Cumnock capitalises on strata management plan

Staff Reporter

A comprehensive new ground control program, and maintenance and cost reduction initiatives, are starting to pay off at the Hunter Valley mine.

Cumnock No.1 Colliery produced a record 2.225Mt in the 12 months to June 30 1999, which equated to an increase of 36% on the previous fiscal year. Most impressively, the mine’s daily output was lifted from 7000 tonnes per day in April 1998 to 11,200tpd in September 1999, representing a 38% improvement.

The underground operation, in the New South Wales Hunter Valley, is run by listed entity Cumnock Coal, which is 81%-owned by Glencore Coal Australia, credited with turning around what was once an ailing asset.

Many factors are attributable for the higher output, including an increase in face width from 140m to 220m and expanded coal clearance capacity from 1500t per hour to 2400tph. Mine management has also paid attention to a range of areas, including strata management, maintenance and cost reduction regimes — three areas in which Cumnock made notable progress in 1999.

Since 1993 mining has been only 35-40m above old workings in the underlying Liddell Seam, which included pillar extraction and longwall panels. Roof and floor strata are typically weakened by the subsidence of the underlying seam. These conditions have necessitated the development of a detailed strata management plan designed in conjunction with consulting group Coffey Geosciences.

In a conference paper on Cumnock’s strata management, Coffey manager underground coal mining, Gang Li, said initial investigations at Cumnock examined the feasibility of mining the Lower Pikes Gully Seam utilising surface drilling and field observations. Drilling was done at 10m spacings along a cross-section of the boundary between goaf and solid coal. “This drilling program has proven to be highly successful, as it provided vital information about the rib edge conditions where maximum differential subsidence, thus strata disruption occurred,” Li said.

One of the main achievements of the Coffey research has been the development of a geotechnical model which defines the expected range of strata conditions, from the unaffected A-zone to the D-zone, which is most subject to strata bending and difficult mining conditions. “Strata control in D-zones represents the key aspect of the SMP implemented by the mine,” Li said.

The geotechnical model predicts that a D-zone would show characteristics including open mining-induced fractures and re-activated joints. “In D-zones with predominantly delaminated roof, beam building and pre-tensioning were the main features of the support design,” Li said. “The A-B-C-D-zone concept was adopted as a simple and effective way of communicating a complex geotechnical situation in simple terms. The process of activating D-zone support was easy to understand ... and was therefore highly effective. This is one of the key issues for the success (of a) mining operation in a geotechnically complex environment.”

In normal operating conditions Cumnock has begun using pre-tensioned five-foot roof bolts — at least a foot shorter than what was traditionally used for support. Mine manager Neville McAlary said the new system, designed by Seedsman Geotechnics, had delivered good results. No roof bolt falls were occurring and regular testing indicated the bolt consistently measured at design specification of greater than 8t.

“It’s not only the foot of steel you drop out of the bolt, it’s the 10 drill bits for every pillar you do and drilling those extra four feet for every strap; (that adds up to) an overall downstream effect on the savings of consumables without compromising the integrity of the roof.”

McAlary said the next step was to explore single-pass drilling.

Incremental cost reductions are critical at Cumnock which operates at a cutting height of only 1.8-2.3m, a distinct disadvantage in comparison with miners cutting at plus-4m. “For every shear the West Wallsends, the North Moranbahs, the Dartbrooks cut, we have to do two. The only way to do that in the same time is to go faster,” McAlary said.

One strategy Cumnock has embraced in order to better compete is the “American-style” operating method. McAlary said in comparison with many south coast mines which had drum speeds of between 26-32rpm, Cumnock clocked 46rpm on the shearer and a haul speed of 16m per minute. “When you have fast spinning drums and fast hauling rates for the shearer, combined with bi-directional shearing, like the Americans, your productivity levels go up,” he said.

An intrinsic aspect of Cumnock’s day-to-day management is a sophisticated system of condition monitoring (CM) undertaken by Pacific Power. In the past six years Pacific Power has won CM contracts through subsidiary company, Powercoal, and now provides CM services to various Hunter Valley mines, including West Wallsend and Mt Owen.

At Cumnock, on-site CM manager Gary Horner has been carrying out a wide range of testing, analysis and consultancy services for the past two years. Horner said the aim of the partnership was to ensure Cumnock met production targets with a reduction in overall maintenance costs.

“Pacific Power has accomplished this by specialists working with Cumnock management through the CM manager to fully integrate advanced CM techniques with Cumnock’s overall operating and maintenance strategy,” Horner said.

While Horner assists in planning outages and repairs to defective plant, one of the more significant aspects of the CM program has been to ‘close the feedback loop’ between CM, production, and operations and maintenance. “In short, Pacific Power’s vision for CM at Cumnock is to provide the critical links between the current plant’s condition and corrective actions between maintenance and the overall CM system effectiveness,” he said.

McAlary said the system of CM as applied at Cumnock was one of the more sophisticated maintenance regimes in the Hunter Valley. “We now have a database which provides us with a base level footprint of condition monitoring information which we use to stay on top of critical components,” he said.