The crippling impacts of unmanaged seismicity on mine production and sustainability

Mining engineering leader, Atlantech Technical Services Manager (Western Australia) Guy Simpson has more than 30 years’ global experience in both underground and open-pit mining across operational and technical disciplines. In this insightful article Guy shares valuable lessons learned at a mine where the pursuit of production over engineering ultimately crippled the mine’s sustainability.

By Guy Simpson, Atlantech Technical Services Manager, Western Australia

Some 23 years ago I was hired as the Underground Manager of a sub-level cave underground gold mine which had high tonnages for an Australian underground mine. This was due to having a very wide and moderate grade sub-vertical orebody.

The orebody was pushing 3.2g/t with 30 to 40m wide stopes, which would have made it a rainmaker at today’s gold price. Remembering that the gold price at that time hovered around AUD $500/ounce, at 90% mill recovery we had a value of about $46/tonne. With a total operating cost – including processing sitting about $41/tonne – there was not much room for capital expenditure, and we had to keep the ore tonnes high and operating costs tight.

As one can imagine with such low gold prices, we were fortunate to have some very talented people on the team with considerable experience. It was a period of high unemployment in gold mining and employers had their pick of the crop. We grimly held onto our jobs. On the flip side, things were nowhere near the costs of today.

The mine plan was to produce at a rate 1.8Mtpa and utilised R2900 loaders, Cat 773 trucks, and Simba 4356 longhole drills. Ore drives and stopes were accessed via level crosscuts from the decline, footwall drives and mill holes to maximise flexibility and production. Due to the size of the trucks, loading ramps off the footwall drives were created.

To reduce capital expenditure, a decision was made to reduce the advance of the decline. This resulted in the stoping front advancing deeper after a time and the sub-level cave front becoming quite flat. Whilst the mine was relatively shallow (<550m) it had been dealing with very high in-situ stresses almost from the start of mining the first upper-level stopes. Much of the development faces would spit small rocks from the faces during development. The mine was in a near constant state of rock noise.

As the cave became flatter (fewer levels being stoped) the area for the stresses to re-distribute became smaller and smaller and, as a result, the stresses at the cave front (bottom production level) became very concentrated with minimal area to disperse. This resulted in rock bursts occurring in abutments around the closure pillars, footwall drives, and passing bays. These rock bursts almost exclusively occurred immediately following a stope firing which resulted in a sudden re-distribution of stresses to localised pillars. Effectively the stresses exceeded the strength of the rock mass. Stope firings only occurred when mine personnel were on the surface.

A seismic monitoring array had previously been installed and it allowed the occurrence of rock bursts to be monitored in real time, often resulting in a direction to not return underground until the system showed little or no rock burst activity following a stope firing.

These sudden events would sometimes result in localised rock bursts which would cause significant damage to footwall drives and associated infrastructure. Significant down-time was then needed while the area was rehabilitated to regain access to the production level.

To reduce the impact of the rock burst events, an early type of grouted dynamic ground support was designed and installed with a very high strength mesh. This allowed the ground to decelerate during a rock burst and contain the burst rock as opposed to sudden ejection and significant damage and rehabilitation. This conical-headed ground support acted as a brake and would allow movement as it was dragged through the grout.

Ultimately, it was determined that the cost of this additional seismic support including materials, manning and machinery was simply not sustainable at prevailing gold prices. Also, the retreat pillar mining method and impact of seismicity following stope firings was not sustainable and changes had to be made.

A decision was made to abandon the existing method of accessing the orebody via central crosscuts and footwall drives and millholes. The decline was started again, and new level accesses were developed from the decline to the northern and southern extents of the orebody and ore drives driven along the full strike of the orebody. Stoping would now start from the centre of the orebody retreating towards the level accesses. This eliminated closure pillars and abutment areas and the impact of rock bursts effectively ceased.

Seismic activity remained a risk usually within 24 to 48 hours after stope firing, so all stope bogging was undertaken using teleremote loaders to reduce exposure to personnel at the cave front. The overall impact of this new extraction methodology was that the flexibility of millholes and footwall drives was lost, and mine production dropped from a rate 1.8Mtpa to 1.2Mtpa once stoping began again. This caused fixed costs to increase significantly as a proportion of overall unit costs.

Efforts to reduce operating costs were accelerated and all unnecessary expenditure was cut. A change of mine ownership happened shortly after this with an aggressive campaign to reduce costs further and attempt to increase production. But ultimately it was not sustainable, and the mine closed soon thereafter.

The clear lesson learned was that the control of the stresses was lost when the decline was stopped, creating a cave front that did not allow enough surface area to dissipate the stresses. A lack of foresight and drive for ounces led ultimately to sizeable financial losses and the closure of a quality asset.

The geotechnical engineers at the operation were first class and were always coming up with innovative ideas for ensuring safety and improving production. They cut their teeth on that operation and ultimately went on to very successful careers due to receiving such a baptism of fire and the learnings they took from it. Personally, this was the hardest operation I ever worked on yet also the most beneficial due to those same learnings.

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