Bulga Coal environmental coordinator David O’Brien presented a paper on managing and monitoring the impacts of mine subsidence at Beltana at the recent subsidence management conference organised by The Mine Subsidence Technological Society (MSTS).

Located 15km southwest of Singleton in New South Wales, Beltana is a punch longwall mine which started longwall mining in June 2003. Beltana is mining from the Bulga open cut mine pit and forms part of the Bulga Coal complex.

Mining targets the Lower Whybrow seam, ranging in thickness from 2.3m to 3.4m, with extraction from 2.65m to 3m. Panels are typically 264m wide and 2.7km to 3.3km in length.

Faced with managing the effects of subsidence on multiple surface features, including both natural and man-made, Beltana adopted a proactive approach, going well beyond what is required under mining law. The outcome, while costly and time-consuming, demonstrates the viability of longwall mining in a rural environment.

A key issue for the project was protecting the water resources of the Wollombi Brook, a perennial stream and shallow surface aquifer. The brook is incised into the alluvial deposits in the bottom of a valley.

Working with the various government departments Bulga Coal designed a buffer between mining panels and the alluvium. This was established as a function of the angle of draw to the alluvium, calculated at up to 26.5 degrees, equating to a horizontal distance of roughly 100m. An additional 40m was added as contingency. In other words, longwall panels were designed to stop short of undermining the alluvium by roughly 140m.

Model simulations commenced in 1990 and a computer mathematical model was developed in 2002 covering 250km2 around the project areas to understand the likely groundwater impacts of the project.

Future mining from 2002 to 2030 was simulated across all four seams. The modelling indicated mining would have a negligible impact on the water resources. Leakage rates are estimated to rise from zero to about 105 Kilo liters per day, during mining, about 0.00015% of the total daily storage in the alluvial aquifer of 70,000ML.

As mining progresses the groundwater monitoring program that has been in place at the Bulga operations will be expanded to confirm the predictions in the Environmental Impact Statement. This will include nine piezometers and speciation of groundwater.

In planning to undermine public roads a series of major strategies were put in place to manage the damage to roads. These included pre-mining inspections, public notices and having repair crews on hand to maintain the road.

It was expected extraction of longwalls one and two would affect a length of road of approximately 300m in length and to date the road has been successfully undermined with no safety incident.

Beltana also undermines a fibre optic telephone cable. This was a standard six fibre cable capable of withstanding a maximum tensile strain of 0.8% or 8mm/m. As strain was expected to be in excess of this, Beltana put in a new 300m section of cable in 50mm conduit to cater for the predicted levels of strain. Pre-mining testing was undertaken and multiple monitoring approaches adopted.

After being undermined by the first longwall the signal in the new section of fibre optic has been unaffected by subsidence. Replacing the fibre in conduit has been adopted for the next four panels and is seen as the best outcome for all parties.

The most cutting-edge work in terms of subsidence monitoring at the site is being done in relation to seven commercial vineyards, covering about 107 hectares, which will be undermined by the Whybrow Seam. No vineyard in Australia has yet been undermined by a longwall.

While minimal impact from subsidence was expected a comprehensive monitoring program has been carried out over the last year at two sites to be undermined. This initial assessment is crucial to establish baseline information against which any subsidence effect may be compared.

The monitoring program is unique in that it draws together a number of different technologies, some of which are at the cutting edge of techniques being used in vineyards around the country.

These include satellite imagery of the vineyard to identify vine vigour across all vineyards; measurement of any trellis distortion; monitoring of soil characteristics and sugar content mapping (to detect variations in quality).

A yield monitor coupled with GPS monitoring will produce yield maps to detect variations and identify areas of low yield. If vine vigour changes this reduces pruning and cane weight, and eventually yield. If subsidence does impact negatively on vine growth this will become evident within three growing seasons.

O’Brien said a positive spin-off of this was that the vignerons (people who run vineyards) said they have learnt far more about their operations than if they had not had a longwall mining coming through beneath them.

“While it is acknowledged this approach comes at a cost, one asks the question: what is the cost of not doing it?” O’Brien said.