New Denmark is owned by Anglo Coal is one of three operating longwall mines in South Africa.

The mine experiences windblast events predominantly at the start of longwall blocks where the roof tends to hold up for the first 30m to 60m. As the longwall advances past this critical point goaf caving occurs normally with occasional roof hang up.

New Denmark conducted its first successful goaf monitoring project in July 1996, when the West Longwall mined through a pre-mined dyke. The monitoring was achieved through surface boreholes, using ISSI as the specialist contractor.

The result of this work was that a definite lead time was identified between a major fracture occurring over the longwall and the physical “goaf” happening on the face.

The development of early warning systems, using technologies such as geophones and seismic sensors, was however somewhat delayed in South Africa due to the need for intrinsic safety of all components and some skepticism of new technology.

Now, New Denmark is currently installing a new technology on its next longwall panel that is due to start mining during the second week of December 2003. The goaf fall is expected on December 17 or 18.

The tool, known as Goafwarn, was developed by South Africa’s mining research organization, CSIR, for that country’s extensive bord and pillar operations. Goafwarn was developed to provide the continuous miner operator with feedback during remnant pillar extraction, where continuous goafing is part of the mining process.

In bord and pillar mines the Goafwarn device is installed in the roof and provides constant feedback to the operator about the likelihood of goafing occurring. The operator receives a sound and colour warning to his belt.

Three colour zones are delineated: green, orange and red. In green zone, where 80% of operating occurs, the chances of goafing are 0%; in orange where 80% of operating occurs, goafing will occur 20% of the time; in red zone, goafing likelihood is 80%.

Van Zyl Brink, an engineer from South African research organization, CSIR, is involved with adapting the technology for use in a longwall environment. Brink was involved with aspects of the windblast management plan developed at the New South Wales Moonee longwall mine.*

Brink said the Goafwarn system being implemented at New Denmark is a much simpler version that that used at Moonee, with an important improvement – the data is analysed at the face, not at a remote surface computer. In Moonee’s case, the processing instrumentation was located at the surface but with the CSIR system all the processing power and intelligence is centralised in the instrument itself.

With such seismic systems, one needs history to predict the future. Goafwarns have been installed at New Denmark in two instances to date, and as the required history was being collected an attempt was made to use it in “real time” though the system is far from being proven.

Conceptually, in a longwall, four Goafwarn devices are installed, two in the maingate and two in the tailgate, roughly 60m apart, that are relocated as mining proceeds. The processing of incoming data related to microseismic events occurring in the roof is performed within the Goafwarn device itself.

The analysis of incoming data and the process of generating an alarm is very similar to what was developed at Moonee, except that it is intended for this to take place at the face, which means no communication backbone is required. Once fully functional workers on the face, within a certain limited range will receive sound and colour-coded warnings like the green, orange, red zones used in bord and pillar.

In the case of a longwall these alarms will not be absolute alarms, Brink said, but rather serve to kick in a heightened risk management strategy. Like Moonee this may involve workers staying out of roadways when the goaf is hanging up and knowing how to reach safe havens.

The development of Goafwarn will potentially offer a step change in addressing the needs of mine operators, both longwall and stooping, and provide an opportunity to use technology to better manage the goafing cycle process.

* The Moonee operation – near Newcastle in NSW - closed a year ago and had enormous problems with windblast since 1996. Moonee mined the Great Northern Seam with a 100m wide longwall face under 40m thick massive conglomerate structure. The goaf could hang up for up to 300m and then cave en masse, generating windblast events of 123m/sec. In 1998, after a number of windblast events and injuries to workers the mine developed a windblast management plan.