Stress corrosion cracking (SCC) has been reported to be a significant cause of premature failure of rockbolts in Australian coal mines [1]. At some mine sites, failure of rockbolts due to SCC has occurred well within the twelve to eighteen month design life of longwall gateroads. Failure is often associated with installations in coal and carbonaceous shales containing clay bands. The use of very high tensile grade steel is commonly used in many of these mines and it is thought that the low impact toughness of this steel may be a significant contributor to SCC failure. While laboratory corrosion studies have been undertaken to assess the metallurgical influence on SCC, the results do not correlate with field observations. A possible contributor to the enhanced incidence of premature failure in recent years may be due to the increase in extraction width from 250 m to 400m in many Australian longwall mines that can lead to an increase in the loading on the rockbolt. This paper examines the current state of knowledge concerning rockbolt failure in Australian coal mines.
Australia is the world's largest exporter of coal worth nearly $A22.5 billion in export earnings in 2006-07. While longwall mining accounts for approximately only 18% of total coal production, this proportion is likely to increase as surface open cut operations reach their economically viable limits [2]. The incidence of stress corrosion cracking (SCC) of rockbolts has been reported in several of the thirty-odd longwall mines operating in Australia, with at least three mines reporting a high incidence of premature failed bolts [1].
The School of Mining Engineering at the University of New South Wales has recently commenced a research project into SCC of rockbolts in Australian coal mines.
The objectives of the project are to identify the environmental causes of SCC and ultimately provide a tool for the identification of SCC high risk environments within a coal mine. The subsequent knowledge and understanding gained about the SCC environment will be applied in building a laboratory facility for testing of various rockbolts. The laboratory facility will enable the development and testing of new coal mine rockbolts to withstand SCC.
The extraction of coal using the longwall coal mining system involves large areas of coal some 250m wide by 2000m long which need to be initially blocked out. These areas are delineated by development tunnels or roadways driven within the coal using Continuous Miners or similar coal cutting machinery. The roadways are cut into the near horizontal coal seam forming a rectangular profile. Weak to moderate strength sedimentary rock layers are present in the roof and floor of the roadway, typically these include shales, claystones, siltstones and sandstones.
Over the years various means have been employed to ensure the stability of these roadways and other underground excavations as the surrounding rock mass is subjected to high levels of stress.
