Premature Rock Bolt Failure: Stage 2

A previously funded ACARP project (C8008), conducted by the same collaborative research team, investigated the problem of premature failure of rock bolts due to stress corrosion cracking. That project was reported in April, 2002.

Stress corrosion cracking is defined, in layman's terms, as "Stress corrosion cracking is slow, progressive crack growth under the application of a sustained load (either residual or applied) in a mildly corrosive environment, with failure occurring below the ultimate tensile strength of the material."

This project was proposed to pursue the recommendations of the previous project. In particular, the project proposal initially set out to investigate the following five key areas.

• Non-destructive bolt integrity testing
• Metallurgical Database and Test Procedures
• 'Bug' Investigations
• Brittle bolt failures under heavy-duty end loading
• Industry guidelines

In response to the project proposal, the ACARP Committee reduced the project scope along the following lines:

"Whilst recognizing the merit of the preceding project C8008, the committee limited funding to $50,000 to help the research team consult industry and rework the full proposal. The full proposal should attempt to define the extent of the problem, and list supporting mines. It could also answer the question, is the solution to simply move to recently developed high fracture toughness steels?"

This final report from the Part 2 project addresses the findings of this modified scope of work.

This second stage ACARP research project has provided a number of new and supplementary insights into both the mechanism of premature bolt failure by SCC, and the occurrence of it within Australian coal mines. The following is a summary of the major conclusions reached.

• The research has provided a further clarification of the type of bolt failure mechanisms that occur, both in Australia and elsewhere.
• There continues to be further evidence of SCC failure at a number of mine sites, some of which are additional to those investigated in the previous ACARP project.
• These examples of failure included some relatively recently installed bolts.
• The use of the DMT ultrasonic testing device has been successful in identifying failed bolts within the resin-grouted horizon, i.e. failed bolts remaining up the hole with a their effective bolt length significantly reduced.
• Final proof of failure following the ultrasonic testing would require overcoring. Unfortunately this was not able to be achieved at the mine sites in question.
• The extent to which further testing could be conducted using the DMT device was limited by lack of access to mine sites.
• It has not been possible to provide any statistically -valid estimate of the extent of the SCC-induced bolt failure problem, other than the evidence found in the first project, plus further isolated cases, including detection of failures within the grouted horizons at sites where the DMT device was used. The problem does still exist in multiple mine sites and coalfields, but the extent of the problem remains unclear, due to difficulties in gaining site access.
• On a positive note, there has been very emphatic evidence of a change in SCC failure experience by one of the test mines from the first study. Since the original work was reported, the mine which had experienced a large number of SCC failures changed the type of steel used in their roof bolts to a higher fracture toughness steel. Since that change took place, although mining in the same areas with similar geology and conditions, there has not been a single reported SCC bolt failure in nearly three years. This is very encouraging evidence for the fact that higher fracture toughness steel can be an effective means of combating the SCC problem.

The research team remain of the opinion that, subject to achieving the opportunity to access and survey multiple mine sites in a range of different locations, further work would be beneficial for the industry, essentially in the areas originally proposed.