaft is complete while the other is due for January 2009. Visit www.raisedrill.comThe Bogong power station itself, which will be 30 metres in height, will be partially buried below the existing ground level and will be almost completely covered (bunkered) with reclaimed earth and replanted once the project is completed. View pictures here. 37/08.



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Beakaert Maccaferri: bm-underground.com