Researchers investigating the causes of premature rock bolt failures in Australian coal mines are awaiting funding to broaden and speed up the work, which has serious mine safety and cost implications.

A report lodged last month by the researchers - led by University of New South Wales Professor Bruce Hebblewhite, Strata Control Technology's Mike Fabjanczyk and Ground Support Services' Dr Peter Gray - highlighted the widespread problem of stress corrosion cracking (SCC) in rock bolts, the need for improved field evaluation of bolts and the gap between Australia and other, lesser users of mine rock bolts in the area of bolt material and performance standards. Britain, for example, which uses about a quarter of the rock bolts used in Australian coal mines, has had a bolt-material testing standard for six years and is currently updating its guidelines to incorporate modern performance-testing procedures.

Australia does not even have a standard to apply to the steels being used in rock bolts manufactured here, used in both coal and metalliferous mines, raising questions about quality control. This is known to have surprised at least one of the major suppliers of steel to rock bolt suppliers. It is currently investigating the path to creation of an Australian standard with other suppliers.

Presenting the results of two years of research at an industry briefing in Sydney last month, Professor Hebblewhite, Fabjanczyk and Dr Gray said SCC-related rock bolt failure was a more common problem than the industry had previously envisaged and was occurring under a range of different circumstances, not in specific mine conditions as previously thought. Bolt failure could have “catastrophic consequences”

Lack of consistent, formal reporting systems was retarding efforts to get a better handle on the causes and extent of bolt failure problems, they said. A further problem was the absence of a proven, affordable tool for routinely testing the non-protruding portion of bolts in mines.

“The current project has effectively characterised the problem and focused on some of the key metallurgical and environmental conditions under which the problem has to date been detected,” the researchers said.

“A real concern is that the database, to date, only consists of bolts that have failed below the grouted horizon and dropped out onto the floor. Because of the nature of the problem, there is a very real likelihood that, under the right conditions, some bolts are failing within the grouted horizon but remaining in the roof.

“At least two examples of broken bolts exposed in falls has indicated premature failure of bolts within the encapsulated section of the bolt.

“These give the appearance of a competent bolt but may in fact be failed insitu and effectively only offering a greatly reduced bolt length. The safety implications of this problem are considerable and warrant priority future investigation.”

Dr Gray told Australia's Mining Monthly that while the use of higher-fracture-toughness-steel bolts by coal mines appeared to be helping to alleviate SCC-related problems, the level of fracture toughness required to prevent premature bolt failures required further investigation. He could not say how many mines were using the new steel bolts, nor could he comment on the number of mines using low-fracture-toughness bolts.

Dr Gray said SCC-related problems, including safety concerns, were almost certain to extend beyond the nine collieries included in the initial research database. Because of the sporadic occurrence of premature bolt failure and the low level of record keeping at local mines, only a limited sample was available for the database.

“A much more thorough examination is needed,” Dr Gray said.

“Industry representatives at the briefing were positive about this (conducting further research) and we would expect, subject to ACARP (Australian Coal Industry Research Program) funding approval, to start work on the new project early next year.”

Apart from the safety implications - a senior NSW mines inspectorate official has already warned mine managers they are in an “indefensible position” if they don't take steps to better understand and manage rock bolt failures - coal mine owners will be spurred to act by the significant though at this stage unspecified cost of widespread premature bolt failures. They may even take a more pro-active role in the development of a bolt material and performance standard, something invariably driven by suppliers.

In the meantime, one solution to the problem of inadequate measurement of insitu bolt integrity that has emerged is the German-developed ultrasonic non-destructive bolt testing device. Approved for use in Germany, the tool has been evaluated at several Australian mines and is expected to be the focus of further examination by the ACARP-funded research team.

Dr Gray said the group also aimed to:

- Further develop metallurgical and corrosion surface test procedures and database expansion (and in particular focus on development of a simplified steel “toughness” test).

- Investigate and document properties of new steel products in the marketplace.

- Investigate the extent of potential bacterial “bug” corrosion of bolt steel and possible remedies.

- Document the extent of brittle failure of bolts in threaded sections, especially with regard to bolts used for hanging structure, monorails, etc.

- Provide the industry with guidelines for minimising SCC problems, including bolt and steel traceability.

- Assist in the introduction of an Australian standard for rock bolts.

Parallel work in the Australian metalliferous mining sector was initiated by the Western Australian School of Mines. The coal industry researchers said it appeared SCC was surfacing as a “significant problem in a number of Australian metalliferous mines”

“In their case the prevalent use of point anchored bolts results is an even more severe problem due to 100% loss of capability on failure,” they said.