Published in March 2008 Australian Longwall Magazine

Pilot-scale trials of a polymer-based alternative to steel mesh for coal mine strata reinforcement are expected to begin later this year under an ACARP-funded project by the University of Wollongong. The pilot will lead to full-scale trials which have been pencilled in for late 2009 with commercialisation of a product hoped for the end of 2010.

If all goes to plan, industry will have at its fingertips a polymer-based alternative to steel mesh that provides an effective skin confinement measure equivalent or superior to steel mesh; can be spray-applied; requires minimal human intervention in its installation; removes personnel from the immediate face area; enables higher underground roadway development rates to be achieved; is safe to use; and is cost effective.

The project is a result of the underground coal industry’s need to increase roadway development rates, and is a piece in the puzzle to fully automate the roadway development process.

An ACARP review that looked at current practices in Australian roadway development highlighted the need for the development of an alternative skin reinforcement and confinement measure to eliminate the use and handling of steel mesh. The study found whilst mesh was installed under current manual practices, automated self-drilling bolt technology could not effectively go ahead.

The investigation into an alternative for steel mesh began in mid-2006 through a six-month feasibility study jointly funded by the University of Wollongong and six longwall coal mining companies.

The result of the study was the recognition that a viable polymeric alternative to steel mesh could be developed. With that confidence under the researchers’ belts, they continued with the project for a further eight months with 100% coal industry funding.

The project was picked up in September 2007 by ACARP, which has backed the research since.

Currently researchers at Wollongong University in charge of the project – including Ernest Baafi, Chris Lukey, Ian Porter, Jan Nemcik, Geoff Spinks and Dylan Riessen – have identified the major property requirements of the mesh replacement, and developed a number of polymeric systems with a range of mechanical properties. In addition, they have investigated the effect of reinforcing additives, and developed a number of tests for flexibility and adhesion to coal and rock substrates.

So what is liquid mesh? The product consists of two liquid components which are mixed immediately prior to spraying, with the addition of reinforcement, and hardens to a rigid solid within a few seconds (researchers are aiming for 10–30 second hardening time). The solidified material adheres to the rock or coal surface, and is then bolted through.

The researchers said it is envisaged the mesh will be applied as a two-component liquid via a bank of fixed spray heads mounted on the front of the continuous miner.

Some modification to the continuous miner will be required in order to accommodate the fixed spray heads and the automatic bolters will need to be repositioned further behind the spray heads.

While the liquid mesh will do the same job as steel mesh – in stopping smaller roof parts from injuring miners – the researchers said the material also adheres to the substrate, so there is potentially an additional reinforcement mechanism.

“Also, because the material is applied conformably [that is, it adopts the profile of the underlying cut surface], any irregularities provide additional internal strengthening to support the strata prior to bolting. Because of the rapid hardening and conformal application, the material may also find uses in other applications such as gutter stabilisation,” researcher Chris Lukey said.

For the research team, the major challenges have been to find a material that solidifies in the required timeframe, has the right mix of mechanical properties, and most importantly does not have any toxic or irritant emissions before, during or after application.

“A geotechnical investigation of the role of steel mesh in underground roadways has commenced, and the results of that analysis will determine the appropriate property mix of the polymeric alternative,” Lukey said.

While the team has not done any detailed analysis of comparing application times of steel versus liquid mesh, they said they believed substantial improvements in development rates would be achieved using the liquid mesh.

Whilst productivity gains are key to this project, increased safety is also a consideration. Because the goal is to fully automate the roadway development process, it means personnel will be removed from the immediate face area; and manual lifting and twisting will be eliminated.

Researchers are yet to take the mesh to site trials, but will be conducting laboratory-scale spray and bolting tests soon to identify any potential problems with liquid viscosity and set time. They will also investigate fire retardant and anti-static additives in order to further improve the safety features of the new material.