In collaboration with consulting company SCT, a range of field and laboratory studies were conducted at the mines to measure the impact of reduced bolt effectiveness.

 

The research not only identified the gloving mechanisms, but also found gloving to be a systematic and widespread phenomenon throughout Solid Energy coal mines. Results suggest that around 10-30% of primary reinforcement dollars are wasted.

 

Trials at the five mines showed the very process of installing bolts resulted in systemic quality issues. Design modifications partly overcame the problem, but researchers could not find a complete solution and research and development work is still ongoing in this area.

 

Traditionally, gloving has been attributed to poor installation methods, issues related to equipment, poor handling of consumables, or geological conditions. However, Solid Energy’s research has discounted these assumptions.

 

An extensive investigation into the installed quality and effectiveness of roof bolts in coal mines showed that gloving and unmixing occurred across the range of resin and/or bolt manufacturers, and in a variety of roof types. Gloving was found in bolts installed using either hand-held pneumatic or continuous miner mounted hydraulic bolting rigs, under both run of mine conditions by operators and under controlled manufacturer’s ‘best practice’ conditions.

 

Solid Energy’s results indicate that regardless of bolt type, lithology, or how the bolts were installed, an average of 500mm of the bolt's length is affected by gloving, with 200mm typically glove and mixed and 300mm gloved and unmixed. The data sets also show a wide range in values, from 30mm to 790mm, with over 70% having over 400mm unmixed length.

 

Further SCT investigations identified a pressure front build-up as the bolt encountered the resin cartridge and was spun up the hole. Over-pressurisation and expansion of the resin cartridge then resulted in an increase in cartridge diameter. The increased diameter allowed the bolt to be spun up inside, without making sufficient contact to shred the cartridge. This culminated in a portion of the bolt being encased in the cartridge and a combination of mixed and unmixed resin mastic and catalyst.

 

That these were the causes of gloving were confirmed in test bench simulations undertaken in clear 28.5mm internal diameter polycarbonate tubing which allowed direct observation of the gloving mechanics.

 

Once the extent of the problem was identified, nine trials using bolt tip modifications and alternative profiles were conducted in an effort to develop a solution. Initial trials in June 2002 were carried out using the chamfered, horn, welded, paddle, peeled and threaded bolts. The spiralled wire, 200mm wiggled and off-centre type of profiles were trialled in May last year.

 

Of the modifications tested, only the chamfered, wiggled and off-centre bolts offered any significant improvement, with each achieving over 90% effective bolt length.

 

“The widespread nature of the gloving problem led to the realisation that the current roof support design assumptions were compromised along with the subsequent deformation trigger levels which form a major component of most strata management plans,” researchers said.

 

Numerical modelling showed that a shorter effective bolt length had a significant effect on the design assumptions. “In the case presented, the shorter bolts could not interact as a pattern and the reinforcement afforded to the roof was reduced, compared to the design assumption of 1800mm-long bolts.

 

“The result was an increased height of softening at lower levels of deformation, leading to the isolation and over stressing of the immediate and secondary roof sections,” researchers said.

 

The findings of the modelling led to trigger levels being revised and incorporated into an updated strata management plan.

 

For the immediate future, researchers advise mine operators to regularly inspect the appearance of bolts in roof falls and around goaf edges. Gloved bolts will also give off a resin smell when exposed. This will give an indication of the prevalence of gloving at an operation and should be included in the mine’s strata management plan.

 

The research was presented at the 5th Underground Coal Operator's Conference in Wollongong in February. The paper, “Investigation into the extent and mechanisms of gloving and un-mixed resin in fully encapsulated roof bolts,” was authored by Richard Campbell, geological engineer, (SCT).