Underground » Strata Control and Windblasts
During a 1997 ACARP-funded mission to South Africa, an acrylic-based polymer membrane called Evermine (formerly known as Everbond) was identified as having considerable potential for Australian coal mining environments. Evermine was developed in South Africa for the CSIR Mining Technology Division as a spray-on rib support material in deep level gold mines. The product was purpose-developed to have a high flexibility, an extremely high tensile strength and elasticity, high adhesion properties, quick set time and low flammability and toxicity levels.
In March 1999, the School of Mining Engineering at the University of New South Wales carried out a series of underground trials to determine the potential of Evermine as an alternative to conventional rib support systems. The mine selected was Angus Place Colliery, reputed to have some of the worst ground conditions of any Australian underground coal mine. Ribs at Angus Place are supported by conventional pattern bolting supplemented by wire mesh.
The trials indicated that, from a support performance perspective, there is potential for the introduction of Evermine or similar polymer membranes into the underground coal mining industry. However, further product development is required in order to realise this potential. The positive features of the product were that:
- it adheres strongly to coal
- it is safe to use provided that basic precautions are taken
- it is simple to apply using basic equipment
- it is a white to light grey colour, thus improving illumination and reducing the need for stone dust
- it resists unravelling or spalling of coal
- initial results in a cut-through trial indicated that ribs that were sprayed exhibited less displacement and spalling than unsprayed ribs, particularly when subjected to abutment loads from an approaching longwall
- in a development heading trial, the application of the membrane allowed the rib to displace significantly more than the unsprayed control sites but the Evermine membrane resisted unravelling of the outer rib skin.
On the other hand, the following are currently impediments to the use of Evermine as a stand-alone rib support system or as a complementary rib support measure:
- the product was slow to set and vulnerable to damage especially in the first 24 hours after application.
- field trials showed that, without the complementary use of mesh, an Evermine skin is inadequate to reinforce highly disturbed ribs such as those at Angus Place. It may be adequate for mines with good ribs where mesh is not required. However, in these cases there may not be a need for Evermine rib support.
- the speed of application is too slow for commercial operation. There is no doubt however, that bulk delivery to the face and rapid application of the product at the face is possible
- doubts remain about its flexible material properties in the long term
Trials on a "link'n lock" chock were also conducted to determine if applying a confining membrane could improve the chock's post failure characteristics. The results were inconclusive but observations indicated that the chock continued to carry load after the longwall face had passed. Therefore, it appeared as if Evermine could enhance a chock's residual strength and load-bearing capacity.
A parallel series of laboratory and underground trials also tested the potential of Evermine as a sealing agent on ventilation stoppings. In the laboratory, the tests indicated that the membrane itself could tolerate pressures of approximately 28 kPa (4 psi), exceeding the failure criterion of the commonly used plasterboard stoppings but below the Type B standard of 35 kPa (5 psi) recommended by the Queensland Department of Mines and Energy. The tests indicated that the membrane could deform to extraordinary shapes while maintaining loading before eventually failing.
The effect of an explosive event on a ventilation stopping sprayed with Evermine was simulated in underground trials consisting of three detonations of explosive charges in a testing chamber. The explosions generated theoretical pressures of 3, 4 and 5 psi respectively but did not destroy the functionality of the stopping. The pressures generated were sufficient to separate the plasterboard sheets, enabling leakage of air to occur. Maximum deflections of the plasterboard for the three tests were 8.5, 26 and 45 mm respectively.
Plasterboard stoppings are nominally rated to withstand a 2 psi explosion pressure. Due to instrumentation failure, tests to confirm this in the blasting chamber were not completed. However, the application of Evermine appeared to enhance the ability of the stopping to withstand considerably more explosive pressure than would a conventional plasterboard stopping while still maintaining its integrity. Of particular note was the resistance exhibited to separation at the roof and corners of the rib where it would be expected that stopping failure would be initiated. The trials demonstrated that there is potential for Evermine to provide a cost-effective seal to existing plasterboard stoppings used in the industry for ventilation control.
The impacts on health caused by the use of Evermine were also investigated in this project. The trials revealed that formaldehyde was emitted during spraying. Formaldehyde is a colourless gas with a pungent odour and exposure at high levels can lead to coughing, eye or skin irritation or allergic reactions. The levels detected were below the 8-hour time weighted average national exposure standard and the short-term exposure limit.
Before Evermine can be used on a routine basis, the following is recommended:
- further product development on the part of the manufacturer is necessary
- at the moment, the product should not be used to replace steel mesh
- any future trials should be in a "tamer" environment where mesh is not used and focus on controlling spall from between rib bolts and straps for example.