Occupational safety and health

Definitions

A seismic event is the transient motion and release of kinetic energy caused by the sudden failure of the earth's crust. This release of energy is usually felt as shaking or tremors in the rock mass. This can be extended to include mining-induced seismicity which is the failure of the earth's crust or rock mass as a result of mining-induced changes in rock stress levels. Seismic events range in size from barely discernable ground motions to very large tremors.

There are three types of mining-induced seismicity:

• failure at pre-existing geological weak-nesses such as faults, dykes and joints which results in medium to large events often far away from workings;
• failure of the intact rock mass in the form of shear fractures that results in larger events close to workings; and
• localised bursting or failure of brittle rock types often referred to as strain-bursting or face bursting (small events at the working face).

The mines are equipped with state-of-the-art seismic monitoring systems. These vary from mine-to-mine, depending on the extent of the seismic problem. ISS International, a leader in the development and manufacture of seismic systems as well as seismic research and analysis, runs the seismic networks on a contract basis and provides a round-the-clock service in terms of network management and analysis. Seismic information is used in terms of short-, medium- and long-term seismic hazard assessment as well as for back-analysing the effectiveness of different mine designs and layouts. The performance of different support types is also monitored.

A rockburst is caused by a mining-induced seismic event of sufficient magnitude to cause obvious damage to mining excavations and support or widespread simultaneous falls of rock. This may be accompanied by injury or more seriously, fatalities.

Technology and Innovation Fund

This fund focuses on developing and implementing knowledge, rather than on fundamental research, on issues which are specific to operations in the South Africa region. This research and development is over and above that undertaken by the industry as a whole and government. Rock engineering related projects overseen by the fund over the past few years include:

• investigation into the development of support systems that can be transported by pipeline, eg packs superfill and shotcrete;
• development of technology for the rapid dissemination of seismic data via the intranet;
• optimisation of the daily seismic hazard rating system;
• development of a testing and quality assurance programme for support; and
• integration of rock engineering audit data into the integrated risk management system (IRMS).

Around R6.5 million ($1.01 million) was spent on these projects in 2003 and 2004 and an additional R1.5 million ($0.23 million) has been budgeted for 2005.

Implementing the fall of ground management strategy at Mponeng

A good example of the successful management of seismicity is Mponeng Mine. Mponeng mines to a depth of up to 3,800 metres underground and is one of the most seismically active mines in the industry. Its history has been marred by high injury rates, caused largely by seismicity in the 1980s and early 1990s. Since the late 1990s, Mponeng has implemented several strategies to reduce the incidence and effects of seismicity. These have resulted in a 70% reduction in the number of potentially damaging seismic events per square metre mined since 1996, and a 75% reduction in the seismic-related lost-time injury rate since 1998.

The strategies adopted fall into three main areas: mine design (prevention), support design (protection) and monitoring.

The management of seismicity will continue to be the backbone of the Mponeng risk management strategy with the increased use of backfill.