Top performance: These ROC F7 rigs, using blast hole diameters of 89 mm and 102 mm, have exceeded all expectations, helping the project to get ahead of schedule.

The second spillway is needed because the dam was originally designed to handle peak outflows of 12,700 cubic metres per second when it was built by Sydney Water in 1960. More recent assessments of the Probable Maximum Flood, however, suggest that peak outflow could be over three times that level, which would overwhelm the dam, collapse its retaining wall and send a cataclysmic torrent of flood water crashing down the river.

Previous solutions to the flood risk included a proposal to raise the dam's retaining wall 23 metres, which outraged environmentalists. Not only would upstream wilderness areas and the scenic Kowmung River be threatened, they argued, but massive expansion of housing in environmentally sensitive areas downstream of the dam would be encouraged. Since the spillway project would only divert water away from the dam, however, it would not shrink the floodplain downstream. More land for development, therefore, would not be created.

The Atlas Copco crawler rigs in action, drilling up to 750 metres in a single shift.

Artificial channel

About 2 million cubic metres of rock will be excavated from the area just east of the dam's retaining wall to create an open channel beside it, which will be concrete lined. Structures known as "fuse plugs" will be constructed at the entrance to the spillway to prevent its operation unless a major flood event occurs.

During a flood, the pressure of the water would cause the fuse plugs to be breached. Flood waters would further erode the plugs, allowing even more water to bypass the dam and exit into the Warragamba Gorge 300 metres downstream, securing the safety of the dam - and Sydney's water supply.

Although the decision to build a spillway was generally applauded by environmental groups, issues remained, including how best to preserve the wilderness where construction is now taking place. Since some of the native flora and fauna are considered unique to the region, the area being used for fill material has been saved for future restoration. Top soil and plants have been removed and will be replaced and replanted at the conclusion of the project.

The operation must also operate under strict rules to limit noise, due to the proximity of residential housing. Drilling hours are 7 am to 6 pm Monday to Saturday, with no night time drilling or blasting.

Some 96 people from 56 contractors are currently on site. The lead contractor, Abigroup Limited, has contracted Precision Blast Hole Drilling Pty Ltd for the drilling of blast holes. The USD 56 million, 18-month contract will be completed by September, 2000.

Strong performance

After removing the rock in the first year of the project, up to 60,000 tonnes of concrete and 5,000 tonnes of reinforcing material will be used to line the spillway. In the first three months, to May 1999, Abigroup moved 10,000 cubic metres of material per day. That figure has now climbed to 14,000 m3 and the strong performance of Atlas Copco ROC F7 drill rigs is an integral part of this picture, drilling up to 750 metres in a single shift.

Precision Blast Hole Drilling purchased their first Atlas Copco surface drill rig two years ago. The company's co-owner, Andrew Lyell, says that the decision was initially based on the technical features of the machine, and adds: "Since then, the performance, combined with the technical service and support that we have received, has more than justified our choice.

Cabin with a view: Operator Ken Miller, at left, with Abigroup Safety Officer Matthew Moon, appreciates the ROC F7's ergonomics, including its increased visibility.

"Precision drilling, for example, is very important in minimising the possibility of dust and rock falling into the water supply. And the Atlas Copco service team is available all hours to keep the machines up and running. In fact, we are expecting to finish drilling ahead of schedule."

The power of the rigs has also impressed at least one of the operators, who said: "These drill rigs have the most drifter (rock drill) power that I have seen. The machines are more complex than other drill rigs but they drill a lot faster."

The company added a second ROC F7 to their fleet in 1998. Their third started work at the site in September.

The ROC F7 is part of the new Power Generation range of surface drilling rigs. Features include:
-Increased cabin ergonomics, with the operator controls placed in the armrests
-Increased visibility during drilling and tramming, with a full-height front window and
-Adjustable operator's seat and air conditioning.

Ken Miller, one of the drillers operating the equipment, has high praise for these ergonomic features, saying: "In 35 years of operating drilling equipment, I've worked for a total of 14 months on the Atlas Copco ROC F7. It is definitely more operator-friendly than any other rig I've operated. The cabin ergonomics are great."