Thick membrane support in the form of shotcrete (sprayed concrete) has been used in underground mines since the 1950s. It is generally applied at a thickness of 25-100mm in mines, depending on need. Thin membranes were applied at a typical thickness of only 2-6mm, MBT said. “Thin membranes are not designed to replace shotcrete, but are a suitable alternative to mesh. Membranes can also be used in conjunction with shotcrete as part of an overall support regime,” the company said.
MBT Australia’s John Gelson said the company had developed a unique membrane, Masterseal 840 R01A, which was a two-component, highly reactive polymer that was sprayed onto a rock surface to provide a skin support and a safety barrier. It could be used equally effectively in metalliferous and coal mines, and in civil tunnelling applications.
“It is the only reactive system presently on the market that does not contain isocyanates and is considered non-toxic,” Gelson said. “Several long-term studies have shown the material to be non-carcinogenic.”
Even though certain thin membranes had been available for several years, intense mining industry interest in them had only grown in the past few years. Gelson said the interest was mainly in their potential to safely reduce mining cycle times and hence improve productivity.
Among its many uses underground, a key use in coal was for rib-side support where it secured fretting ground. The thin membrane also sealed coal from air and gases.
“Masterseal 840 R01A has undergone considerable testing in Australia, USA, Europe and South Africa for a variety of operational health and safety issues,” Gelson said. “The tests have proved the product is safe, viable and an economic solution for use as a rock support membrane.”
Gelson said the Masterseal membrane offered several advantages over similar types of support, such as mesh and lacing. These included:
* Reactive loads are generated with far less rock movement. The membrane reinforces the surface structure.
* High application productivity can be achieved.
* Remote application is possible.
* The membrane support is not prone to corrosion.
* It is easier to apply concurrently with the face advance.
* The membrane is less prone to blasting damage.
* Weathering of the rock surface beneath the membrane is virtually eliminated.
* The membrane cures in less than five minutes.
Masterseal 840 R01A provided further advantages over other “superskins”, including:
* It sets and cures in 3-5 minutes.
* Application equipment is less costly.
* Only low volumes are required, hence the product is easy to transport and apply.
* It can be shotcreted over at a later and more convenient time, if required.
* Masterseal 840 R01A is white, which enhances its illumination underground.
Gelson said there were a wide range of typical uses for the product underground. Apart from its use as rib-side support to stop fretting and to seal coal from air and gases, it could be applied to provide face support during tunnel development, scat and loose rock control, and pillar reinforcement. It sealed rock prone to weathering, and could be used as a support component to help reduce rockburst damage. The thin membrane could also be used as a ventilation sealant.
“Thin membranes are not generally a replacement for quality shotcrete, where a thickness of 25mm or more is required,” Gelson said.
“The combination, however, offers good potential in severe rock instability areas as the membrane can give rapid support and the shotcrete — generally fibre reinforced — provides the high compressive strength and durability. For this to be practical, the shotcrete needs to have good adhesion to the specific membrane, and few membranes are suitable for this.
“There is little doubt that thin support membranes such as Masterseal 840 R01A hold great potential as a support element as part of a support system, mainly to replace screen and wire mesh.”
Originally published in the March 2001 edition of Australia's Longwalls.