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Japan working with South Africa to reduce risks in deep-level mines

26th September 2014

By: Keith Campbell

Creamer Media Senior Deputy Editor

  

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Japan is assisting South Africa with research into seismicity and mining.

“We have joint research on mitigating seismic risks in South African mines,” reports Japanese ambassador to South Africa Yutaka Yoshizawa.

“Japan experiences a lot of earthquakes, so Japanese universities have a lot of good technology regarding seismicity that can be used in South African mines. South African institutions involved in this research include the Council for Scientific and Industrial Research.”

The Council for Geoscience (CGS) is also a participant, as are the University of the Witwatersrand and, on the Japanese side, Ritsumeikan University, Tohoku University and the University of Tokyo.

This joint research project is a five-year programme that has been funded by the Japan International Cooperation Agency (Jica), the Japan Science and Technology Agency and the South African Department of Science and Technology. The project is part of the Science and Technology Research Partnership for Sustainable Development (Satreps) and is entitled Observational Studies in South African Mines to Mitigate Seismic Risks. It will conclude next year.

Japan is located where four of the plates which compose the earth’s crust meet and push against each other. The country has suffered a number of very severe earthquakes (and associated tsunamis) over the past few centuries, in which thousands and even tens of thousands of people have died. In the past 100 years alone, these have included the 1923 Great Kanto earthquake, in which 99 000 perished, and earthquakes in 1948 (more than 5 000 deaths), 1995 (more than 6 000 people killed) and 2011 (the Tohoku earthquake and tsunami, which killed more than 20 000). There have been many more smaller and less deadly earthquakes as well. Unsurprisingly, then, Japan is one of the world leaders in research into seismicity and earthquake prediction.

Further, although not a major mining country, Japan does have a mining sector, long dominated by coal mining (although this is now in decline). Possessing underground mines and experiencing extreme seismicity mean that Japan’s experience is relevant to South Africa.

One Japanese technology derived from these experiences and now being tested in South Africa is the use of acoustic emission (AE) sensors to warn against rockbursts in deep-level mines. “Rockbursts, as well as natural earthquakes, are caused by sudden slip movements inside the rock mass,” explained University of Tokyo associate professor Dr Masao Nakatani in a Jica newsletter in March. “Faults are big fracture surfaces in the rock mass extending from metres to tens of kilometres in size. They serve as slip planes for both natural earthquakes and relatively large rockbursts in mines. In addition, in mining environments where excavation at depth squeezes the surrounding rock mass severely, large fracture zones up to tens of metres in size form in the intact rock near to the mining stopes. These new fracture zones may also serve as sites for violent slip movements.”

Laboratory research on rock samples has revealed “precursory” submillimetre-scale microfractures. These exist on both already existing faults and new fracture zones. The process of using a network of these microfractures to outline a developing and large rupture is designated the AE Method and this has already been applied in a number of geotechnical areas, including the development of petroleum and geothermal reservoirs and the monitoring of bridges and tunnels. In a research project at the Cooke 4 mine at Westonaria, 24 AE sensors were placed in boreholes drilled in the mine’s tunnels, 1 km underground. These detected the faint vibrations caused by microfactures.

Over two months, these sensors recorded an average of ten thousand microfractures a day. (No rockbursts took place during this time.) On the other hand, many of the already well-known faults in the area being monitored were “quiet”. Nakatani and his team suspect that a fault that “talks” a lot is highly stressed, near to the point of failing. This monitoring “has revealed strong localisation of microfractures at a large scale comparable to damaging rockbursts, both on geological faults and in the intact rock near the stope,” he reported. “. . . [T]he clear and strong localisation revealed so far suggests that AE monitoring has a high potential for tracking the development of large-scale damage structures in mines.”

In another part of the same project, involving cooperation between Jica and the CGS, a network of ten seismic stations was installed on the surface in the Western Deep mines area to complement existing surface and underground networks. Set up in 2012, the Jica/CGS network has improved the monitoring of mining-induced seismic events in the Western Deep area.

Also under the aegis of Satreps, Japan is providing seismic-related training and research opportunities for South African scientists and engineers. These have included AE experiments by South Africans at Japan’s National Institute of Advanced Industrial Science and Technology.

Edited by Martin Zhuwakinyu
Creamer Media Senior Deputy Editor

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