Download files
Abstract
The work describes a study of stress corrosion cracking (SCC) failure of rock bolts in underground coal mines. It focuses on understanding the environment in which failures occur and the mechanism behind failure. SCC tests which could be used in-situ in underground mines were also evaluated and validated in the laboratory.
Analysis of the ground water was conducted for two mines (Mines A and B) which were known to have undergone a substantial number of SCC failures. Both mine waters were found to be alkaline and corrosive. The alkalinity was almost exclusively due to the presence of bicarbonate ions. The bicarbonate levels in Mine A were substantially higher than in Mine B. Microbiological analysis of water from Mine B showed that it contained both iron- and sulphur-reducing microorganisms.
An investigation into the failure mechanism in rock bolts retrieved from these two mines was also conducted. This indicated that SCC in underground coal mines can occur by two different mechanisms. In high alkalinity mine water, SCC occurs by an anodic mechanism involving carbonate/bicarbonate cracking, whereas in low alkalinity mine water, a cathodic mechanism occurs involving hydrogen induced cracking. The two different SCC mechanisms are similar to the high pH carbonate/bicarbonate SCC and the near-neutral pH hydrogen induced SCC observed in buried pipelines. The occurrence of carbonate/bicarbonate rock bolt SCC has not been reported previously.
The use of a pin-loaded slotted coupon SCC test was examined which was suitable for use in rock bolt bore holes while also retaining the physical and metallurgical features present at the surface of rock bolts. The test methodology was validated in the laboratory using an acidified sodium chloride sodium sulphide solution. Laboratory testing was subsequently conducted for 3 months in ground water extracted from Mine B using the pin-loaded specimen. Alternating 8 hour wet/8 hour dry cycling was used for the tests. No SCC failures were observed over the duration of the test. This may indicate that microorganisms, which did not survive when the water was removed from the mine, may be involved in the in-situ SCC process.