Optimisation of Roof Bolt Length Based on Improved Resin Performance

This study compared the performance of two types of resin capsules used for full encapsulation of roof bolts in underground coal mines. The first, a 15:1 mastic to catalyst capsule which is the predominant system used in Australian mines, despite some well-documented problems. The second, a 2:1 mastic to catalyst capsule, is commonly used in US mines. The basis for the study is that if the 2:1 resin capsules are found to perform better than the current Australian resin capsules, then shorter bolt lengths may be possible with no associated increase in geotechnical risk. The cost and productivity benefits are obvious.

Australian researchers identified the endemic problems of gloved and poorly mixed (uncured) resin in Australian underground mines more than a decade ago, yet, the widely used 15:1 mastic to catalyst resin bolt systems have not significantly changed in response. This study tested whether 2:1 mastic to catalyst resin systems used in the US will improve the integrity and reliability of Australian resin bolts.

Published research into gloving, uncured resin, roof bolt installation pressure, and load transfer guided the surface testing method developed as part of this project. This methodology replicated installations of fully encapsulated resin-anchored roof bolts as used in the Australian underground coal industry. Two parameters were tested:

· The resin capsule mastic-to-catalyst ratios of 2:1 and 15:1; and

· Bolt hole diameters of 28 mm and 30 mm (to represent a typical bolt hole diameter, and a bolt hole subject to overbreak).

Data was collected and analysed for gloving, uncured resin, installation pressure, and load transfer performance. Key results included:

· The magnitude of gloving was up to eight times greater for the 15:1 resin bolts (12-15% of the bolt) compared with the 2:1 resin bolts (2% of the bolt);

· The magnitude of uncured resin was far greater for 15:1 resin bolts installed into both the in 28 mm and the 30 mm diameter holes (5% and 26% respectively), than for 2:1 resin bolts (0.2% for both diameters).

The results clearly indicate that the 2:1 catalyst to mastic resin is a far more reliable in terms of the endemic problems of gloving and uncured resin, and that it performs far more robustly across two 'typical' bolt hole sizes.

The implications are that if industry wants a roof bolt resin system that offers both reliable mixing and minimal gloving of the capsule film, then it should adopt the use of 2:1 resins as an industry standard. Further, the resin system can be readily introduced into Australian coal mines without major changes to bolting systems or procedures.

An e-newsletter has also been published for this project, highlighting its significance for the industry.