Investigations into the corrosive environments contributing to premature failure of Australian coal mine rock bolts

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Abstract

University of New South Wales (UNSW Australia) had been involved in the study of premature failure of rock bolts in Australia coal mines from the initial identification of the problem in 1999. Rock bolt steel changes over the last decade appear to have not reduced the incidence of failures. A broadened UNSW research project funded by the Australian Research Council (ARC) and Industry has targeted finding the environmental causes through extensive field and laboratory experiments. This paper describes the field studies conducted in underground coal mines, in particular attempts to measure the contribution to corrosion from groundwater, mineralogy and microbial activity. Various underground survey techniques were used to determine the extent of broken bolts, with the presence of both stress corrosion cracking (SCC) and localized deep pitting making no single technique suitable on their own. Groundwater found dripping from bolts across various coalfields in Australia were found to be not aggressive and known groundwater corrosivity classification systems did not correlate to where broken bolts were found. In-hole coupon bolts placed in roof strata containing claystone bands confirmed the clay as being a major contributor to corrosion. Microbes capable of contributing to steel corrosion were found to be present in groundwater, and culturing of the microbes taken from in-situ coupon bolts proved that the bacteria was present on the bolt surface. An ‘in-hole bolt corrosion coupon’ development by the project may have multiple benefits of (1) helping quantify newly developed corrosivity classification systems, (2) providing an in-situ ground support corrosion monitoring tool, and (3) for testing possible corrosion protection solutions.

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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