Ridgeway, which is located 3km north-west of Cadia in NSW, is expected to be commissioned in early 2002, building up to ore production of 4 million tonnes per annum in 2003.

At that level, Ridgeway will be Australia's fourth biggest underground mine behind Olympic Dam, Mount Isa, and Northparkes. Peak annual metal production will be 350-370,000 ounces of gold and 26,000t of contained copper in concentrate.

Ridgeway's development was approved by the Newcrest board in November last year, but it moved into top gear in June when the company awarded an underground development and mining contract to Peabody Mining Services Pty Ltd. The $138 million job includes trial mining works, underground development activity, and ore production, from project commissioning through to December 2003.

Peabody got the nod ahead of other contenders, including WA contractor Brandrill, which built the exploration decline at Ridgeway.

"They [Peabody] did most of the work at Cannington [BHP silver mine] and a lot of the Roxby Downs [WMC] expansion," said Newcrest director Bryan Davis. "So they are not short on mining experience. But they do a lot of civil work as well."

Newcrest runs its own fleet at nearby Cadia and the Telfer gold mine in WA, and looked seriously at owner mining for Ridgeway.

"During the first phase of mining we'll be evaluating in more detail those issues, including owner mining, whether we stay contracting, what equipment we'll use and so on," Davis said.

"That is why we've left it flexible. We'd have needed to recruit an underground workforce [for owner mining], and while we're developing, it makes sense to use a contractor to evaluate all your options. Do we stay with diesel loaders or use electric? Do we use fully automated or semi-automated? We'll use that period with the contractor in there to evaluate all those issues."

Newcrest has chosen to develop Ridgeway using a bulk mining method called sub-level caving, which is ideally suited to the vertical pipe-like orebody. It has a typical diameter of 180m that extends from 475m to 1,025m vertical depth, beneath which it is open.

According to CIBC World Markets analyst, Tony Lethlean, it should provide low operating costs over the budgeted 14-year mine life.

"This bulk stoping method, together with the grade of Ridgeway, should see cash costs better than $250/oz," he said in CIBC's latest Gold Book.

Tim Lehany, general manager of Ridgeway, said sub-level caving was chosen after consideration of a number of alternative methods.

"We've had opinions from a number of people who worked on the feasibility with us that they haven't seen an orebody better suited to the method," he said.

"It has a very good footprint which enables us to establish quite a number of production draw points. At its best it is around 136,000 tonnes per vertical metre. It is a really good size orebody for a large tonnage, sub-level cave operation."

Newcrest had completed a lot of geotechnical work on the overlying country rock, which confirmed it would cave successfully.

"And we have been very conservative in our approach," Lehany said.

"The footprint we are developing to establish it is larger than the dimension that is required to establish caving. So we are not taking any chances."

According to Davis, sub-level caving is also used at LKAB's Kiruna iron ore mine in northern Sweden.

"This is the same mining method, but on a much reduced scale."

He said the ready "caveability" of the ore and host rock should not cause any serious headaches in terms of ground support. The mine will use 2.4m resin encapsulated rock bolts, along with 50mm steel reinforced fibrecrete for surface support.

"Provided you only open them up in small dimensions in a drive, they [the rocks] are quite stable," Davis said.

"This is non-entry mining. You just put the loaders in there and keep mucking it out. You don't put anybody under ground that has been blasted. It is all in front of you. So you are working under supported roof all the time. When it is empty you go down to the next level. And the thing just keeps caving up to the surface, and eventually reaches the surface in terms of a depression."

At the end of 14 years, Newcrest predicts the underground mine will have created an ellipsoid cave zone on the surface of dimensions 800m by 600m, with a depression of around 120m.

The nature of the mining method lends itself to automation, such as the computerised loaders being used at Kiruna. But Davis said nothing had been finalised in that regard.

"Peabody will start using manned loaders, and we'll just suck it and see," he said.

"So many things are happening with computerised equipment, and more than one supplier can offer you the same result. We haven't made up our minds yet, but will look at all options from manual to completely automatic. Nothing we're doing will preclude using one or the other."

General manager Lehany said Newcrest would install a voice and data communications backbone in the mine, with future automation in mind.

"Our philosophy is we build it so we don't rule out any future options in terms of remote controlled loaders and so on. All of that is being scoped and will go through a design phase. The contract runs until end-2003 which gives us time to do our technical evaluation and put our plans in place."

To read more on the Ridgeway development see the August edition of Australia's Mining Monthly. Fax: +618 9381 1848