Investigations into the corrosive environments contributing to premature failure of Australian coal mine rock bolts
Introduction
The problem of premature rock bolt failure in Australian coal mines was published in 2002 and 2004 by UNSW Australia from Australian Coal Association Research Program (ACARP) funded projects. The majority of the broken bolts examined had steel Charpy impact toughness values of 4–7 J, and the failure mechanism was most often stress corrosion cracking (SCC). Steel fracture mechanics predicts that an increase in impact toughness will increase the length of the crack before sudden brittle failure. In the final report of 2004, anecdotal evidence from one coal mine indicated that the problem may be eliminated in some environments by a change to steel grades with higher Charpy impact values of 16 [1], [2].
Between 2004 and 2010, many Australian coal mines had reported further SCC premature rock bolt failures and these now included the higher Charpy impact toughness steels of approximately 16 Joules. In 2010, the current UNSW ARC and industry funded linkage research project LP100200238 commenced with significantly more resources than previous projects.
The UNSW ARC linkage project had three main areas of investigation towards achieving its aims: (1) laboratory bolt corrosion experiments aimed at re-producing SCC failures, (2) metallurgical examinations aimed at defining the causes and mechanisms of coal mine SCC, and (3) coal mine data collection to identify the extent and environmental contributors to the problem. This paper discusses the coal mine data collection and analysis on environmental contributors to bolt corrosion.
Section snippets
Broken bolt database
Approximately 200 broken rock bolts have been collected from twelve Australian coal mines and received into UNSW laboratories for various analyses. All of the rock bolts were 21.7 mm core diameter, “X” grade steel which is typically >600 MPa yield and >840 MPa UTS. Three main failure modes are visually evident as shown in Fig. 1. Rebar SCC represented 63% of the broken bolts, localized pitting corrosion represented 30% and thread SCC was 7%. It was obvious from mine sites with an adequate number
Mine 1
Mine 1 represented 20% of the bolts within the broken bolt database at UNSW Australia. The portion of main roadways visually inspected for broken bolts is schematically shown in Fig. 2 with respect to some major features. The support pattern included 2.1 m rock bolts and a mixture of 4 and 8 m long cable bolts. The right hand side of the main roadways was known to contain the majority of broken bolts, and heading #5 was known to have water dripping from many of the roof and cable bolts. A total
Groundwater
Thirty-eight rock bolt dripper groundwater specimens have been collected across twelve Australian coal mines, spread across eight coal seams and five coalfields. Six of the mines had recorded premature rock bolt failure due to corrosion related mechanisms. Corrosivity studies typically focus on a limited number of groundwater features including pH, alkalinity, total dissolved solids (TDS), aggressive anions (Cl- and SO42−), dissolved oxygen (DO) and temperature.
The groundwater analyses were
Corrosion coupons
Dorion, Hadjigeorgiou and Ghali completed steel coupon testing in mine groundwater of Canadian and Villaescusa, Hassell and Thompson in Australian hard rock mines respectively, with the aim of correlating groundwater characteristics to the general corrosion rate in millimeters per year [8], [9]. The coupons were carefully prepared specimens to the ASTM standard G4 as shown in Fig. 7, with the surface mill scale removed. These carefully prepared steel coupons have been essential in building
Conclusions
The current UNSW Australia study into premature failure of coal mine rock bolts has collected over 200 broken bolts from twelve Australian coal mines. An extensive underground survey revealed that the number of broken bolts collected and supplied to UNSW may represent only 12% of the total number visually found underground. Non-destructive load testing in the very worst affected roadway containing 10% visually broken bolts has revealed up to 24% premature failure rate of the remaining intact
Acknowledgments
Industry partners have made significant cash and in-kind contributions to the UNSW ARC funded project, these companies include; Anglo American Coal, BHP Billiton, Centennial Coal, Glencore, Jennmar Australia and Whitehaven Coal.
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